Denitrification as a Means of Addressing Nitrate-Contaminated Groundwater
on Cape Cod, Massachusetts
by
ARCHINES
MASSACHUSETTS INSTITUTE
OF TECHNOLOLGY
Kenneth M. Motolenich-Salas
B.S. Chemical Engineering
University of Notre Dame
(1995)
JUL 02 2015
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Submitted to the Department of Civil
and Environmental Engineering in Partial
Fulfillment of the Requirements for the Degree of
MASTER OF ENGINEERING
In Civil and Environmental Engineering
at the
Massachusetts Institute of Technology
June 1997
@Massachusetts Institute of Technology
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01F6-Department
of Civil and Environ ental Engineering
PI
9 May 1997
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Professor, Civil and Environmental Engineering
Thesis Supervisor
n
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Accepted by
Joseph M. Sussman, Chairman
Departmental Committee on Graduate Students
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Denitrification as a Means of Addressing Nitrate-Contaminated Groundwater
on Cape Cod, Massachusetts
by
Kenneth M. Motolenich-Salas
Submitted to the Department of Civil and Environmental
Engineering on 9 May 1997 in partial fulfillment of
the requirements for the Degree of
Master of Engineering in Civil and Environmental Engineering
ABSTRACT
The residents of Cape Cod face a problem of nitrate contamination of their
groundwater (their primary source of drinking water) and their coastal and aquatic
environments. Groundwater is the only source of drinking water on Cape Cod and
the aquifer is defined as a "sole source aquifer" by the Safe Drinking Water Act.
While many activities contribute nitrate (NO 3 -) contamination to groundwater,
nitrate contamination from land application poses the greatest threat on Cape Cod.
Only a few small areas on Cape Cod are sewered, and the majority of homes and
businesses rely on septic systems. Increased urban development has increased the
frequency of installation of septic systems. In many locations, the density of septic
systems is greater than the natural ability of the subsurface environment to receive
and purify system effluents prior to their movement into groundwater. Many of
Cape Cod's environmental resources, including coastal receiving waters, marine
embayments threatened with eutrophication, endangered wetlands, and Areas of
Critical Environmental Concern (ACECs), are also threatened by nitratecontaminated groundwater flowing into the coastal waters of Cape Cod, which are
extremely sensitive to eutrophication from excess nitrogen loading.
In order to address nitrate-contaminated groundwater on Cape Cod, solutions
based on biological denitrification should be considered. In this work, these
solutions are discussed and explored. First, the major sources of contamination and
possible health and environmental effects are discussed. Second, the fate and
transport of nitrate in the subsurface environment is analyzed, with a detailed
discussion of the factors governing biological denitrification. Third, the current
status of groundwater nitrate contamination on Cape Cod is detailed. Fourth,
possible options, alternative septic systems and in-situ remedial schemes, which all
use biological denitrification as a means of attenuating nitrate in septic system
effluent, are presented. Lastly, a proposal for action to deal with nitrate
contamination on Cape Cod and suggestions for future study and long-term action
for domestic sewage are given, based on my opinion of the scientific and
engineering aspects of the circumstances of the contamination.
Thesis Superviosr: Harold F. Hemond
Title: Professor of Civil and Environmental Engineering
2
Acknowledgments
My time here at MIT has had its ups and downs. Throughout it all, I could
rely on certain individuals and institutions to provide me with a kind word, good
advice, a smile.
I would like to first thank the GEM (National Consortium for Graduate
Degrees for Minorities in Engineering and Science), based at my alma mater; my
sponsoring company, E.I. duPont de Nemours, Inc.; the Department of Civil and
Environmental Engineering at MIT; the United States Navy; and the University of
Notre Dame. GEM provided to me the opportunity to pursue a Master's degree in
Environmental Engineering. E.I. duPont de Nemours gave me invaluable work
experience, an undergraduate scholarship, and, through their benevolent giving,
funding for graduate school. The Department of Civil and Environmental
Engineering, especially Cynthia Stewart and Pat Dixon, helped me through the
tough Summer of 1996. The United States Navy has also provided me with
numerous opportunities, including a R.O.T.C. scholarship to Notre Dame, allowing
me to enter the Navy debt free, and for granting me a leave of absence to pursue
graduate studies immediately after my undergraduate years, so that I did not have a
chance to forget all that I had learned. Lastly, I want to thank my beloved alma
mater, the University of Notre Dame, for educating me and being more than an
exemplary institution of higher learning: Notre Dame continues being a source of
inspiration for me.
However, my true source of inspiration throughout my life is people whom I
love. My mother and father, Peter and Nilda Motolenich, have always provided for
me: financially, spiritually, emotionally, and always lovingly. Finally, I owe my
happiness in life to the love of my life, my future wife, Anita Varma. Before she
3
came into my life, I was alone walking a journey with no definite destination. With
Anita, I have a loving companion with whom I can share all the joys of life. It is to
Anita, the woman I love and want to spend my life with, that I dedicate this thesis.
4
Table of Contents
Page
Abstract
Acknowledgments
3
List of Tables
8
List of Figures
9
Chapter 1. Introduction
12
1.1 Scope of the Problem
12
1.2 Sources of Nitrate
15
1.3 Health and Environmental Effects
16
1.3.1 Health Effects
16
1.3.1(a) Methemoglobinemia
17
1.3.1(b) Gastric Cancer
17
1.3.1(c) Other Health Effects
18
1.3.1(d) Cancer on Cape Cod
18
1.3.2 Environmental Effects
19
1.3.2(a) Algal Blooms
20
1.3.2(b) Case Study: Waquoit Bay, MA
20
21
1.4 Conclusions
Chapter 2. Nitrate in Groundwater
22
2.1 Nitrate in Groundwater: Introduction
22
2.2 Nitrate Transport
24
2.2.1 Physical Processes
25
2.2.2 Sinks
27
2.3 Microbial Activity in Groundwater
28
2.4 Biological Denitrification
29
5
2.4.1 Conditions Needed for Denitrification
2.4.1(a) Oxygen
2.4.1(b) Microorganisms and Organic Carbon (OC)
2.4.1(c) Nutrients, Temperature, and pH
2.4.1(d) Redox Potential (EH)
2.4.1(e) Depth in Aquifer
2.4.1(f) Depth of Aquifer
2.4.1(g)
Losses of Nitrate in the Unsaturated Vadose Zone
2.5 Denitrification Rates in Groundwater Environments
2.6 Conclusion
Chapter 3. Nitrate Contamination on Cape Cod
3.1 Introduction
3.2 Cape Cod Nitrate Contamination
3.3 Use of Septic Systems
3.4 Nitrate from Septic Systems
3.5 Groundwater on Cape Cod
3.6 Effects of Nitrate on Water and Environmental Quality on Cape Cod
3.6.1 Water Quality
3.6.2 Environmental Quality
3.6.2(a) Wetlands
3.6.2(b) Areas of Critical Environmental Concern
3.7 Urbanization and Nitrate Contamination
3.8 Conclusion
Chapter 4. Remediation Technology
4.1 Introduction: Motivation for Addressing Nitrate Contamination
4.2 Title V
4.3 Zones of Groundwater Protection
6
4.4 Environmental Motivation for Action
76
4.5 Remediation Technology
78
4.5.1 Permitted Alternative Septic Systems
80
4.5.1(a) The RUCK System
84
4.5.1(b) The FAST System
88
4.5.1(c) Ekofinn Bioclere TM
90
4.5.1(d) Recirculating Sand Filter (RSF)
93
4.5.1(e) Sequencing Batch Reactors (SBR)
95
4.6 In-Situ Treatments
96
4.6.1(a) Daisy System
97
4.6.1(b) Peat
100
4.6.1(c) Reactive Porous Media Barriers
104
4.6.1(d) Bioremediation via Autotrophic, Hydrogen-Oxidizing,
Denitrifiers on Cape Cod
106
4.6.1(e) Sulfur/Limestone Denitrification
111
4.7 Conclusion
112
Chapter 5. What Should Be Done For Cape Cod's Nitrate Contamin ation?
113
5.1 Introduction
113
5.2 Immediate Action for Domestic Sewage Disposal
114
5.2.1 Homes Near the Coast
115
5.2.2 Large Cluster of Homes
117
5.2.3 Other Domestic Sewage
118
5.3 Large-Scale Contamination Remedial Action
118
5.4 Conclusion: Future Work and Long Term Action for
Domestic Sewage Disposal
120
127
References
7
List of Tables
Page
Table 1: Microbial Growth Requirements in Subsurface Environments
28
Table 2: Field Estimates of Denitrification Rates in Aquifers
40
Table 3: Towns on Cape Cod and Percentage of Homes with Sewer Connections
57
Table 4: Physical Properties and Constituents in Cape Cod's Groundwater
62
Table 5: Major Wetlands on Cape Cod
65
Table 6: Waste Water Loading Credits For Four Alternative Septic Systems
83
Table 7: Effluent from Porter's Orchard Partnership
87
Table 8: Denitrification Efficiencies for the Ekofinn Bioclere TM Septic System
92
Table 9: Summary of Construction Details for Experimental Peat Filter Bed
103
Table 10: Treatment of Septic Tank Effluent by Sphagnum Peat Filter Beds
103
8
List of Figures
Page
Figure 1: Sources and Pathways of Nitrogen in the Subsurface Environment
23
Figure 2: Movement of Fertilizer Nitrate Through Sandy Loam Soil
26
Figure 3: Oxidation of Organic Carbon in the Saturated Zone with the
Sequence of Electron Acceptors and the Resulting Reduced
Inorganic Compounds
32
Figure 4: Stability Diagram of Nitrogen Species Showing the Predicted Species
of Most Groundwater at 25 'C and 1 atm
39
Figure 5: Nitrous Oxide Production by Slurried Core Material at 1.5 m
Beneath the Water Table versus Time
42
Figure 6: Vertical Profile of Rates of Denitrification for Slurried Core Material
42
Figure 7: Time Course of Nitrous Oxide Production by Core Samples and
Well Water Samples from Two Depths
43
Figure 8: Frequency Distributions of Denitrification .Rates Measured in
Sediment Cores
44
Figure 9: Vertical Profiles of Denitrification Rates, Dissolved Oxygen, NitrateNitrogen, and Dissolved Organic Carbon Measured in Sediment
Cores
45
Figure 10: Nitrous Oxide Production in Sediment Slurries with Nitrate and
Carbon Addition
47
Figure 11: Political Map of Cape Cod, Massachusetts
49
Figure 12: Landfills and Sewage-Disposal Sites on Cape Cod
50
Figure 13: Nitrate Concentrations and the Frequency Distribution on Cape
Cod, 1980-1984
53
Figure 14: Approximate Populations Using Septic Tanks
55
Figure 15: Schematic Cross-Section through a Conventional Septic Tank Soil
Disposal System for On-Site Disposal and Treatment of Domestic
Liquid Waste
56
9
Figure 16: (a) Schematic Cross Section of a Conventional Septic System,
Including Septic Tank, Distribution Pipe, and Groundwater Plume
(b) Sequence of Simplified Redox Reactions in the Two Major
Zones of a Conventional Septic System: the Septic Tank and the
Drain Field
56
Figure 17: Cape Cod Regional ACEC Locus Map
66
Figure 18: Changes in Eelgrass Distribution in Waquoit Bay, 1951-1987
68
Figure 19: Growth in Housing Units in Massachusetts Counties, 1980-1990 in
Relation to Distance From the Sea
69
Figure 20: Nitrate Concentrations in Groundwater Below Areas of Cape Cod
Having Different Densities of Buildings
69
Figure 21: Housing Density on Cape Cod, 1985
70
Figure 22: Median Nitrate Concentration as a Function of Housing Density
72
Figure 23: Median Nitrate Concentration as a Function of Building Density
72
Figure 24: Recharge Areas to a Pumped Well in a Valley-Fill Aquifer
77
Figure 25: Ocean Sanctuaries of Massachusetts
79
Figure 26: Alternative Onsite Septic Systems Installed to Date in Barnstable
County: Estimates as of August, 1996
82
Figure 27: RUCK System
86
Figure 28: Smith and Loveless Single Home FAST System
89
Figure 29: Sectional View of BioclereTM Components
91
Figure 30: Final Effluent Nitrogen Component Concentrations
91
Figure 31: Principal Components of a Recirculating Sand Filter System
94
Figure 32: Schematic Description of In-Situ Denitrification
98
Figure 33: Schematic Description of the Daisy System
98
Figure 34: Schematic Diagram of System One and Two Peat Filter Beds
102
10
Figure 35: Killarney and Borden (Horizontal) Denitrification Layers Showing
Chemical Profiles (mg/L) After One Year of Operation
107
Figure 36: Long Point (Vertical) Denitrification Wall with Chemical Levels
Up- and Downgradient of the Wall
108
Figure 37: Time Course of Nitrate Concentration (mM) in Sediment Slurries
110
11
Chapter 1
Introduction
1.1 Scope of the Problem
Groundwater is a vital resource. Over 50% of drinking water in the U.S. is
supplied by groundwater and is the source of drinking water in 95% of rural farm
areas (Follett, 1989). In Massachusetts, groundwater consists of 22% of public
drinking water supplies and 97% in rural areas (Persky, 1986). Groundwater is the
only source of drinking water on Cape Cod.
Many activities of modern society contribute nitrate (NO 3 -) contamination to
groundwater. Nitrates are mobile in soil and will often present a potential threat to
groundwater whenever they are used as a fertilizer or when nitrogen is discharged
onto land surfaces by septic tanks or under feedlots. Wells and groundwater
exhibiting nitrate contamination have been noted in every state in the USA (Follett,
1989). Nielsen and Lee (1986) from the U.S. Department of Agriculture's Economic
Research Service noted that wells with greater than 3 mg/L occurred in 29% of all
the counties in the U.S.A. Nitrate is the most frequently reported health-related
contaminant in the world's groundwater systems (Spalding and Parrott, 1994).
Nitrate is also an environmental contaminant of concern.
Groundwater contributes
to the nitrate contamination of surface waters through base-flows to streams and
lakes. In coastal zones from Cape Cod to Texas, groundwater conveys land-derived
nitrogen to receiving waters (Valiela and D'Elia, 1992). High nitrate concentrations
in groundwater could cause a serious environmental quality problem.
The conventional septic tank-soil absorption system is the most convenient
and economical method for home sewage disposal. Increased development of
12
housing and mobile home parks in rural areas and in small towns without
domestic waste treatment plants has increased the frequency of installation of septic
systems for sewage treatment and disposal. Groundwater degradation has occurred
in many areas having high densities of septic systems, including Cape Cod, with the
degradation exemplified by high concentrations of nitrates, bacteria, and other
contaminants. Septic system problems are magnified by the fact that in many areas,
a substantial reliance on subsurface disposal systems is paralleled by a reliance on
private wells for drinking water supplies. Hence, nitrate from septic system effluent
can have a detrimental effect on health by contaminating the drinking water supply
and the environment by groundwater flow to surface waters.
Water quality problems may occur when the amount of nitrogen released
from septic systems exceed health and ecological limits of water resources. While
nitrogen from one septic system may not be a problem, a concentration of septic
systems in a resource area may threaten water quality by exceeding safe limits. What
is even more critical for Cape Cod is the extreme sensitivity of marine recharge areas
to excess nitrogen loading. A marine embayment is very sensitive to nitrogen
loading, depending on the shape and depth of the embayment. Its nitrogen limit
can be exceeded at a housing density as low as one house per three acres in its water
shed or marine water recharge area (Cape Cod Commission, 1996).
Nitrate in groundwater is subject both to reactions within aquifer systems and
to effects from surface land use and reactions within the unsaturated zone above the
aquifer (Follett, 1989). Biological denitrification is important in many aquifers for
removing nitrate. The rate of denitrification is governed by redox conditions, the
existence of denitrifier populations, and available carbon. Increased dissolved
organic carbon (DOC) in a contaminated aquifer would probably increase
denitrification.
This, coupled with the development of more anaerobic conditions
13
from microbial activity, could be used as a remediation approach to nitratecontaminated groundwater.
There is a possibility that the rates of denitrification in aquifers may be greater
than previously estimated. A number of studies conclude that rates of
denitrification may be relatively high in groundwater, based on three lines of
evidence:
Larger than expected concentrations of dissolved organic matter:
Concentrations of labile organic matter in groundwater (0.7-27 mg
DOC/L, Fiebig et al., 1990) may be greater than was thought to occur.
Such concentrations make denitrification more likely in groundwaters
than in unsaturated zones, because the carbon might support microbial
activity which would result in lower oxygen concentrations.
*
Downgradient lowering of nitrogen concentrations: Correlational
evidence that nitrate losses occur downgradient (in waste water or
fertilizer plumes) has been obtained by examining distributions of
solutes (Trudell et al., 1986; Smith and Duff, 1988; Robertson et al.,
1991). In a sand and gravel aquifer on Cape Cod, denitrification is the
predominant nitrate-reducing mechanism within the aquifer (Smith
and Duff, 1988).
*
Tracer experiments: Experimental injections of nitrate into aquifers
showed that nitrate disappeared downgradient faster than expected
based on dilution of a conservative tracer (bromide, Br-). Further,
nitrate consumption was matched by increases in bicarbonate (HCO3)
and decreases in dissolved oxygen (02), as expected from carbon
mineralization attributable to denitrification (Trudell et al., 1986).
One could thus use denitrification as a possible means of addressing nitrate
contamination in groundwater on Cape Cod.
It should be noted that many researchers report nitrate concentrations either
as nitrate or as nitrate-nitrogen (N0 3 -N).
The drinking water standard, to which I
will be referring later, is an example of the latter. To clarify, the drinking water
standard is
14
10 mg N0 3 --N/L (nitrate as nitrogen) = 44.3 mg N0 3 ~/L (nitrate)
It is necessary to establish a framework by which to investigate possible
remediation technologies and/or management options for groundwater
contamination. One must answer the following questions initially in order to
understand the motivation for addressing contamination:
1) What are the major sources of contamination?
2) What are the possible health effects?
3) What are the possible environmental effects?
I will answer these questions in this chapter, with reference to the sources and
environmental and health effects specific to Cape Cod and its residents.
1.2 Sources of Nitrate
Nitrate occurs naturally in the environment at low concentrations; around
0.05 mg/L from the decay of vegetation and the infiltration of nitrogen-laden
precipitation (Frimpter et al., 1990). Nitrate in groundwater can also arise from
deposits laid down during geologic times (Boyce, 1976). Concentrations of N0 3 ~-N
greater than 0.5 mg/L probably reflect anthropogenic inputs. Anthropogenic sources
of nitrate that contribute to groundwater quality problems include typical point
sources, such as sites related to the disposal of human and animal sewage, in
particular areas with a high-density of individual septic systems; industrial sites,
related to food processing, including dairy and poultry feedlots; munitions, or some
polyresin facilities; and sites where handling and accidental spills of nitrogenous
materials may accumulate (Vomocil, 1987). A long-range source is atmospheric
deposition. On Cape Cod, the major source is from domestic septic systems.
15
1.3 Health and Environmental Effects
Increased attention nationwide on nitrate as a contaminant has been
prompted by health and environmental studies and an increasing number of
groundwater sources experiencing contamination. For example, the
Commonwealth of Massachusetts Department of Environmental Protection has set
a planning goal of 5 mg N0 3--N/L, which is one-half of the federally mandated
drinking water standard for nitrate. The Commonwealth of Massachusetts has
adopted this as a goal since a consistent background concentration of 5 mg N0 3--N/L
has been shown to correlate to isolated exceedances of the drinking water standard.
The planning guideline must be met for Zone II (hydrogeologically defined
wellhead protection areas for public supply wells) as described in the Massachusetts
Water Supply Regulations 310 CMR 22.21(2)(d). However, concentrations of N0 3 -N 10 to 100 times less than the drinking water standard can cause adverse
environmental effects (Geist, personal communication).
1.3.1 Health Effects
The importance of groundwater is illustrated by data which shows that
groundwater is the source of drinking water for about half of the population and for
about 85% of the rural population (CAST, 1985). Cape Cod depends almost entirely
on groundwater for its drinking water, making it necessary to ensure the protection
of supplies from present and future contamination. Nitrate is one of the primary
contaminants of concern. Nitrate poses a health hazard. Public health standards for
nitrate in public drinking water supplies have been set at 10 mg N0 3 -N/L. Concern
arises when nitrate accumulates in groundwater because, when ingested in high
enough amounts by humans and animals, potential adverse health effects may
16
occur. These health effects are reported to include methemoglobinemia, cancer, and
possibly others.
1.3.1(a) Methemoglobinemia
Ingestion of water exceeding the nitrate standard has been shown to cause
varying levels of methemoglobinemia (also known as blue baby syndrome) and
sometimes death in infants younger than six months. Methemoglobinemia results
when ingested nitrate is converted to nitrite (NO 2 -) in the oral cavity and the
stomach. The nitrite is absorbed through the intestinal wall and combines with
hemoglobin in the blood, making it incapable of absorbing and transporting oxygen.
The blood is then unable to transport oxygen, which results in the infant's death
due to oxygen starvation. The public health standard of 10 mg N0 3 --N/L was
chosen because it was the concentration below which no cases of infant
methemoglobinemia had been identified (Walton, 1941). Adults are immune to
this condition because they have developed bacteria in their intestines which
prevent the absorption of the nitrite.
1.3.1(b) Gastric Cancer
An association between nitrate intake and gastric cancer mortality has been
suggested by Fine (1982) based upon the correlation of stomach cancer mortality rates
against previously published data on daily nitrate intake in different countries
(r=0.88). Both for healthy individuals and for special risk groups, the possible
correlation between nitrate and nitrite intake and stomach cancer is based upon
intake of possible exogenous sources as well as endogenous formation of
carcinogenic nitrosamines. It has been proposed that nitrate in drinking water
could, under certain conditions, react with compounds in foods to form
17
nitrosamines. Nitrate can be transformed in the gastrointestinal tract to nitrite,
which, in the presence of secondary or tertiary amines (R-NH2 , with R an organic
moiety), to form nitrosamines:
N03- -+ gastrointestinal tract -+ N02- + Tertiary/Secondary R-NH2 -+ nitrosamines
However, it is presently impossible to make a scientifically reliable estimate of the
risk of human cancer posed by exposure to nitrate in drinking water and the possible
formation of nitrosamines.
1.3.1(c) Other Health Effects
Other studies (Scragg, et al., 1982) have shown a positive correlation between
nitrate in groundwater supplies and birth defects. Cattle which were fed high nitrate
levels exhibited inhibited growth, shortening of life spans, increased abortion rates,
and reproductive difficulties. Although it has not been proven that the nitrate is
the cause of the malformations, the association with elevated nitrate in drinking
water has been proven. Also, high nitrate levels indicate that the water supply is
intercepting septic system leachate and may be contaminated with other harmful
chemicals contained in waste water. About half of the waterborne disease outbreaks
in the United States are related to contaminated groundwater; septic systems are the
most frequently reported cause of this contamination (Yates, 1985).
1.3.1(d) Cancer on Cape Cod
On Cape Cod, Federal and State health officials have reported an increased
incidence of certain cancers, other illnesses, and symptoms (Cape Cod Times,
January 8, 1997). The Department of Public Health of Massachusetts feels that there
is an association between environmental factors and cancer incidence on Cape Cod.
18
5,654 Upper Cape residents from Bourne to Barnstable reported having cancer from
1982 to 1990 compared with the 4,543 cases expected. That represents 24% higher
rates than the state average, and Massachusetts has the twelfth highest cancer rate in
the country (Cape Cod Times, January 8, 1997). It has not been conclusively shown
that the elevated rates of cancer are the result of increased exposure to waste water
in the drinking water, of which nitrate can be used to detect. However, when the
effect at issue is a serious and irreversible one such as cancer, even a largely
speculative risk may be seen as justification for some policy action. In this case, the
soundest way to reduce the potential risk of cancer is to reduce exposures to waste
water and, since nitrate can serve as an indicator of waste water presence, reduce
exposure to nitrate.
1.3.2 Environmental Effects
Coastal and estuarine water are the most nutrient-enriched ecosystems on
earth (Valiela et al., 1992). The principal alteration of coastal and estuarine
ecosystems today is eutrophication brought about by increased nitrogen loads
derived from human activities on land (National Academy of Sciences, 1994) and
transported by freshwater to coastal waters (Cole et al., 1993). Nitrogen transport
rates are of critical importance because rates of coastal production, as well as many
other key processes coupled to production, are set by nitrogen supply (Nixon et al.,
1996). Nitrate, a plant nutrient, is often assumed to be the limiting nutrient for
phytoplankton activity in marine systems, which could contribute to eutrophication
(a pervasive problem along the east coast of the U.S.) in estuarine environments
(Nixon and Pilsen, 1983). A marine embayment is very sensitive to nitrogen
loading, depending on the shape and depth of the embayment. Its nitrogen limit
can be exceeded at a housing density as low as one house per three acres in its water
shed or marine water recharge area (Cape Cod Commission, 1996). Nutrient
19
loading has led to increased abundance of algae and plants, reduced oxygen content
of water, decimated shell and finfish, and many other changes in structure and
function of aquatic communities.
1.3.2(a) Algal Blooms
Today the health of large marine ecosystems- their diversity, productivity,
and resilience- is threatened by coastal algal blooms. Algal blooms (red, green,
golden, brown, bioluminescent), covering vast expanses of marine, estuarine, and
inland water have now been described from points as diverse as California, North
Carolina, Guatemala, Iceland, Japan, Thailand, and the Tasman Sea. This increase
in blooms is a direct consequence of human activities. The major anthropogenic
influences are pollution, over-harvesting of shellfish and finfish, and loss of
wetland habitats. Excess nutrients from sewage and fertilizer effluents is a primary
cause of marine eutrophication. Algal blooms have been linked to cholera
outbreaks (Epstein et al., 1993), which seriously threaten both the environment and
pubic health. The degradation of marine ecosystems increases the risk of diseases
emerging. Changes along coastlines contribute to pubic health hazards and can
cause hpyoxia in the breeding grounds of marine animals and plants.
1.3.2(b) Case Study: Waquoit Bay, MA
Waquoit Bay is a shallow bay on Cape Cod which exhibits symptoms of
eutrophication, largely attributed to septic nitrogen inputs. Valiela et al. (1992)
found that groundwater is the major mechanism that transports nutrients to
Waquoit's coastal waters. Nitrogen delivered to the watershed surface can be taken
up by vegetation, be denitrified in the soil, or percolate through soil. On Cape Cod,
domestic waste water from septic tanks provides more nitrogen than precipitation
20
(atmospheric deposition) or use of fertilizers. The waste water releases nitrogen
deep in the subsoil, which eventually enters the aquifer. The groundwater seeps or
flows through the sediment-water boundary and sufficient groundwater-borne
nutrients, nitrogen in particular, traverse the boundary to cause significant changes
in the aquatic ecosystem. These loading-dependent alterations include increased
nutrients in water, greater primary production by phytoplankton, and increased
macroalgal biomass and growth. For example, Valiela noted that even small
increases in nitrogen loads markedly decrease eelgrass cover area and leads to
increased biomass and productivity of benthic seaweeds. Macroalgal photosynthesis
largely controls oxygen supply in bottom waters and the more nitrogen loading, the
greater the fluctuations of oxygen concentrations, with increased frequency of
hypoxia within the bay and its estuaries (D'Avanzo and Kremer, 1994). Thus,
nitrate contamination in groundwater can adversely affect ambient aquatic
ecosystems. I will be referring to Waquoit Bay throughout this thesis to highlight
the environmental impacts of nitrate on Cape Cod.
1.4 Conclusion
In Chapter 1, I established a framework by which to investigate possible
remedial efforts for groundwater contamination by citing the major sources of
nitrate contamination and the health and environmental effects of nitrate.
Cognizant of this information, I now address the fate and transport of nitrate in the
subsurface environment.
21
Chapter 2
Nitrate in Groundwater
2.1 Nitrate in Groundwater: Introduction
Nitrogen is ubiquitous in the natural environment and its conversion to
nitrate is part of the natural functioning of any ecosystem. Nitrogen is an essential
nutrient for plant and animal growth. In the natural environment, nitrogen is
cycled through plants and animals in a complex of series of biological and chemical
processes. Figure 1 is a summary of the sources, sinks and pathways that are
integrated into the nitrogen cycle into the subsurface environment.
Nitrate can be discharged directly from septic system soil absorption drainage
tiles into the subsurface environment. It can also form in the soil by nitrification of
other nitrogen compounds, such as ammonium (NH4 +). Nitrates can reach
groundwater from a variety of sources. Nitrogen is a primary component of
inorganic and organic fertilizers (Scarsbrook, 1965), transforms rapidly to nitrate
under normal soil conditions (Alexander, 1965), and is primarily transported in the
water infiltrating the soil (groundwater), not the water moving over it (runoff, or
surface water).
The widespread appearance of nitrates in groundwater is a consequence of a
number of factors. The nitrate ion is negatively charged; consequently, it is repelled
rather than attracted to negatively-charged clay mineral surfaces. This is contrasted
with the ammonium ion which, in groundwater, adsorbs to aquifer solids that have
a significant cation exchange capacity (CEC). Thus, nitrate is very mobile in most
soils, and is easily displaced from its point of origin by water additions. It is stable in
soil except when biologically
22
Precioitation
NH 3
NO~
NH
3
NO~
Mineral
fertilizer
Plant residue,
compost
Organic-N
N H-3
NH 3
NO
Organic -N
proteins
Sewage
NH 3
FN 2
Plant
proteins
N2
Nitrogen
Fi xa3t ion
I7NK
NZ
(Denitrif ca i
ston
Nitrif icationecm
N
4
(N
I7
Prens
rfiaon
NHI
n
nNHD
-Asr7
NIO
Decomposition
N03
) Ni
rfia=
4
(Adsorption)
n
Denitrification
Nitrification
NO-
NO~
1
L e ach *ng
GroundwaTer
N1O1
Denitrification in reducing zones}
N03
N2 a
0
Figure 1: Sources and Pathways of Nitrogen in the Subsurface Environment (From Freeze and Cherry,
1979)
23
transformed by denitrification, which only occurs in very wet soil or inside of soil
aggregates at high moisture content (Broadbent, 1973). Although nitrate is taken up
rapidly by plant roots, this removal mechanism only occurs near the surface in a
region known as the soil root zone. Thus, wherever there is a source of nitrogen
and an excess of water applied to the soil, nitrates have the potential to reach
groundwater. Since two of the major sources of nitrogen addition to soil, irrigated
agriculture and septic tanks, are also sources of excess water, it is easy to see why
these operations have been associated with nitrate pollution.
If one wishes to use a specific (bio)chemical transformation of a chemical
compound in order to address its contamination, one must investigate the
phenomena which affect the transport and transformation of that chemical
compound. In this chapter, I will present biological denitrification of nitrate to
gaseous nitrogen species, which has been suggested as a means of addressing nitratecontaminated groundwater. I will do this in the context of fate and transport of
nitrate in groundwater. Again, I will address denitrification and nitrate fate and
transport in groundwater with respect to the specific case at hand: nitrate
contamination of groundwater on Cape Cod.
2.2 Nitrate Transport
In general, delivery of nitrate to groundwater is via vertically percolating
recharge and typically exceeds the denitrification potential of aquifer material.
Nitrate can move with groundwater flow with minimal transformation. It can
migrate long distances from input areas if there are highly permeable subsurface
materials which contain dissolved oxygen. Movement of any dissolved ion such as
nitrate through soil is governed by two mechanisms: convection of the chemical
with moving soil solution and diffusion of the chemical within the solution. The
24
amount of nitrate found at a point of groundwater is a function of physical and
(bio)chemical factors.
Nitrate concentrations can be modified by physical processes which do not
change the total mass of nitrate but which change the concentration in solution.
Nitrate can be removed permanently or temporarily from aquifers by (bio)chemical
conversion and a decline in the redox potential of the groundwater to other forms
of nitrogen.
2.2.1 Physical Processes
Recharge of water with lower nitrate concentration causes dilution. Other
physical processes which can modify nitrate concentrations in aquifers are advection
(transport by motion of the flowing groundwater), diffusion (movement of nitrate
from areas of high to low concentration), and dispersion (mixing caused by
microscopic differences in flow rate through porous media). Figure 2 shows the
movement of nitrate fertilizer through a sandy loam soil. Nitrate will have an
increasingly broad concentration peak as it moves with water through a soil. This
spreading is caused by diffusion and hydrodynamic dispersion. The peak moved
downward with time and gradually became less sharply defined as a result of
diffusion and dispersion. Movement into less permeable areas or carbon-rich
portions of the aquifer can promote nitrate reduction through denitrification, which
is discussed later.
Although these physical processes can modify nitrate concentrations in
homogeneous aquifer systems, their effects can be small compared to dilution
caused by heterogeneities in aquifer flow patterns, storage, recharge, and discharge.
For example, in settings such as sand and gravel aquifers (Cape
25
NITRATE-N BULK DENSITY (g/m2 /cm)
0.2
0.4
0.6 0.8
0.0
0.2
0.4
0.0
15-
15
15
30
30-
30-
30-
45
45-
60
60
30-
60-
75
75
75-
75-
90
90
0-
90-
-
105
-
1 35-
105-
120
- 120
-
1 20-
120-
135
-
135
-
1:
150
-
-
DAY
0.4
45
105
150
0.2
-
0.0
-
1.0
135
50
'
Uj
0.8
3
15
a.
0.6
DAY2
-
0.4
-
0.2
-
0.0
150
DAY 3
DAY 4
Figure 2: Movement of Fertilizer Nitrate Through Sandy Loam Soil (From National Academy of
Sciences, 1978)
26
Cod) and fractured bedrock, low dispersion often maintain high nitrate
concentrations for considerable travel distances (Wilhelm et al., 1994). Today, the
effect of dilution on nitrate concentration is coming under criticism. In the past,
many researchers thought that dilution was a major source of nitrate attenuation.
However, tracer tests have now demonstrated that hydrodynamic dispersion in
most sand aquifers is much less than previously thought and the dilution models
which are commonly used to attenuate nitrate from septic systems are probably
physically unrealistic (Robertson and Cherry, 1995).
The nitrate mass balance equation may be written as
d/dt (C) + on = d/dz Js
Il}
where:
9 = soil moisture content
C = mass of nitrate per unit volume fluid
On= a term describing the net disappearance of nitrate mass per volume per unit
time from (bio)chemical transformation (a sink term)
Js = nitrate mass flux in mass of nitrate per area per unit time
Js is the sum of convective and diffusive mass fluxes:
Js = - F(6)Dsw dC/dz + uwC
{2}
where:
E(O) = tortuosity factor as a function of soil moisture content
Dsw = effective diffusion and dispersion coefficient, area per unit time
uw = water velocity, length per unit time
2.2.2 Sinks
It is desirable to identify and evaluate any potential nitrate sinks within the
groundwater regime that may be acting to protect water supplies from
contamination. Below the root zone, there are four possible "fates" (other than
continued leaching) for nitrate: (1) soil retention, (2) assimilatory reduction into
27
microbial biomass, (3) dissimilatory nitrate reduction to ammonium (DNRA), and
(4) denitrification. Only the latter may serve as a major nitrogen sink since the
others only temporarily immobilize nitrogen.
2.3 Microbial Activity in Groundwater
Microorganisms are key factors that control the chemistry of groundwater.
Microbial populations can have profound effects on the transport and fate of both
naturally-occurring and contaminant solutes (Smith et al., 1996). In general, except
for heavily contaminated situations, groundwater microbial processes proceed at
rates that are slower than the rate of groundwater flow; that is, many solutes are
transported faster than they are transformed. This is due to the nutrient and energypoor nature of the groundwater environment.
Nitrogen transformations by groundwater microorganisms largely depend on
redox conditions in the aquifer (Smith et al., 1976). Microbial activity can reduce the
redox potential by oxidizing potential electron donors. It is well established that
substantial microbial populations do exist naturally in many groundwaters. Britton
and Gerba (1984) summarized microbial growth requirements in the subsurface
environment:
Table 1: Microbial Growth Requirements In Subsurface Environments
Comments
Parameter
Inorganic: Carbonates and bicarbonates
Organic: Humic substances and waste water
organics
Mode of Utilization of Organics
Secondary substrates
Other Elements
N, P, S, Na, Ca, Mg may be present in sufficient
quantities to allow growth
Electron Elements
02: absent in most deep aquifers
2
Other Electron Acceptors: NO3- and SO4
-
Carbon E ources
Source: Britton and Gerba, 1984.
28
Specially adapted organisms, such as those which can use electron acceptors
other than oxygen, are able to proliferate in the subsurface environment. Due to
relatively low substrate concentrations and high specific surface areas, bacteria
which form biofilms on the surface of solid particles predominate in groundwater
(McCarty et al., 1984) although most bacteria may remain in solution in sand
aquifers, like those located on Cape Cod (Matthes, 1985).
2.4 Biological Denitrification
Biological denitrification is the only permanent sink for nitrate and the major
pathway by which nitrogen is returned to the atmosphere as gaseous nitrogen
(dinitrogen, N 2, and nitrous oxide, N 20). Biological denitrification is a respiratory
process whereby nitrate is utilized as the terminal electron acceptor in lieu of oxygen
by bacteria which are mainly aerobes (Knowles, 1982). Nitrate is reduced to gaseous
nitrogen and an electron donor, usually organic carbon, is oxidized. The
denitrifying bacteria are capable of normal respiratory growth in the presence of
oxygen. However, under microaerobic and anaerobic conditions they use nitrate,
nitrite, or nitrous oxide as the terminal electron acceptors.
The reduction of nitrate to nitrogen involves several specific biochemical
sequences (the oxidation state of nitrogen in each molecule is shown above it in
parentheses):
(+5)
(+3)
(+2)
(+1)
(0)
N0 3 -> N02- -+ NO -> N 2 0 -+ N 2
Many bacteria are able to perform only one or two of these. Thus, the denitrifying
microflora must be considered as a group of complimentary microorganisms able to
carry out the conversion of nitrate to dinitrogen in its entirety.
29
The following discussion of the factors needed for biological denitrification
refers to heterotrophic denitrification. However, Thurman (1985) stated that the
majority of all groundwaters have dissolved organic carbon (DOC) < 2 mg/L. Thus,
in the absence of DOC, large amounts of denitrification are often only possible in
aquifers with large quantities of reduced inorganic compounds or solid labile OC in
the porous matrix. Thus, with low DOC and high inorganic carbon, autotrophic
(inorganic carbon as energy source) denitrification is possible.
The potential for
autotrophic denitrification is discussed later in Chapter 4 and 5.
2.4.1 Conditions Needed for Denitrification
Biological denitrification rates are controlled by redox conditions, available
carbon, denitrifier populations, and other environmental factors. Denitrification
can be carried out by several common genera of bacteria provided a substrate,
usually organic matter (as a source of energy and electrons), and oxidized nitrogen
are present, but it only occurs under anaerobic or almost anaerobic conditions. At
least 14 genera of denitrifying bacteria have been identified, and they are present in
most soil and aquatic environments (Follett, 1989). Because much of the
groundwater is suboxic or anoxic, denitrification is one of the predominant terminal
electron-accepting processes in the groundwater plume, and it is a key factor in
carbon and nitrogen cycling by microorganisms within the affected portions of the
aquifer (Smith et al., 1996).
2.4.1(a) Oxygen
Denitrification primarily occurs under low oxygen conditions. Oxygen, which
competes with nitrate as an electron acceptor in the energy metabolism of cells, is an
important inhibitor. Considerable changes take place in the energy metabolism of
30
microflora when the availability of oxygen is limited (Hiscock, 1991). The gradual
depletion of oxygen or provision of semi-anaerobic conditions appears to favor
denitrification.
Knowles (1982) explains that in soils there is frequently an inner-
aggregate air-filled porosity surrounding intra-aggregate water filled pores which
become virtually anaerobic permitting denitrification to occur. Aerobic respiration
is then replaced by anaerobic respiration, during which oxygen is replaced by nitrate
as an alternative electron acceptor (see Figure 3). Nitrate is the first compound to be
reduced after oxygen depletion (half reaction):
N0 3 - + 6 H+ + 5 e- -> 0.5 N2 (g) + 3 H20
{3}
Generally, denitrification is not observed at an oxygen concentration above 0.2 mg/L
(Korom, 1992). It has also been observed that the denitrification rate decreases as the
air-filled porosity increases, and denitrification ceases as the environment becomes
oxidizing at air-filled porosities of 11 to 14% (Pilot and Patrick, 1972). However, the
presence of denitrifying enzymes has been detected even in sandy soils which have
20% 02 in the air-filled spaces (Tiedje et al., 1982).
2.4.1(b) Microorganisms and Organic Carbon
Microbial populations in the aquifer can catalyze redox processes and,
therefore, affect speciation of the inorganic nitrogen compounds (Ceazan et. al.
1989). Bacteria capable of denitrification are commonly found in many subsurface
environments, even when denitrification is not actively occurring (Keeney, 1986).
Most denitrifying bacteria are heterotrophic (organic carbon is used both as a carbon
source and electron donor) and are able to utilize a wide range of carbon compounds
(sugars, organic acids, amino acids) as sources of electrons. For example, the
oxidation half reaction of a carbohydrate is:
31
SahuraIed Zone
Organic
Carbon
Min(IV) as f&022
--
-
Mn_(U) as N
-
-~
I
jnC(
L~e Mas FeO0 H~
-
ISE14
t I(II
as FeCO31
)
1h-h(
Figure 3: Oxidation of Organic Carbon in the Saturated Zone with the Sequence of Electron
Acceptors and the Resulting Reduced Inorganic Compounds (From Korom, 1992)
32
CH 2 O+ H2 0 -+ CO2 (g) + 4 H+ + 4e-
{4}
Combining equations (3) and (4) for heterotrophic biological denitrification yields:
5 CH2 O + 4 NO 3- + 4 H+ -+ 2 N 2 + 5 C02 (g) + 7 H 2 0 {5}
As can be seen in equation {5}, the alkalinity and pH of the solution increases during
denitrification. Dinitrogen is unavailable to most organisms.
In groundwater, the concentration of organic carbon is limited by oxidation of
the organic matter to carbon dioxide before reaching the water table, and the general
lack of soluble organic carbon contained in aquifer solids. Bradley et al. (1992) found
a highly significant relationship between potential denitrification and sediment
total organic content. An enhancement of denitrification activity in sediments
amended with glucose indicated that denitrification rates were carbon limited.
According to equation {5}, the denitrification reaction requires 1.25 mmol of carbon
for the reduction of each mmol of nitrate and this stoichiometry has been observed
in situ by measuring excess N2 and C02 gas produced at the Cape Cod MMR site
(Smith et al., 1991).
Denitrification is carbon limited in a sewage-effluent-contaminated glacial
aquifer on Cape Cod (Smith and Duff, 1988) and will most likely be the limiting
factor for denitrification throughout Cape Cod's groundwater. The organic carbon
limitation is potentially complex because a wide variety of organic compounds are
available to denitrifying organisms (Beauchamp et al., 1989). However, only a
fraction of the naturally occurring total organic carbon in soils or aquifer sediments
generally is labile (Lalisse-Grundmann et al., 1988). Solid-phase organic carbon also
may support denitrification (Robertson et al., 1991). However, the amount of
organic carbon in subsurface sediments is often very small, generally less than 0.5%
(by weight) in sand aquifers and much of this solid organic matter may be very old
and resistant to biodegradation (Mackay, 1990).
33
Trudell et al. (1986) found that for denitrification to occur in a shallow
unconfined sand aquifer, soil organic carbon at 0.08-0.16% by weight, was adequate to
denitrify large amounts of nitrate. During recharge events, when the volumetric
water content of the surface soil increases to near saturation, it is possible for organic
carbon dissolved in the surface soil zone to be transported to the water table without
completely undergoing oxidation. At the water table, this carbon may remain
dissolved or precipitate as solid organic carbon. However, each recharge event that
brings dissolved oxygen and nitrate to the water table will deplete the pool of
residual solid organic carbon in an aquifer to some extent. The potential exists for
extended inputs of nitrate to a flow system over a long period of time to deplete the
carbon source to the point at which the aquifer no longer has the capacity to support
denitrification. The net result is the loss of this nitrate sink in the flow system,
giving rise to the possibility of extensive accumulation of nitrate contamination of
shallow groundwater. However, further work is required to evaluate the potential
of this process for the replenishment of DOC in the aquifer for possible
denitrification.
2.4.1(c) Nutrients, Temperature, and pH
The availability of nutrients is an important requirement in sustaining
biological cell growth. On the basis of average cellular composition, the favorable
ratio of C:N:P:S is about 100:20:4:1 (Spector, 1956), although additional organic carbon
is required as an electron donor. Most groundwaters contain adequate
concentrations of the necessary minerals and trace metals to support biosynthesis
(Champe et al., 1979).
Denitrification was reported to be positively related to pH with an optimum
in the range of 7.0-8.0 (Hiscock, 1991). However, Bradley et al. (1992) did not find a
significant correlation between the rate of denitrification and the pH of the
34
groundwater and hypothesized that denitrifiers adapt to in-situ pH conditions.
Temperature is also a significant controlling factor. At a low temperature,
denitrification decreases markedly but is measurable between 0 and 5 'C. A
synergistic effect of temperature and oxygen upon denitrification can be noted. At a
high temperature, oxygen solubility is less, thus increasing the biological rate
process, and vice versa. Generally, a doubling of denitrification rate is possible with
every 10 'C increase in temperature (Gauntlett and Craft, 1979).
2.4.1(d) Redox Potential (EH)
Nitrogen transformations by groundwater microorganisms largely depend on
redox conditions in the aquifer. Microbial activity can reduce the redox potential by
oxidizing potential electron donors. In a groundwater redox investigation of the
Lincolnshire, England limestone aquifer, Gauntlett and Craft (1979) reported that
nitrate levels were lowered dramatically when the groundwater EH dropped below ~
+0.25 V. Spalding and Parrott (1994) found that in both domestic and irrigation
wells in Nebraska, N03--N concentrations greater than 10 mg/L occurred at a much
higher N03--N frequency in groundwater with measured EH >
-
+0.28
0.02 V than
in groundwater with lower EH values. They concluded that removal of nitrate via
bacterial reduction (heterotrophic denitrification) occurred below +0.28 V. They
observed that N03--N and EH decrease both with depth and proximity to the redox
front, which is due to the more reducing conditions at greater depths in most
aquifers. Spalding and Parrott thought that there would be no reason to suspect that
the low redox groundwater they investigated was unique to that geographical
location so that, worldwide losses via this mechanism could be high and that
communities plagued by high nitrate groundwater areas could utilize this in-situ
nitrate removal systems by locating their municipal wells on the reducing side of
the EH front.
35
2.4.1(e) Depth in Aquifer
A feature of nitrate contamination of groundwater that has been noted in
studies worldwide is the general, inverse relationship between nitrate concentration
and depth below the land surface or within an aquifer (Freeze and Cherry, 1979).
This is generally attributed to greater rates of biological denitrification in the less
oxygenated (i.e., deeper) parts of the aquifer (Trudell et al., 1986). Bradley et al. (1992)
found that deep sediments exhibited a rate of denitrification three times that of
shallow sediments. Trudell et al. (1986) observed a striking similarity in decreasing
nitrate and dissolved oxygen concentrations, which extended from a depth of 1.5-2.1
m. This is believed to be the zone of ongoing denitrification. It has also been noted
that the concentration of organic carbon, both solid and dissolved, declines
dramatically with depth in the subsurface, which would imply oxidation via
denitrification (Thurman, 1985).
Figure 3 is a schematic representation showing the fate of organic matter, the
most common electron donor in nature, in the presence of a variety of electron
acceptors in the saturated zone. The figure additionally depicts a generalized
progression of reactions with depth below the water table. Nitrate is the next
electron acceptor, after 02, to oxidize organic carbon. This is heterotrophic
denitrification. It helps explain why nitrate is often found in much greater
concentrations near the saturated surface than at greater depths in the aquifer.
2.4.1(f) Depth of Aquifer
Gillham and Cherry (1978) studied several unconfined aquifers in southern
Ontario, Canada and found that aerobic conditions and nitrate contamination were
generally limited to depths of one to two meters below the water table in aquifers
36
whose water table was less than two to three meters below ground surface. Starr and
Gillham (1989) found that the common pattern of denitrification occurring in
shallow but not deep water table aquifers is controlled by the decline in organic
carbon availability with depth. They provided results which showed that there was
sufficient labile organic carbon under the ground surface at a site in Canada. This
shows that under shallow water table conditions, there is sufficient labile organic
carbon present to reduce the dissolved oxygen, followed by denitrification. These
results provide strong evidence that denitrification can be an important process in
shallow unconfined aquifers.
In contrast, Gillham and Cherry (1978) also observed that aerobic conditions
and high nitrate concentrations persisted for substantial depths below the water
table in aquifers whose water table was deeper than two to three meters below
ground surface, indicating that denitrification was not an important process in these
situations. Under deeper water table conditions, there was insufficient labile organic
carbon to provide a substrate for dissolved oxygen and nitrate reduction. The
evidence suggests that dissolved organic carbon originating in the soil zone and
transported down by infiltrating water is an important source of labile organic
carbon in unconfined aquifers. If this were the case, then the occurrence of
denitrification in groundwater would probably be more closely related to the
residence time of infiltrating water in the vadose zone than to the depth of the
water table. Soil texture and infiltration rate affect residence time in the vadose
zone, and could therefore be important factors in addition to water table depth.
2.4.1(g) Losses of Nitrate in Unsaturated Vadose Zone
The fate of nitrate as it traverses unsaturated vadose sediments above the
water table remains unclear. Keeney (1986) argues that denitrification is unlikely in
the aerobic vadose zone of sediments. Travel distances are much less in vadose
37
zones than in aquifers (m vs. km) and conditions favoring denitrification are
somewhat more likely in aquifers than in unsaturated sediments. Smith and Duff
(1988) reported that biological processes (in particular, denitrification) did not seem
to have any significant effect on effluent nitrate concentrations during transport
through the unsaturated zone.
2.5 Denitrification Rates in Groundwater Environments
From a consideration of thermodynamic principles, the most stable nitrogen
species within the pH-EH range encountered in the majority of groundwaters (see
figure 4) is predicted to be gaseous dinitrogen (N2). For, example, Freeze and Cherry
(1979) state that in an ideal situation, thermodynamics predicts bacterial
denitrification with an organic carbon electron donor will occur in water at pH - 7.0
and EH +0.25 V. The observed departure from equilibrium is explained by the
catalyzing effect of bacteria in accelerating the biological reduction of nitrate at lower
redox potentials. Denitrification is not an equilibrium process because, as a
microbially-catalyzed reaction, it is irreversible due to the absence of a suitable
catalyst (microbe) for the reverse reaction. Thus, one must address denitrification in
groundwater from a standpoint of kinetics, not thermodynamics.
The following table is a sampling of some of the denitrification rates reported
in the literature:
38
I
I
I
,
I
i I
-
i
+1200
Water oxidised
+1000
+800
N 2 (g)
NO
-
+600
+400
pH-Eh
field
representing
+200
-
E
0
+H+most
-w
-
.=
groundwaters
4
-200
-400
-600
Water reduced
-800
I I
-1000
0
2
I
I
4
I
I
I
6
I
8
I
I
II
I
I II
10
12
14
pH
Figure 4: Stability Diagram of Nitrogen Species Showing the Predicted Species of Most
Groundwater at 25 *C and 1 atm (From Hiscock et al., 1991)
39
Table 2: Field Estimates of Denitrification Rates in Aquifers
Reference
Location
Aquifer
Material
Temp. (*C)
Sample
Depths
Initial
[N0 3--N],
mg/L
Kolle et al.
(1985) and
Bottcher et
al. (1989)
Hannover,
Germany
Sand
Multiple
8-10
Depths to
12 m (water
table at 2
m)
Trudell et
al. (1986)
Ontario,
Canada
Sand
Van Beek
and Van
Puffelen
(1987)
Starr and
Gillham
(1989)
Holland
Coarse
Sand with
Gravel
3 (water
table at 1
m)
44m
Ontario,
Canada
Sand
Korom
(1991a)
Utah
Clay, Silt, 35 m
Sand
(water
table at 1
m)
NR
10
NR
below
water table
which was
1m below
ground
surface
6-8
2
Denitrification
Rates
up to 40
first order
reaction
with a
half-life of
1.2-2.1
1
years
13.0
0.19-3.1
(mg N/L)/d
2.1
0.09-0.15
(mg
3
N/L)/d
6.4
12.5 and
23.7
0.58 (mg
N/L)/d
up to 0.73
(mg
N/L)/d 3
Source: Korom, 1992.
1: Denitrification rates are claimed to be by autotrophic denitrification.
2: Not reported.
3: Denitrification rates are claimed to be by heterotrophic and autotrophic denitrification.
Smith and Duff (1988) reported rates ranging from 2.3 to 260 pmol of N 2 0
produced (g of wet sediment)-1 h-1 . Smith et al. (1996) estimated in-situ rates of
-
denitrification in the range of 0.60-1.51 (nmol N 2 0 produced/cm 3 aquifer)/d (0.017
0.042 (mg N/L)/d). They reported rates highest nearest the contaminant source,
with decreasing rates with increasing distance downgradient. In incubated sediment
samples, denitrification in samples in which the nitrate concentration was low was
stimulated by nitrate; denitrification in samples in which the nitrate concentration
was high was stimulated by glucose addition (see figure 5). Near the contaminant
40
source, N0 3 --N concentrations up to 24 mg/L were measured. The center of the
plume 0.25 km downgradient had no detectable N0 3 --N concentrations. Figure 6
shows vertical profile of rates of denitrification and, as stated earlier, follows the
general occurrence of greater rates of denitrification at shallow depths.
Smith and Duff (1988) found that the bulk of the denitrifying activity was
associated with sediment core solids, not in the interstitial groundwater (see figure
7). The population of denitrifying organisms must then be attached to particulate
surfaces rather than existing as free-living organisms in the interstitial groundwater.
This was consistent with the findings of Harvey et al. (1984) for the entire subsurface
microbial community: 96% of the organisms within the contaminant plume are
particle bound. Thus, groundwater movement past the particulate surfaces controls
the nitrate supply to particle-bound microorganisms.
DeSimone et al. (1995) measured denitrification rates in sediment cores from
the anoxic zone of a sewage plume on Cape Cod ranging from 0.2 to 32 (ng
N/cm 3 )/d (2 x 10-4 - 0.032 (mg N/L)/d), with a mean of 9.6 7.4 (ng N/cm 3 )/d
(0.0096 0.0074 (mg N/L)/d) ( one standard deviation) (see figure 8). Denitrification
rates were high where oxygen was depleted and concentrations of nitrate and
dissolved organic carbon were high (figure 9). They also found that nitrous oxide
production in unamended slurry incubations was enhanced significantly with the
addition of glucose only. The mean denitrification rate calculated from dinitrogen
production, 3.0 (ng N/cm 3 )/d (0.003 (mg N/L)/d), corresponds to a nitrate reduction
of 0.015 mg N/L per foot of travel distance, using the groundwater flow velocity of
0.5 ft/d and aquifer characteristics. This is equivalent to an attenuation in nitrate
concentration of about 1.5 mg N/L per 100 feet of travel distance under anoxic
conditions. This quantity is less than 5% of the nitrate concentrations in the center
of the plume. Interestingly, the addition of nitrate had no effect on denitrification
41
-
7
Glucose
6-
NO 3
-
5
C
E
4
Glucose
2
0
5
10
15
20
25
30
35
40
HOURS
Figure 5: Nitrous Oxide Production by Slurried Core Material at 1.5
m Beneath the Water Table
versus Time (From Smith and Duff, 1988)
0
10
0.91
Z
30
0.0
0.5
1.0
1.5
2,,-
2.0
42.
2.5
3.0
3.5
DENITRIFICATION RATE (nMOLES N (g SED)-' DAY-')
Figure 6: Vertical Profile of Rates of Denitrification for
42
Slurriedi Core Material (From Smith et al., 1991)
3D0
-
Sediment
a
6.1 m
2.5-
2.0 L
E
_
1.5 m
-
1.5
1.0 r
W.
-
Groundwater
6.1 m
1.5 m
0 C.
0
15
30
45
HOURS
Figure 7: Time Course of Nitrous Oxide Production by Core Samples
and Well Water Samples from
Two Depths (From Smith and Duff, 1988)
43
A
18
16 -A.
MEAN: 9.6 MEDIAN:
8.8
STANDARD DEVIATION: 7.4
C,,
z 14
COEFFICIENT
OF VARIATION:
LUJ
LUJ
12
C,,
10
77%
LU
8
0
LUI
6
z
4
2
0
0
4
8
12
16
18
24
28
I
~..B.
16
32
36
40
I
MEAN: 2.8 MEDIAN:
1.3
STANDARD DEVIATION: 3.2
z 14
COEFFICIENT
OF VARIATION:
LU
LU
20
12
110%
CC,
10
LU
0
LU
z
8
6
4
0
0
4
8
12
16
'I
20
, I
24
28
, , ,
32
36
,
2
40
DENITRIFICATION RATE, IN NANOGRAMS
NITROGEN PER CUBIC CENTIMETER PER DAY
Figure 8: Frequency Distributions of Denitrification Rates Measured in Sediment Cores (From
DeSimone and Barlow, 1995)
44
.L DENITRPIFICATION
EM LOG
DISSOLVED
NITRATENITROGEN
2 OXYGEN
-f
20
RATE
WATER.
-
-}
TABLE -
DISSOLVED
ORGANIC
CARBON
-AR-ON
40-
60
80-
1 00-
A.
EM LOG
2
~
DENITRIFICATION RATE
-
20 -Water tanle
DISSOLVED
OXYGEN
-
-
NITRATENITROGEN
--
40-
DISSOLVED
ORGANIC
-- CARBON
CARBON
60
80
---
100-
-
-B.
0
200
400
600
ELECTRICAL CONDUCTIVITY. IN
MICROSIEMENS PER CENTIMETER
AT 25 DEGREES CELSIUS
0
4
8
12
16
DENITRIFICATICN
20
24
RATE. IN
NANCGRAMS NITROGEN PER
CUBIC CENTIMETER PER DAY
0
4
a a
20
40 0
Figure 9: Vertical Profiles oi Denirrificanon Rates. Dissolved Oxygen, Nitrate-Nitrogen,
and
Dissolved Organic Carbon Measured in Sediment Cores (From DeSimone
and Barlow, 1995)
45
1.0
CONCENTRATION. IN MILLIGRAMS PER LITER
2.0
rate. These data suggest that the denitrification rate in the septage-effluent plume
primarily was carbon limited (figure 10).
2.6 Conclusion
In Chapter 2, I examined the fate and transport of nitrate in groundwater,
with an emphasis on biological denitrification as a sink of nitrate in the subsurface
environment. Studies, including those conducted on Cape Cod, of naturally
occurring denitrification in the groundwater environment demonstrate that
populations of denitrifiers do exist in both shallow and deep aquifers systems, and
emphasize the dependence of denitrification upon the provision of a source of
oxidizable material, usually organic carbon. The extent to which limited natural
denitrification can ensure groundwater supplies with a low nitrate concentration
will depend on the continued supply of organic carbon and other nutrients for
biosynthesis. In a sandy aquifer, such as Cape Cod, the amendment of soil with
labile organic carbon is probably required for the enhancement of denitrification as a
means of addressing nitrate-contaminated groundwater.
I have summarized the sources, effects, and fate and transport of nitrate in the
subsurface environment. In order to address possible remedial actions for nitrate
contamination on Cape Cod, one has to address the nature of nitrate contamination
on Cape Cod and the ramifications that this contamination has on Cape Cod's
population: their environment, drinking water, and rate of expansion. This will be
the topic of Chapter 3.
46
6.0C
5,00
-
z
z
/-
-
0
4,00
0
Cn
3,00
0
-
0
-
0
0
-
0
0 z
LU
-
z
LII
/-
-
-
4:
..---
2,00
C0
a/
X
0 z
w
z
1,00
C
0
3
2
1
4
5
6
TIME, IN DAYS
EXPLANATION
A
NO ADDITIONS
*
WITH ADDED GLUCOSE
o
WITH ADDED NITRATE
o
WITH ADDED GLUCOSE AND NITRATE
Figure 10: Nitrous Oxide Production in Sediment Slurries with Nitrate and Carbon Addition (From
DeSimone and Barlow, 1995)
47
Chapter 3
Nitrate Contamination on Cape Cod
3.1 Introduction
Figure 11 shows the political map of Cape Cod. Groundwater is the only
source of drinking water on Cape Cod and the aquifer is defined as a "sole source
aquifer" by the Safe Drinking Water Act. Nitrate contamination from land
application (the disposal of primary or secondary treated waste water effluent to the
land) of municipal wastes and contamination from septic systems poses the greatest
threat to groundwater on Cape Cod. In this chapter, I will specifically address nitrate
contamination on Cape Cod. This requires a summary the major sources of nitrates
on Cape Cod, a discussion of Cape Cod hydrogeology as it pertains to nitrate
contamination, and a review of the impact of nitrate on water and environmental
quality on Cape Cod. Lastly, I will discuss the impending urbanization on Cape Cod
and its possible adverse effects on groundwater quality with respect to nitrate
contamination.
3.2 Cape Cod Nitrate Contamination
On Cape Cod, there are both widespread point sources of nitrate
contamination from septic systems and large nitrate plumes. Figure 12 shows the
landfills and sewage-disposal sites on Cape Cod. The Tri-Town Septage Treatment
Facility in Orleans (see figure 11) has disposed treated septage effluent since February
1990. The Tri-Town Septage Treatment Facility is a municipal facility owned and
operated by the Orleans, Brewster, and Eastham Groundwater Protection District.
The treatment produces a nitrogen-rich effluent that is disposed of through
48
71 0 0a'
73 0O
72 00e
A TLA NTIC
Provincetown
42 0 Wa...
420W0
OCEAN
-.
MASSACHUSETTS
'70OWQ
K Js-sCAPE
42 000
0
0
20
20
40
60
80
TruH
42000
COE)
ao MILES
'0
70 00
41 0 30'
100 KILOMETERS
TRI-TOWN
Eiaimam
SEPTAGE-
TREATMENT
0
CAPE COD BAY
7004a.
22
BMW 21
Dennis
',
ORLEANS
nrwster
Sannno.Chatham
Masfnmas
Falmouth
NANTUCKET SOUND
0
4
I
II
0
4
8
I
8
12
12 MILES
I
I
16 KILOMETERS
Figure 11: Political Map of Cape Cod, Massachusetts (From DeSimone and Barlow, 1995)
49
Provincelown
42*00'-
42*00'
Wellfleet
EXPL ANA TION
A
70-00'
Truro
L ANDFILL
0
Eastham-A
70*30'
Orleans
41615'-
Bourne
A
A
01,Dennis\
A
Sandwich
-- _
~---
/-
i CA
/Harwich
SBarnstable
A
0
/1Yarmouth
MashpeeChatham
A
Falmouth70*15'
Famoth/0
5 10
MILES
I
0
70*30'
I
5
'
II
10 KILOME TERS
fu*0'
Figure 12: Landfills and Sewage-Disposal Sites on Cape Cod (From Office of Water Resources, 1994'
infiltration into the sandy aquifer underlying the facility. Infiltration occurs within
700 feet from the coastal Namskaket Marsh. Namskaket Marsh has been recognized
by the Commonwealth of Massachusetts as an Area of Critical Environmental
Concern (ACEC), and the tidal creek that drains the marsh is designated as an
Outstanding Resource Water. Thus, the effluent discharge from the septagetreatment facility may affect groundwater quality and valuable coastal resources.
There are currently five (active and inactive) municipal land application sites
on Cape Cod. Barnstable has been in operation for almost 55 years. The waste water
receives secondary treatment before being discharged onto sandbeds. The Chatham
Municipal Treatment Facility also uses secondary treatment and has been in
operation for ten years. Falmouth and Hyannis also have small municipal waste
water treatment plants.
Otis Air Force Base used a rapid infiltration site starting in 1941. It is currently
inactive, having been shut down in 1995. The result of waste disposal at Otis is a
plume of sewage-contaminated groundwater characterized by elevated
concentrations of dissolved solids, boron, chloride, sodium, phosphorus,
ammonium, detergents, and nitrate (Garabedian and LeBlanc, 1991). A maximum
nitrate concentration of 16 N0 3 -N mg/L has been detected. The concentration in
the center of the plume immediately downgradient of the disposal beds in below
detection (Ceazan et al., 1989) owing to microbially-mediated denitrification (Smith
et al., 1991). Within 1.5 km of the disposal beds, the predominant nitrogen species
in the plume is ammonium (NH4+). Beyond 1.8 km from the beds, the
predominant nitrogen species changes to nitrate, with concentrations of about 3 to 4
mg N0 3 -N/L. This is caused by adsorption of ammonium onto the aquifer
sediments (Ceazan et al., 1989). The zone of nitrate at the upper boundary of the
sewage plume was found to be anoxic, contain high concentrations of N 2 0, and
coincide with a zone of active denitrification.
51
The concentration of degradable dissolved organic carbon is low in the sewage
plume. Interestingly, as a result of this low concentration, part of the denitrifying
population appears to be autotrophic (inorganic carbon fixation) and capable of
oxidizing molecular hydrogen as an alternative to organic carbon as an energy
source (Smith and Ceazan, 1991). I discuss this further in Chapter 4.
Figure 13 shows the widespread nitrate problem on Cape Cod. This is
primarily the result of contamination from septic systems, which are used
throughout Cape Cod. Coincidence of areas of high nitrate concentrations with
areas of high housing density is particularly notable along the densely settled
southern shore in Hyannis, Yarmouth, Dennis, southwestern Harwich, and
Chatham. High nitrate concentrations also are apparent in the town centers in
Wellfleet and Eastham.
Housing density is not the only factor that determines the nitrate
concentration in groundwater; the presence of sewage- or septage-disposal sites and
use of fertilizer have effects. Municipal sewage and septage facilities are associated
with areas with high nitrate levels in north-central Falmouth, Bourne, central
Dennis, west-central Barnstable, northwestern Wellfleet, and Provincetown.
Elevated levels in Brewster also may be the result of septage disposal. Fertilizer use
at golf courses may be responsible for high nitrate levels in northern Dennis and
along the southwestern coast of Barnstable. High levels in central Truro may result
from sewage disposal at an Air Force station, fertilizer use at an adjacent golf course,
or both. An inactive landfill probably is the cause of high levels in southwestern
Falmouth. Elevated nitrate levels at Otis Air Base may a be result of fertilizer
application, leaking sewer lines, or both (Persky, 1986).
52
1096
1039
1000
~
~
800
--
~
~
~
r------------··----·-·-------
600 --·-
0
D
a::
LLI
~
=>
z
.WO
NITRATE, IN MILLIGRAMS
PER LITER OF NITROGEN
-----
0-1
1-5
200 ........•..
>S
0-0.1
1-2
0.1-1
2-5
5-10
NITRATE, IN t-41WGRAMS PER LITER OF NITROGEN
>10
I
L___~I
AREA OUTSIDE OF QWSYMAP
BOUNDARY FOR
BARN ST ABLE COUNTY
• EACH DOT REPRESENTS 60 HOUSES
0
0
53
5
5
Figure 13: Nitrate Concentrations and the Frequency Distribution on Cape Cod, 1980-1984 (From
Persky, 1986)
10 MILES
10 KILOMETERS
3.3 Use of Septic Systems on Cape Cod
The United States uses individual on-site septic systems to dispose of
approximately one-third of its domestic waste water (Canter and Knox, 1985). Figure
14 is a summary of the approximate populations in the United States utilizing septic
tank systems. The greatest densities of usage occur in the east and southeast, as well
as the northern tier and northwest portions of the United States. In rural and urban
fringe areas of North America, almost all domestic liquid waste is disposed into inground septic systems. Of all groundwater pollution sources, septic systems rank
highest in total volume of waste water discharged directly to soils overlying
groundwater, and they are the most frequently reported sources of contamination
(U.S.E.P.A., 1977).
The conventional septic tank-soil absorption system (figure 15) is the most
convenient and economical method for home sewage disposal. A conventional
system is defined as a system composed of a standard septic tank for solids removal
and decomposition, a distribution box for dispersing effluent into field lines, and a
field distribution system consisting of trenches or beds, 60 to 180 cm wide, placed in
natural soils at a depth of 60 to 120 cm.
The septic system serves several important functions such as solid-liquid
separation, storage of solids and floatable materials, and anaerobic treatment of both
stored solids as well as nonsettleable materials. In the buried tank, waterborne
wastes are collected, and scum, grease, and settleable solids are removed from the
liquid by gravity separation. A subsurface drain system allows clarified effluent to
percolate into the soil. System performance is essentially a function of the design of
the system components, characteristics of the wastes, rate of hydraulic loading,
climate, areal geology and topography, physical and chemical composition of the soil
mantle, and care given to periodic maintenance (Canter and Knox, 1985).
54
Percentage of
Households Using
Septic Tanks
Over 35%
25% to 35%
Under 25%
Figure 14: Approximate Populations Using Septic Tanks (From Canter and Knox, 1985)
55
PRODUCTION
PRET
REATMENT
DISPOSAL
evapotranspiration
we 11
septic tank
60 cm
I
sail absorption
purification
ground water
streams. lakes
Figure 15: Schematic Cross-Section through a Conventional Septic
Tank Soil Disposal System for
On-Site Disposal and Treatment of Domestic Liquid Waste (From
Canter and Knox, 1985)
(a)
---
N~ ,..c~rI
ZONE
FIELD~
4
I
I
IrrnTTI
I
GROUNDWATER
IO
FLOW
SEPTIC
TANK
ORGANIC C -
CO2, CH 4
ORGANIC N
NH 4
--
---
f CH4
C02
DRAIN
FIELD
ORGANIC C+0 2 - CO 2
N
+ 0
N H 4 + 0 2 --- N O 3
+
(b)
OF GASES
DRAIN
SEPTIC
TANK
UNSATURATED
-WASTEWATER
~ FLOW
MOVEMENT
C02
02
SATURATED
ZONE
Figure 16: (a) Schematic Cross Section of a Conventional Septic System, Including Septic Tank,
Distribution Pipe, and Groundwater Plume (b) Sequence of Simplified Redox Reactions in the Two
Major Zones of a Conventional Septic System: the Septic Tank and the Drain Field (From Wilhelm
et al., 1994)
56
In the past, when this waste disposal technique was used in areas of low
populations, few problems were noted. However, septic system densities in some
areas and public awareness of groundwater contamination have increased the extent
of groundwater degradation caused by septic systems that it has become an issue in
environmental management and suburban planning. Also, a significant number of
septic tank systems fail to operate properly; many fail during their first year of
operation. These failures create potential health hazards, impair water quality, and
are usually aesthetically unpleasant. Since the domestic waste water in septic
systems contains many environmental contaminants, septic systems in the United
States and Canada constitute approximately 20 million potential point sources for
groundwater contamination (Wilhelm et al., 1994).
One must be cognizant of the magnitude of septic system use on Cape Cod in
order to suggest feasible remedial options. The problem of nitrate contamination
from septic system on Cape Cod is magnified due to the fact that few households
have connections to public treatment facilities. Table 3 highlights the problem.
There are few sewered homes in the larger communities of Cape Cod and none in
the smaller towns. Municipal sewage discharges to surface waters off Cape Cod are
prohibited by the Massachusetts Ocean Sanctuaries Act (see Chapter 4).
Table 3: Towns on Cape Cod and Percentage of Homes with Sewer Connections
Town
%
Population
Truro
0
1573
Wellfleet
0
2493
Dennis
0
13864
Harwich
Orleans
0
0
10275
5838
Otis
Eastham
Mashpee
Brewster
Provincetown
Chatham
Barnstable
Bourne
0
0
0
0
0
1-25
1-25
1-25
1073
4462
7884
8440
3561
6579
40949
16064
Source: Commonwealth of Massachusetts, Title V (1995)
57
3.4 Nitrate from Septic Systems
Of concern in terms of groundwater pollution is the quality of the effluent
from the septic tank portion of the system and the efficiency of constituent removal
in the soil underlying the soil absorption system. N0 3 --N has a maximum
permissible drinking water concentration of 10 mg/L. However, properly designed
and constructed septic systems frequently cause N0 3 -N concentrations greater than
10 mg/L in the underlying groundwater. For example, Valiela et al. (1992)
determined that 29 mg N/L from each household reaches the water table from septic
systems in Waquoit Bay.
Septic systems typically develop two major redox environments in which the
waste water is transformed. Figure 16 shows a schematic cross section of a
conventional septic system and the sequence of simplified redox reactions in the
two major zones of a conventional septic system: the septic tank and the drain field.
Anaerobic digestion of organic matter and production of ammonium from
organic nitrogen predominate in the septic tank:
Urea (CO(NH 3 +)2 ) + H 2 0 --+ 2 NI-4+ + CO 2
{6}
In this zone, the total nitrogen content of the waste water may be reduced by roughly
10 to 30%, mostly due to organic nitrogen storage in the sludge (Winneberger, 1984).
The septic tank effluent flows into the aerobic unsaturated sediments of the
drain field, which constitute the second major redox zone. The domestic waste
water undergoes its most significant geochemical changes in the drain field, where it
flows downward to the water table. In this zone, microorganisms use oxygen as the
electron acceptor in the nitrification of NH 4 + to N03- (two-step process):
NH4 + + 1.5 O2 -- NO2- + 2 H+ + H2O
{7b}
N02- + 0.5 02 -> NO 3 -
58
{7a}
Nitrification (NH4 + -> NO2 - -> NO 3 -) is an aerobic reaction performed primarily by
autotrophic organisms and nitrate is the predominant end product (rarely is there
any nitrite accumulation). Nitrification is dependent on the aeration of the soil.
Effluents from septic systems located in sandy soils, such as Cape Cod, can be
expected to undergo predominantly aerobic reactions. Ammonium oxidation in
this zone creates nitrate concentrations in the effluent that are roughly two to seven
times the drinking water limit.
The most complete treatment of waste water occurs when a natural anaerobic
setting, which is capable of denitrification, follows the aerobic treatment zone. In
this case, not only organic carbon but also nitrate is removed from the waste water.
Unfortunately, nitrate removal from waste water by denitrification is rare in
aquifers below septic systems. In particular, as discussed in Chapter 2, denitrification
in most aquifers is limited by the lack of a long-term supply of labile organic carbon.
Regulations should recognize the substantial addition of nitrate to groundwater
from conventional septic systems and work to protect valuable groundwater from
nitrate contamination. Either another form of waste water disposal should be
required or alternative systems which provide the proper sequence of redox zones
and organic carbon supply to remove nitrate should be installed in nitrate sensitive
ground water areas, such as Cape Cod. I will address this issue specifically in
Chapter 5.
In addition to nitrate, septic systems also release other contaminants, human
pathogens, and toxic organic chemicals from household products (Wilhelm et al.,
1994). The dilution of nitrate and these other constituents in the groundwater
below septic systems is generally much less than assumed in previous decades.
These findings raise issues beyond those of improved septic-system functioning and
point to the possible need to diminish the reliance on conventional septic systems
59
as a common means of waste water disposal. A discussion of alternative septic
systems currently in use on Cape Cod is presented in Chapter Four.
3.5 Hydrogeology of Cape Cod
There are two major types of geological formations on Cape Cod. Glacial
moraine is the geological formation left by deposits of the rock matter brought by the
Ice Age. Glacial moraine is composed of till. Till is a poorly sorted, heterogeneous
mixture of sand, gravel, silt, clay, and boulders. Outwash plain is the second major
type of geological formation on Cape Cod. Outwash plains are made up of a wellsorted combination of sand and gravel. The outwash plains areas were deposited by
sediments in streams and small rivers of meltwater during the time when Cape Cod
was formed.
Most of Cape Cod serves as a recharge area (in the form of precipitation) and
the predominant form of geological material, sands and gravel, has a high
permeability. Because of the highly permeable nature of Cape Cod soils, the aquifer
is susceptible to contamination from septic wastes and other sources. Nitrogen from
septic systems, municipal sewage systems, and fertilizer leachate have caused
increased levels of nitrate in groundwater on Cape Cod. Water supplies have also
been threatened by naturally occurring substances such as iron, manganese,
hydrogen sulfide, and seawater. There have also been incidences of water supply
contamination from road salts, leaky underground storage tanks, and landfills
(CCPEDC, 1978). However, nitrate from domestic septic systems is the greatest threat
to the quality of groundwater on Cape Cod.
60
3.6 Nitrate Effects on Water and Environmental Quality on Cape Cod
The presence of elevated concentrations of nitrate in ground and surface
waters stimulates two major concerns: a risk of adverse health effects on humans or
animals through drinking water and the likelihood of enhanced biotic productivity,
with a potential for eutrophication in aquatic ecosystems.
3.6.1 Water Quality
Freshwater in the porous and relatively permeable deposits form a fresh
groundwater reservoir that is the source of supply for nearly 100 municipal wells
and thousands of private domestic wells. This water source receives waste water
and landfill leachates generated by an expanding population of Cape Cod residents.
Most groundwater on Cape Cod is of good chemical quality for drinking use.
It is characteristically low in dissolved solids, soft, and virtually free of toxic heavy
metals and organic compounds such as insecticides and herbicides. Constituents
thought to be problems or threats to water supplies for drinking water are salt
(NaCl) from seawater intrusion, coastal flooding, and highway deicing salt; nitrogen
from domestic and municipal sewage; and iron and manganese, which occur
naturally. Iron and manganese concentrations also can be increased locally by
landfill leachates. Hydrogen sulfide (H2 S) gas and ammonia (NH3 ) also occur
naturally in organic sediments in a few areas (Frimpter and Gay, 1979). Table 4 lists
the physical properties and constituents in Cape Cod's groundwater. One should
note that the values listed for total organic carbon (TOC) represent a pool of
subsurface organic carbon including solid organic carbon, which is more resistant to
biodegradation, and does not account for the necessary replenishment of this pool if
in-situ denitrification, using subsurface organic carbon, is to be used as a means of
addressing nitrate contamination on Cape Cod.
61
Table 4: Physical Properties and Constituents in Cape Cod's Groundwater
Constituent
Maximum1
Median
Minimum
Specific
conductance
(micromhos per cm)
FH
Hardness (Ca +
Mg as CaCO3)
Sodium
Bicarbonate
Carbon dioxide
Sulfate
Chloride
Dissolved Solids
(sum of
constituents)
Nitrate (N)
Ammonia (N)
Total Organic
Carbon
Iron (gg/L)
1760
123
46
90% of analyses
contained less than
value indicated
173
7.6
185
6.1
4.1
7.0
264
100
41
61
480
41
13.2
11
5.7
6.6
19
9.1
3.5
0
'0.4
1.1
5.8
3.2
23.8
6.3
0.91
7.6
0.12
0.01
1.3
0.07
5.5
0
0
0.07
8800
41
0
1702
20
5
38
22
19
16.4
38
12.8
Source: Frimpter and Gay (1979)
1: All concentrations in mg/L
2: 75% of analyses contained less than this value
Drinking water for Cape Cod residents is supplied almost entirely from the
groundwater aquifer that underlies the land surface of Cape Cod. Precipitation is the
only means of replenishing the groundwater supply. The high permeability makes
the Cape Cod aquifer a highly productive source of drinking water. It has been
designated a sole-source aquifer, as defined in the Safe Drinking Water Act, because
it is the only potential source of drinking water for Cape Cod residents. Hence,
protection of the aquifer from nitrate contamination is a major concern for residents
of Cape Cod.
For Cape Cod and in similar areally-extensive water table aquifers, the U.S.
Geological Survey (USGS) has identified incompatible land uses as the greatest
threat to water quality. On-site sewage disposal in septic tanks and cesspools has
been identified as the most important non-point source of groundwater quality
degradation in residential areas in the U.S. (Grady, 1993) and as one of the most
serious threats to water quality on Cape Cod. Over 90% of Cape Cod residents
dispose of domestic sewage in cesspools and septic systems and Table 3, which shows
62
the percentage of residents in Cape Cod towns served by municipal sewers,
highlights the problems of septic system contamination on Cape Cod.
Drinking water for about 80% of Cape Cod residents is provided by 18 public
or private water supplies, which draw on about 130 groundwater wells and one
surface water source. Public water supply system customers are the primary water
users on Cape Cod, with a base off-season average day demand of 18.68 million
gallons per day (mgd) and an in-season average day demand of 38.73 mgd (Office of
Water Resources, 1994)
The disposal of human waste is a major concern for water-supply planners on
Cape Cod. Only a few small areas on Cape Cod are sewered, and the majority of
private homes, motels, restaurants, and businesses rely on septic systems. All public
sewage systems on Cape Cod rely on land disposal. Municipal landfills on Cape Cod
(figure 12) include septage-disposal facilities. Local officials are concerned that the
population expansion will result in widespread degradation of groundwater quality,
hence, affecting drinking water quality.
Waste water is the primary source of impact to drinking water supplies on
Cape Cod and nitrate measurements can be used as an indicator of waste water
impact. The Cape Cod Commission has listed the following as reasons nitrate is a
useful indicator: (1) it is continuously discharged from septic systems, making it a
reliable indicator of how much effluent is being introduced to the water table; and
(2) nitrate is routinely tested in both private and public wells, so data are available
for many wells across Cape Cod (M. Geist, Personal Communication). Background
levels of nitrate in the Cape Cod aquifer are well below 0.5 mg/L (Frimpter and Gay,
1979), and measurements at or above this level are considered indicators of impact
from local land use, primarily from residential on-site waste water disposal. Nitrate
measurements in public wells during the 1970s and 1980s range from 0.05 to about 6
63
mg/L, and in private wells about 30% of tests show levels above 1 mg/L (Janik,
1987).
3.6.2 Environmental Quality
Cape Cod's rich water resources are used for a variety of non-consumptive
uses. Numerous kettlehole ponds left by the retreating glaciers dot the landscape,
wetlands and coastal streams provide habitat for fisheries and wildlife, and a wide
range of freshwater recreation opportunities exist. The following discussion
provides an example of the environmental resources that can be affected by nitrate
groundwater contamination on Cape Cod. Homes and businesses near these vital
environmental resources which could load nitrate to groundwater via septic
systems should be subject to different septic system nitrate effluent limits and/or
remedial technologies.
3.6.2(a) Wetlands
The wetlands on Cape Cod provide important wildlife habitat and
recreational opportunities. The major wetlands on Cape Cod are listed in Table 5.
These delicate environmental areas could be adversely impacted by nitrate loading
in excess of their natural ability to attenuate and incorporate nitrate. One must be
cognizant of the importance and existence of Cape Cod's wetland resources in order
to address groundwater contamination of nitrate due to domestic sewage. For
example, homes near to major wetland areas may be subject to stricter effluent
limits from their septic systems or be required to use a system capable of reducing
nitrate to lower limits.
64
Table 5: Major Wetlands on Cape Cod
Wetland Name
Location
Acres
Clapps Pond Wetland
Shank Painter Pond Wetland
Pilgrim Lake Wetland
Little Pamet Wetland
Truro River Wetland
Herring River Wetland
Duck Harbor Wetland
Namskaket Creek Wetland
Chathamport Wetland
Grass Pond Wetland
Swan Pond River Wetland
Ware Creek Wetland
Parkers River Wetland
Witcomb Swamp
Provincetown
Provincetown
Truro
Truro
Truro
Truro/Wellfleet
Wellfleet
Brewster/Orleans
Chatham
Harwich
Dennis
Dennis
Yarmouth
Mashpee
70
80
290
80
220
350
400
35
50
90
140
80
80
55
Source: USDA, Soil Conservation Service, 1978
3.6.2(b) Areas of Critical Environmental Concern
The Areas of Critical Environmental Concern (ACEC) Program was
established in 1974 to protect certain areas that are of central importance to the
welfare, safety, and pleasure of all Massachusetts citizens (MCZM, 1989). An ACEC
is an area containing concentrations of highly significant environmental resources
that has been formally designated by the Secretary of Environmental Affairs.
Designation of an area as an ACEC provides a higher level of environmental review
for proposed activities. The seven designated ACECs on Cape Cod are: Bourne Back
River, Pocasset River, Waquoit Bay, Sandy Neck, Pleasant Bay, Inner Cape Cod Bay,
and Wellfleet Harbor (figure 17).
Waquoit Bay is one of the seven ACECs on Cape Cod. Valiela et al. (1992)
observed significant changes in the aquatic ecosystem in Waquoit Bay, including
increased nutrients in water, greater primary production by phytoplankton, and
65
4.
Tam\
Well fleet
Harb or
ACE C
-J
FLYWOM~
x
Herring
liver
Ilisville Harbor
ACEC
Inner
,Af~
Cape Cod Bay
ACEC
ACEC
4IC
7
-V
Sandy Neck/
Bourneack hier
s
.CCEC
Pocasset liver
~
*,*
'-~
,.
Barnstable Harbor
CIC
-Pleasant
.
-
M
/
-
//
YAR1MI
ANWrD[
,LE
KARWEH
04&DAM
N J-4
NI
'
I
A
-
-~"
Wmngtit law ACEC
y
.e-,ILN
61.1
Cape Cod Regional
ACEC
Locus Map
Scale
1:461,400
Figure 17: Cape Cod Regional ACEC Locus Map (From Office of Water Resources, 1994)
66
slo
increased macroalgal biomass and growth. The increased macroalgal biomass
dominated the bay ecosystem through second and third order events, such as
alterations of nutrient status of water columns and increasing frequency of anoxic
events. They recorded increases in seaweeds that have led to a decrease in the area
covered by eelgrass habitats (figure 18). Thus, even the delicate ecosystems that are
protected as ACECs are subject to excess nutrient loadings that can lead to significant
changes in the ecosystem. Greater effort to protecting these areas and the
aforementioned wetlands from nitrate contamination of groundwater should be
exerted.
3.7 Urbanization and Nitrate Contamination
A major concern in many locations is that the density of septic systems is
greater than the natural ability of the subsurface environment to receive and purify
system effluents prior to their movement into groundwater. Increased
development of housing and mobile home parks in rural areas and in small towns
without domestic waste treatment plants has increased the frequency of installation
of on-site sewage treatment and disposal. Urbanization, which is high on Cape Cod
and similar coastal areas increases release of waste water, which, in turn, increases
delivery of nutrients to groundwater, streams, and coastal waters. Barnstable
County (Cape Cod), which is the fastest growing county in New England, had a
35.3% rate of housing unit growth between 1980 and 1990 (figure 19). Valiela et al.
found that concentrations of nutrients in groundwater seem more closely related to
the density of septic systems in the watershed. Their study also found that median
nitrate concentrations increased as building density increased (figure 20). Figure 21
shows the housing density on Cape Cod. Housing density can be assumed to equal
septic system density in unsewered areas; hence, figure 21 gives a rough estimate of
septic system density on Cape Cod. In addition to an increase in the direct input of
67
r
1951
197
1978
Figure 18: Changes in Eelgrass Distribution in Waquoit Bay, 1951-1987. Black areas are eelgrass
beds with cover near 100%. (From Valiela et al., 1992)
p987
S NontucKet
40
*6orrnsCce
Cx
,Cz0e
u'es (Mcr*c s vineycr:
30 -
I-
4
S20U
0 Worcester
PlmothNorfolk
Frankline
MOmpshire
Berkshre 9
*
40Bristol \
[
.Suffolk
0
ddies
Hampden e
Essex
60
80
100
t20
Distance away from cocsan
20
40
440
480
460
(Km)
Figure 19: Growth in Housing Units in Massachusetts Counties, 1980-1990 in Relation to Distance
From the Sea (From Valiela et al., 1992)
3
-0
CO
C
C
2
C
0
z
*
0
V
0
S
0
2
3
5
4
6
Building density (nouses
7
8
9
0O
* .)
Figure 20: Nitrate Concentrations in Groundwater Below Areas of Cape Cod Having Different
Densities of Buildings (From Valiela et al., 1992)
69
--
Wellfleet
Center-
Newcomb
Hollow
.
.
2*
EXPLANATION
EACH DOT REPRESENTS
60 HOUSES
North
Sandwich
LJ SAMPLE AREA AND IDENTIFIER
Eastha
South.
Eastha
.07030'-
.
Brewster t.**
/.
.
West.
'
.* --' Pond
. .J, .15'
- .
- -..
^yni
.*~~~0
NWesk
/-
-
,
Scorton
.'
Barnstable
--.-
s.
1
.
:.f
0
5
E''I
South
Harding
10 MtL ES
-VDilsla-ge
L15sNck'00
70We00
Figure 21: Housing Density on Cape Cod, 1985 (From Persky, 1986)
.
'
'
S70'00'
42*00'-
nutrients via septic system, urbanization may also increase loading to groundwater
by removing forests that intercept precipitation-borne nutrients.
Persky (1986), in a study which included chemical analyses from over 3,468
private and public supply wells from 1980 to 1984, correlated median nitrate
concentration in groundwater with housing density for 18 sample areas on Cape
Cod, yielding a correlation coefficient of 0.802 (figures 22 and 23). In five of nine
sample areas where housing density is greater than one unit per acre, nitrate
concentrations exceed 5 mg N0 3 --N/L (the Barnstable County planning goal) in 25%
of wells. On the other hand, nitrate concentrations exceed 5 mg N0 3 -N/L in 25% of
the wells in only one of nine sample areas where housing density is less than one
unit per acre. In addition to housing density on Cape Cod, the presence of sewagedisposal sites and fertilizer use are also important factors that affect the nitrate
concentrations.
As mentioned earlier, Valiela et al. (1992) found that groundwater is the
major mechanism that transports nutrients to Waquoit's coastal waters. Figure 21
shows that the greatest density of homes on Cape Cod lies on the coast. One should
note the high density of septic systems along the coast. Septic system effluent from
these systems could greatly impact receiving water quality since the effluent would
undergo far less (bio)chemical transformation as it progresses toward the coast (less
time of travel in the subsurface). Rate of housing growth nearest the coast, coupled
with slow groundwater movement, results in the bulk of septic nitrogen entering
coastal waters lagging behind urban development by nearly a decade. So, even if
residential development is held at present levels, nitrogen input from septic
systems will increase by 36% over the current levels (Valiela et al., 1992). At full
residential development, septic nitrogen loading will eventually increase to more
than twice the current levels (Sham et al., 1995). The Massachusetts Institute for
Social and Economic Research estimates that the off-season population of Cape Cod
71
SLOPE
=
0.752
INTERCEPT = 0.131
CORRELATION COEMTTC:ENT = 0.802
z
-
z
z
0
0
I
2
3
HOUSING UNITS PER ACRE
Figure 22: Median Nitrate Concentration as a Function of Housing Density (From Persky, 1986)
4
SLDPE = 0.756
INTERCEPT = 0.220
CORRELATION CEFFICENT = 0.821
w
-
w-
U20
ZL.
0=
0
a
2
3
BUILDING UNITS PER ACRE
Figure 23: Median Nitrate Concentration as a Function of Building Density (From Pirsky, 1986)
72
will increase from 186,605 (1990 Census) to 229,437 in 2020, an increase of 23%.
Summer population is expected to rise from 512,520 to 594,831 by 2020, an increase of
16% (Office of Water Resources, 1994). Thus, there will be an increased demand for
clean drinking water, increased threat to delicate environmental settings, and
increased loading of nitrogen to groundwater.
3.8 Conclusion
In Chapter 3, I addressed nitrate contamination on Cape Cod. I discussed the
use of septic systems for the disposal of domestic sewage, which has been
determined to be the primary source of nitrate to the groundwater aquifer. I briefly
explained why nitrate can exist in such high levels in septic system effluent before
detailing how nitrate can adversely affect the water and environmental quality of
Cape Cod. The impending rates of urbanization will also have adverse effects on
water and environmental quality unless action is taken to address the problem of
nitrate contamination of groundwater on Cape Cod. In Chapter 4, I look at possible
options that use denitrification as a means of remediating contaminated
groundwater. It is from this set of options that a possible solution to nitrate
contamination on Cape Cod exists.
73
Chapter 4
Remediation Technology
4.1 Introduction: Motivation for Addressing Nitrate Contamination
The major threat to groundwater on Cape Cod which could adversely affect
both environmental and drinking water quality is nitrate from land application of
waste water. So far, I have summarized the sources and health and environmental
effects of nitrate; analyzed the fate and transport of nitrate in the subsurface
environment, emphasizing biological denitrification as a sink; and addressed the
nitrate contamination on Cape Cod by discussing septic system contamination,
effects to water and environmental quality, and rates of urbanization.
One could address the problem of nitrate in Cape Cod groundwater from one
(or both) of these perspectives: the environment and drinking water.
Unfortunately, effective elimination of the risk of eutrophication in coastal
environments, at least insofar as nitrogen is a controlling factor in the process, could
require much more stringent control measures than are needed to meet the 10 mg
N0 3 --N/L drinking water standard for nitrate. In this chapter, I will look at possible
solutions to the problems of nitrate contamination on Cape Cod. I will accomplish
this by:
1. Providing motivations for remedial action
2. Analyzing remediation technology: alternative septic systems,
in-situ denitrification techniques, and denitrification enhancement
techniques
3. Addressing the feasibility of specific remedial actions
74
4.2 Title V
Title V of the State Environmental Code contains the minimum
requirements for the subsurface disposal of sanitary sewage. Contained therein is a
discussion of alternative septic systems. The Commonwealth of Massachusetts is
cognizant of the nitrate problem on Cape Cod and its adverse effects on drinking
water and environmental quality. By January 1, 1998, the Department of
Environmental Protection is to prepare a report in which the following are central
focuses:
*
*
*
*
*
*
a summary of the Department's experience in approving alternative
systems pursuant to 310 CMR 15.000 (310 CMR 15.041)
an assessment of the number and cost of system upgrades which
should be accomplished in order to protect public health, safety,
welfare, and the environment based upon results of inspections
an assessment of critical resource areas and impacts of pollution
the feasibility of developing siting and design criteria that have express
terms addressing pollutant loadings directly rather than through
estimated flow
an analysis of septage disposal to determine if additional requirements
concerning the designation of disposal locations by local authorities is
necessary to ensure adequate septage disposal and proper allocation of
septage capacity in accordance with a septage management plan
an analysis of the application of the nitrogen loading limitations to
commercial uses where the use of both on-site (septic) systems and
drinking water supply wells is proposed to serve the facility
4.3 Zones of Groundwater Protection
Glacial outwash aquifers, such as on Cape Cod, are recharged from the land
immediately overlying them. Groundwater quality is highly dependent on local
land uses. The Commonwealth of Massachusetts has developed an approach to
managing groundwater quality that focuses management efforts on the land that
recharges the parts of aquifers that contribute water to wells.
75
In Massachusetts, the land surface that contributes recharge to a public-supply
well is referred to as Zones II and III by the Department of Environmental
Protection. Zones I, II, and III are defined in 310 CMR 22.20 and shown in figure 24.
Zone I is the protective radius around a public water-supply well or wellfield owned
or controlled by the water supplier, as required by the Massachusetts Division of
Water Supply. Zone II (the Municipal Wellhead Protection Area) is defined in 310
CMR 22.20 as "the area of an aquifer that recharges a well under the most severe
recharge and pumping conditions that can be realistically anticipated." It is bounded
by groundwater divides that result from pumping the well and by the contact of the
edge of the aquifer with less permeable materials such as till and bedrock. Zone III is
defined as:
"that land area beyond the area of Zone II from which surface water and groundwater drain into Zone
1I. The surface drainage area as determined by topography is commonly coincident with the
groundwater drainage area and will be utilized to delineate Zone III. In some locations, where surface
water and groundwater drainage are not coincident, Zone III shall consist of both the surface drainage
area and the groundwater drainage area."
In summary, the delineations of Zone II and Zone III are important because water of
impaired quality recharging the groundwater system within these areas ultimately
will affect the quality of water at the wellhead. Also, the delineations of the
different zones is important because in the future, it is likely that homes and
businesses within certain zones may be subject to stricter septic system nitrate
effluent levels or be required to use alternative technologies to improve nitrate
removal.
4.4 Environmental Motivation For Action
310 CMR 15.215 designates areas which, having been determined by the
Department of Environmental Protection to be particularly sensitive to the
discharge of pollutants from on-site sewage disposal systems, to be designated as
76
PRECIPITATION
DRAINAGE DIVIDE
WELLW
PUMPING>
W . TERp/
4
AUIFER
BEDROCK
ZONE I
..7777.
ZONE II
400 foot protective radius about public-supply well
-
--
Land surface overlaying the part of the aquifer that
contributes water to the well
ZONE III -- Land surface through and over which water
drains into Zone II
DRAINAGE DIVIDE
DIRECTION OF WATER FLOW
Figure 24: Recharge Areas to a Pumped Well in a Valley-Fill Aquifer (From Frimpter et al., 1990)
77
nitrogen sensitive. 310 CMR 15.215 continues, stating that the necessity of providing
increased treatment and reduction in nitrogen and nitrate-nitrogen from on-site
sewage disposal systems warrants the imposition of nitrogen loading restrictions,
which are set forth in 310 CMR 15.214. In addition to land areas, nitrogen sensitive
embayments are also to be classified as such.
Compounding the problem of nitrate contamination from septic systems and
subsurface injection of treated municipal sewage return water to the aquifer is the
Massachusetts Ocean Sanctuaries Act (M.G.L. c. 132A Sections 13-16, 18) which
prohibits any new discharge of municipal wastes into the Ocean Sanctuaries
surrounding Cape Cod (the Cape Cod Ocean Sanctuary, the Cape and Islands Oceans
Sanctuary, and the Cape Cod Bay Ocean Sanctuary: figure 25).
The waters of the outer cape adjacent to Cape Cod National Seashore Park are
also designated as "national resource waters" and no new discharge is allowed in
these waters to preserve "their outstanding recreational, ecological, and/or aesthetic
values." The inland waters of Cape Cod Bay and the waters of outer Cape Cod,
which are not national resource waters, are protected by an "anti-degradation"
clause in the act: "The quality of the waters of the Commonwealth shall be
maintained and protected to sustain existing beneficial use." This policy will reduce
contamination of surface waters from sources of nitrate but will, in turn, increase
the potential for nitrate contamination of groundwater by promoting more
widespread use of land disposal for wastes. Thus, with prohibition of surface water
discharge of waste water, one must turn to remedial action to address the problems
of nitrate contamination of Cape Cod's source of drinking water: its groundwater.
4.5 Remediation Technology
It is apparent from the discussion of natural denitrification in groundwater
that it is not extensive and is limited by the availability of organic carbon. Even
78
Ocean Sanctuaries of Massachusetts
as defined by M.G.L.C. 132A ss. 13-16 and 18
North Shore Ocean Sanctuary
South Essex Ocean Sanctuary
Cape Cod Bay Ocean Sanctuary
Cape Cod
Ocean Sanctuary
Cape & Isfands Ocean Sanctuary
Figure 25: Ocean Sanctuaries of Massachusetts (From Office of Water Resources,
1994)
79
where it does occur, it is not clear whether the rate of denitrification is sufficient to
remove the high concentration of nitrate now present in many aquifer systems. An
approach to securing low nitrate levels in groundwater is to take advantage of the
bacterially mediated process of denitrification. There are two approaches by which
one can use the potential of artificial denitrification as a water treatment process:
*
*
Permitted Alternative Septic Systems
In-Situ Denitrification: Stimulation of denitrification underground by
the injection of suitable nutrients and the utilization of the aquifer for
both supporting denitrification and filtration/re-aeration of the
denitrified water
4.5.1 Permitted Alternative Septic Systems
Local site conditions that may preclude the use of conventional septic systems
include shallow soil cover, percolation rates that are considered too slow or too
rapid, high groundwater, steepness of slope, limited area, or insufficient treatment
prior to infiltration into the groundwater.
As mentioned earlier, the conventional
septic system does not specifically address nitrate as a contaminant of concern.
Removal of organic matter is a primary goal of waste water treatment via septic
systems, but its requirement for abundant oxygen makes the concurrent conversion
of ammonium to nitrate unavoidable.
To address the nitrate problem, several alternative septic system designs for
enhanced nitrogen attenuation have been investigated in recent years. Alternative
systems, when properly designed, constructed, operated and maintained, may
provide enhanced protection of pubic health, safety, welfare, and the environment.
310 CMR 15.282 states that
Alternative systems proposed may include, but shall not be limited to, any of the following:
(1) humus or other composting toilets;
(2) alternative mounded systems designed to overcome limiting site conditions;
(3) any system designed to chemically or mechanically aerate, separate or pump the liquid, semi-solid
or solid constituents in the system; or
80
(4) any system designed specifically to reduce, convert, or remove nitrogenous compounds, phosphorus, or
pathogenic organisms (including bacteria and viruses) by biological, chemical, or physical means.
Alternative systems must be formally proposed, approved for remedial use, and
pilot tested.
For example, Title V stipulates the specific use of an alternative technology in
nitrogen sensitive areas:
"A recirculating sand filter or equivalent alternative technology approved by the Department of
Environmental Protection shall be a required design component of all systems with a design flow of
2,000 gallons per day or more to be located in Nitrogen Sensitive Areas."
310 CMR 15.202 (Use of Recirculating Sand Filters) also allows the use of a
recirculating sand filter to enhance nitrogen removal in systems with a design flow
below 2,000 gpd. The sand filter effluent total nitrogen concentration shall not
exceed 25 mg N/L and shall require, at a minimum, effluent standards for N0 3 -N.
As of August, 1996, there were over 50 alternative onsite treatment systems in
the ground on Cape Cod (figure 26). Waquoit Bay National Onsite Demonstration
Project recently installed an onsite system featuring a reactive porous barriers
beneath the leaching field designed by researchers at the University of Waterloo.
This is covered in more detail in section 4.6.1(c). Some of the alternative systems
are discussed in the following section.
On March 24, 1995 the Department of Environmental Protection, pursuant to
the "Approval of Alternative Technologies" provisions of Title V, issued approval
for several technologies capable of enhanced nitrogen removal. Three alternative
technologies have been issued either General or Provisional Use Approval with
nitrogen removal credits and associated increases in allowable design flow per acre
in designated nitrogen sensitive areas.
These systems are the RUCK, Ekofinn
BioclereTM, and the Smith and Loveless FAST systems.
81
Waterloo
Orenco
Alternative Onsite Septic Systems
installed to date in Barnstable County
(estimates as of August, 1996).
RUCK
Peat
RSF
FAST
Bioclere
5
15
10
20
25
30
Figure 26: Alternative Onsite Septic Systems Installed to Date in Barnstable County: Estimates as
of August, 1996 (From Barnstable County DHE, 1996)
82
The RUCK system has received Certification for General Use with nitrogen
removal credit for residential design flows under 2000 gpd. Both the BioclereTM and
the FAST systems have received Provisional Use Approval with nitrogen removal
credit for residential and nonresidential use. These approvals mean that the RUCK
system is approved and the FAST and BioclereTM systems are provisionally
approved as an alternative technology equivalent to a recirculating sand filter for
installation in nitrogen sensitive areas as defined in 310 CMR 15.202. The
BioclereTM system has also received General Use Approval for use without nitrogen
removal credit and may, if approved by the Board of Health, be installed with a
conventional Title V system (Barnstable County Department of Health and The
Environment, 1995). Table 6 summarizes the waste water loading credits for the
three systems.
Table 6: Waste Water Loading Credits For Four Alternative Septic Systems
RUCK
BioclereTM
FAST
General for
residential only
Provisional
Provisional
Recirc.
Sand Filter
General
660
NA 1
660
660
550
550
550
550
19
19
19
25
nonresidential
NA 1
25
25
25
minimum of
10,000 ft2 but
less than 15,000
ft2
330
330
330
no credit
minimumof
15,0002 but less
440
440
440
no credit
Approval
Category
Nitrogen Credit
gpd/acre
Total
Nitrogen Limits
(mg/L)
residential
nonresidential
residential
Expansion
to Existing
Homes
than 40,000 ft 2
Source: Barnstable County Department of Health and The Environment, 1995.
1: Not Allowed
83
Notice that the nitrogen limits for discharge are total and, if nitrogen is primarily in
the form of nitrate, it is above the drinking water standard. Hence, these systems
could possibly pose a threat to both the health and the environment.
Both the BioclereTM and FAST systems have also received Remedial Use
Approval. In remedial situations for existing homes which cannot meet Title V,
use of either of these systems can allow reduced soil absorption system area, reduced
setbacks to groundwater and other resource areas, and a reduction in the
requirement for four feet of naturally occurring pervious material below the leach
field. These reductions will only be granted when there are no other alternatives for
siting the septic system and must be approved by the Board of Health and DEP.
4.5.1(a) The RUCK System
Denitrification utilizing septic tank carbon is widely considered to be the most
economical and efficient method for nitrogen removal. This is the nitrogen
removal mechanism used in the RUCK system. The entire system consists of two
septic tanks, the RUCK filter, and a conventional leaching facility, all of which are
located below ground surface (with the exception of a vent pipe that is three feet
tall). The system is passive, requiring no pumps or other moving mechanical parts
(Barnstable County Department of Health and the Environment, 1995). There are
both commercial and residential RUCK systems in the ground from Vermont to
Arizona. In the RUCK system, the wash water (greywater) and the nitrogen-rich
toilet and kitchen sink waste (blackwater) are plumbed separately and flow to
separate septic tanks. Blackwater contains most of the household waste water
nitrogen (over 80%) and represents
-
40% of the total flow. Greywater represents
60% of the household flow and contains sufficient organic carbon for denitrification.
The blackwater flows to a conventional septic tank where solids settle. From
there, it flows to the aerobic RUCK filter where it is nitrified. The key element in
84
treating the blackwater is the RUCK filter, which is essentially a vented and buried
sand filtered that is engineered with a system of in-drains, gravel, and vent pipes.
The RUCK filter is designed based on flow rates and the characteristics of the waste
stream. The RUCK filter is composed of several layers of in-drains which are
overlain by layers of sand and filter cloth. The in-drain media and sand support the
growth of nitrifying bacteria. Bacteria will oxidize nitrogen to nitrate, creating
desirable acid conditions within the filter. The acid condition will enhance the
removal of phosphorus and pathogens from the waste stream. The fine sand filters
out suspended solids and pathogens and adsorbs ammonium and potassium. The
in-drains are composed of proprietary material and provide air to the filter. The
filter is assembled on-site by a qualified septic system installer. A schematic diagram
of the RUCK filter is shown in figure 27.
From the RUCK filter, the effluent flows to the anaerobic greywater septic
tank for denitrification. Denitrification of the effluent is accomplished by passive
mixing of the RUCK filter-treated blackwater with the greywater. The greywater is a
source of organic carbon and several genera of bacteria use the organic carbon as an
electron donor to denitrify nitrate to nitrogen gas. The denitrification process
increases the alkalinity of the system, raising the pH to normal levels. The nitrogen
gas escapes to the atmosphere through the plumbing vent, into the soil air, or
RUCK vents. After treatment in this tank, finished effluent flows to the leaching
facility for disposal.
The primary advantage of this system is the long design life. For example, the
oldest system in Massachusetts is ten years old. The filter was excavated as part of
the ongoing research and development of the system. The ten year old filter
showed no signs of degradation and was operating normally. The predicted life of
85
VENT BAC
STROUGH HOUSE ROOF
EQUSE
NITRIFICATION
BLACK
WAE
SEPIC
TANK
DIST.
RUCK
FITRBOX
GREYr WATER
ScT
c
MAN6
J
*
.1
.1
DENITRIFICATION
1.
TITLE 5
SYSTEM
If ground water is greater than
10' deep, no pump is required.
Figure 27: RUCK System (From Barnstable County DHE, 1996)
86
the upper layer of that filter from the observations was for a thirty year life span
(Innovative RUCK Systems, Inc., 1996). Other advantages of this system include (1)
utilization of passive, rather than energy consuming mechanical, methods; (2) the
lack of moving parts and little maintenance and power requirements; (3) the
greywater septic tank would accumulate solids about eight times less than a
blackwater septic tank ad would need pumping only once every 10-15 years; and (4)
no needed injection of organic substrate into the ground, which would increase
operational costs and possible have adverse effects; and (5) increased bedroom count
in nitrogen sensitive areas (Innovative RUCK Systems, Inc., 1996).
Nitrogen removal starts at 55% efficiency and builds up gradually for the first
three weeks of use (Innovative RUCK Systems, Inc., 1996). Table 4-5 shows
ammonia, nitrite, nitrate, and total Kjeldahl nitrogen for one year operation of a
cluster of homes (8 houses located in the Porter's Orchard Partnership), each with a
backwater septic tank, a RUCK filter piped to a greywater septic tank, with all
effluent then piped to a pump station. the concentrations are lower than the
planning goal; however, the influent concentrations were not measured.
Table 7: Effluent from Porter's Orchard Partnership
Date
09/26/91
11/19/91
01/24/92
01/31/92
03/30/92
05/28/92
07/30/92
09/24/92
10/15/92
11/05/92
12/22/92
Mean
STD DEV
95% CV
pH
NH 3
NO 2
NO 3
TKN
7.78
7.42
5.70
6.58
7.26
7.49
7.36
7.23
7.03
6.88
9.46
7.29
0.9101
7.79
2.25
1.96
4.78
7.20
2.49
10.9
2.96
1.52
1.83
1.87
0.940
3.52
3.021
5.17
0.041
0.005
0.005
0.006
0.005
0.011
0.005
0.005
0.005
0.005
0.014
0.010
0.011
0.016
0.645
2.84
0.056
0.01
0.082
0.058
0.010
0.010
0.010
0.017
0.131
0.352
0.846
0.814
1.76
3.73
6.82
9.71
3.71
11.8
3.46
4.73
3.66
7.89
2.72
5.45
3.18
7.19
Source: Innovative RUCK Systems, Inc., 1996.
87
However, because of the number of separate components and the necessary
drop in elevation, the leaching facility would have to be located at a relative deep
elevation. If a 10 foot depth to groundwater does not exist, the installation of a
pump chamber and pump is required, increasing the cost. The cost is high: $800
annually for maintenance, and $7250 and $9250 for a three and four bedroom home,
respectively, above the cost of a Title V system. This is priced out of the range of
many homeowners and is the primary disadvantage of this system (Barnstable
County Department of Health and The Environment, 1995).
4.5.1(b) The FAST System
The FAST system (figure 28) uses a Fixed Activated Sludge Treatment process
to treat and denitrify waste water. The FAST process is a two zone design which
consists of a primary anaerobic settling zone and an aerobic biological treatment
zone. Solids are trapped in the primary settling zone. The aerobic biological zone
consists of a submerged media bed which is colonized by nitrifying bacteria naturally
present in sewage. Waste water is recirculated between the two zones allowing both
nitrification and denitrification to occur. The FAST unit is purchased as a module
which is fitted into a 1500-2000 gallon conventional precast or fiberglass septic
system (Barnstable County Department of Health and The Environment, 1995).
The treatment process is as follows: waste water flows into the primary
settling zone of the septic tank.
Solid matter settles out in the solids collection zone
at the bottom of the tank. The FAST unit sits in the septic tank liquid with its media
bed submerged. An air blower located above ground forces air down a central tube
to the bottom of the submerged media. As the air rises up through the media to acts
as an airlift and carries waste water with it. The waste water flows up through and is
dispersed over the top of the media bed. The media serves as a site for the growth of
88
AJR-LIFT
ufim
AIR
INTAKE
EXHAUST
VENT
HOUS
ECIRCULATION
LIFTS LIQUID
IT THR
DIS
JGH
ARGE
CE
IED
-
FRMAIR
FROMDRAWING
MEDRA BEff)
LINE --...
-S CL 1)S OC LE
- .7..ONE--
CHAMBER~- ---
-01
-
(
MEDIA
=SETTLING
t4
=ZONE =
BED-=
\-
Figure 28: Smith and Loveless Single Home FAST System (From Barnstable County DHIE, 1995)
89
nitrifying bacteria. When the waste water reaches the top of the media, a portion
flows out through a channel back into the primary settling zone of the septic tank.
This zone is anaerobic and is the site of denitrification. With each pass through the
media, a portion of the waste water passes out through a baffle and flows to the
leaching field. Because the air blower runs continuously the waste water is
recirculated many times through the nitrifying media and the denitrifying anaerobic
zone before being discharged. Thus, efficient nitrification and denitrification is
achieved (Barnstable County Department of Health and The Environment, 1995).
The advantages of this system are that the unit is located below the ground
surface and the system is capable of maintaining bacterial growth during periods of
low water use because the media bed is submerged, remaining wet. However, it
does have an air blower which is above ground and it is expensive: approximately
$6500 (not including maintenance), with an optional disinfection unit adding
another $1000.
Again, this it out of reach of most homeowners (Barnstable County
Department of Health and The Environment, 1995).
4.5.1(c) Ekofinn BioclereTM
The Bioclere TM unit (figure 29) is a modified tricking filter which utilizes a
stable fixed film process for simultaneous nitrification/denitrification.
The on-site
treatment system incorporates a septic tank in series with a modified trickling filter
(the Bioclere TM system) and a soil absorption field. The trickling filter is a fixed film
aerobic process in which microorganisms attach themselves to a highly permeable
media, creating a biological filter (slime layer) through which waste water is trickled
allowing organic matter to be absorbed into the slime layer.
The treatment process is as follows: waste water flows to a conventional
septic tank where primary settling occurs. Effluent from the septic tank passes to the
BioclereTM unit and is distributed over a synthetic media bed at varying
90
Distributor
FRn Housing
Recyc le line---
PVC media
Influent
Effluent
Baffle-
Clarifier
Dosing pump
Recycle pump
Figure 29: Sectional View of BioclereTM Components (From Barnstable County DHE, 1995)
14 1
-.
-1
-
12
-
Total-N
10
E
. ...--.........
-.....
a
Z
I
:D Z
NO3-N+NO2-N
4
Orguic-N
NH4-N
...
..
..
..
2
.............
........ .....
0
2
3
5
4
6
7
8
TEM(Week)
Figure 30: Final Effluent Nitrogen Component Concentrations (From Barnstable County DHE, 1995)
91
recirculation rates. Oxygen is introduced to the media chamber through the use of a
fan. Nitrifying bacteria form a biomass in the aerobic environment of the media
bed. As the biomass thickens, it forms aerobic and anaerobic zones which
establishes the conditions for simultaneous nitrification and denitrification.
Approximately 20% of the total nitrogen can be lost through this process. After
flowing over the media bed, waste water returns to the sump portion of the tank is
recirculated over the media. In systems designed for more complete denitrification,
which would be required on Cape Cod, a portion of the water in the sump can also
be sent back to the septic tank through the use of a second pump located in the sump
portion of the BioclereTM. Recirculation of nitrified effluent to the septic tank where
anaerobic conditions and high nutrient levels prevail and, utilizing the raw waste
water enhanced by secondary sludge as the carbon source, allows efficient and more
complete denitrification to occur (Barnstable County Department of Health and The
Environment, 1995).
In a field performance test, effluent total nitrogen concentrations varied
between 8.5-12.1 mg/L, corresponding to a mean removal of 54% and remained in
compliance with the revised Title V regulations specifying the performance of an
alternative system in a nitrogen sensitive zone (figure 30). Table 7 shows removal
efficiencies for this septic system.
Table 8: Denitrification Efficiencies for the Ekofinn Bioclere T M Septic System
% Removal in septic system
Sample
1
2
3
4
5
6
7
8
AVG
N0 3 +N0 2
97.7
99.7
98.9
97.2
97.5
71.6
99.8
99.8
95.3
Total N
54
58
71
46
68
67
31
38
54
Source: AWT Environmental, Inc., 1996.
92
total N in Effluent
mg/L
10.3
9.2
11.1
12.1
11.9
8.5
11.4
11.8
10.8
When designed for full denitrification, the unit operates with two pumps, a
fan, and a timer. Some advantages of this system include the adaptation of existing
septic tanks to form the primary treatment stage of the BioclereTM process. Routine
operation and maintenance is necessary and this is a major disadvantage of this
system. The cost of a unit capable of treating 1000 gpd is $6000, not including
maintenance, additional labor (installation) costs, and conventional Title V leaching
field costs (Barnstable County Department of Health and The Environment, 1995).
4.5.1(d) Recirculating Sand Filter (RSF)
Sand filters are shallow beds of sand (600 to 760 mm) provided with a surface
distribution system and an underdrain system. Treatment of the effluent in a sand
filter is brought about by physical, chemical, and biological transformations.
Suspended solids are removed principally by mechanical straining, straining due to
chance contact, and sedimentation. The conversion of nitrate to nitrogen gas
routinely occurs, resulting in a significant (up to 45%) loss of nitrogen (Metcalf and
Eddy, 1991). Denitrification is brought about by anaerobic bacteria that coexist in
anaerobic microenvironments within the filter bed.
The principle components of a recirculating sand filter system are shown in
figure 31. Effluent leaves the septic tank and enters a recirculating tank large
enough to hold one-half to one-day's flow. A pump located in the recirculation
tank is used to pump waste water from the recirculation tank to the sand filter.
Effluent is applied to the filter for approximately five minutes every thirty minutes.
Treated effluent from the filter returns to the recirculation tank.
There exists a total of five RSF systems in operation in Barnstable County, as
of August, 1996 (Department of Health and the Environment, 1996). Two RSFs
have recently been installed on lower Cape Cod. East Cape Engineering, with
93
Pretreatment
unit
Recirculation
tank
Free access
recirculating
sand filter
To
disposal
I
I-I-
~r7~
Recirculating
pump
Gravel layer
Sand
Float valve.
The simplest float valve is
compnsed of a 4 in tee with
a round ball set below it
in a wire cage.
Figure 31: Principal Components of a Recirculating Sand Filter System (From Metcalf and Eddy, 1991)
94
assistance from the Barnstable County Department of Health and the Environment,
designed a single-family system in Orleans and coastal Engineering of Orleans
designed a larger RSF for cottages in Wellfleet. The high quality effluent produced,
ease of operation, and low maintenance cost are the principal factors contributing to
the popularity of RSF. However, as is seen in figure 31, homeowners need to
purchase a conventional septic system, in addition to the recirculating sand filter.
Also, a separate disinfection unit is often required. Hence, the RSF system is not
economically advantageous compared to a conventional septic system.
4.5.1(e) Sequencing Batch Reactors (SBR)
Sequencing Batch Reactors (SBR) can be contrasted with the majority of
alternative technologies, which treat septic tank effluent in a continuous stream
that passes over or through a media for the nitrification step of the process and then
returns to an anaerobic part of the system for the denitrification step. Batch
technologies alternate supply and deprive batches of effluent with air so that the
nitrification and denitrification steps can occur in the same vessel. The primary
advantage of batch technology is improved process control. The operational details,
such as dissolved oxygen necessary for nitrification and time needed for
denitrification, can be better controlled by the use of timers, fluid pumps, valves,
and air blowers. The challenge for these technologies will be to provide a cost
effective way to treat sewage onsite. These systems display optimal cost effectiveness
when used for flows slightly above the single family use (trailer parks, small clusters
of homes, etc.) (Barnstable County Department of Health and The Environment,
1996).
The SBR was pioneered for use in the remote regions of Australia with little
or no operator attendance. Currently on Cape Cod, officials are focusing on two
types of SBRs: the Amphidrome@ and the Cromaglass@ waste water treatment
95
system. The Amphidrome@ system is installed as part of the Waquoit Bay National
Onsite Demonstration Project, at Stuart's Mall in Swansea, MA, and another two
plans have been submitted for homes in Mashpee. Theoretically, the
Amphidrome@ system should be able to achieve a discharge concentration of < 10
mg N/L. The costs are still undetermined. A 550 gpd residential Cromaglass@
system has recently been installed in Cohasset and there are numerous Cromaglass@
installations in Pennsylvania, New York, and Maine. A basic residential system
costs $8500, including module, installation, electricity for one year, and a
maintenance contract.
4.6 In-Situ Treatments
Known technologies for nitrate removal can be classified into three
categories:
(1)
(2)
(3)
Operational, e.g. mixing
Physicochemical, e.g. ion-exchange, reverse osmosis,
electrodialysis
Biological, based on bacterial denitrification (ex- and in-situ)
Solutions of the first two groups are both expensive and inconvenient because they
require either major changes in water supply sources and main conveyance systems
or construction of expensive treatment plants. The physicochemical processes also
generate concentrated waste streams which present a severe environmental
problem.
The main advantage of in-situ denitrification in comparison to ex-situ
technologies is the fact that supplementary treatment is required after the
denitrification is carried out in the aquifer, including: filtration and bacteria die-off,
removal of organic residuals by biodegradation and adsorption, and re-aeration in
the aerobic environment of the aquifer outside the limit of the anoxic zone.
96
Another important advantage of the in-situ process is its independence of seasonal
temperature variations.
The in-situ denitrification schemes I will discuss focus on the presence of
organic substrate in the aquifer as a requirement for denitrification. The effects of
injecting organic substrate into the aquifer, which forms the basis for all schemes,
are presented schematically in figure 32. Anoxic conditions develop at the vicinity
of the injection well, creating a natural biological reactor at zone I. In this zone, the
denitrification process takes place and the nitrate is reduced to nitrogen gas. Zone II
serves as a sand filter in which turbidity and suspended solids are removed. The
water passing zones I and II is stored in zone III containing now reduced-nitrate
water.
4.6.1(a) Daisy System
The principle of this scheme is presented in figure 33. This scheme involves
only one pumping well, surrounded by a circular battery of small diameter wells
utilized for injection. Organic substrate and phosphorus (for microbial growth) are
introduced to the aquifer through the injection wells and diluted by the
contaminated groundwater converging to the production well. Denitrification
conditions are expected to develop along the flow paths towards the well, resulting
in nitrate reduction in the converging water and lowering nitrate levels of pumped
water. The degree of nitrate reduction is dictated here in major part by geometric
factors, such as the number and spacing of injection wells and, to an extent, on
lateral dispersion. Nitrate reduction may also depend upon substrate type and dose.
97
carban
A PO4 kyectmn
Scouce
*
NOj
OFe
2w*
Zone
-
U
-
-- H -a
@
L
Zone I
-
Bichacai
0.1
Zone 11 - FMter
II
e gend
W - well
RWO - Radxn of Zwae III
q - Rwciarg Row
- Rech
e rne
b - Agquer Thcknes
n - Parsay
bn -ElecIre Dlikness
reactor
Zane III-
hber
-
3 rumued
III
Siriage a/ NOj
free water
R(
[ Qt 1112
fIna
a,..
Figure 32: Schematic Description of
In-Situ Denitrification (From Mercado
et al., 1988)
(jAMW
1IOK
4 POI
CAMW
ra a..
IV
10A"R
PO,*
.......IM
ca .......-.
a Wsmtt
ao
fi
IIl
It
IV
- hIKiucAIs
*
*~
4119
A I
AIa
arIr
4.
MfIUI J
IV
IV~ I
Figure 33: Schematic Description of the Daisy
System (From Mercado et al., 1988)
98
I
The daisy scheme has the following advantages:
* Only one ordinary pumping well is required. Hence, lower costs are
expected.
" Denitrified water is mixed with untreated aerated water, at the
vicinity of the well. As a result, the pumped water is not completely
depleted of oxygen and additional aeration of the pumped water
might not be needed (Mercado et al., 1988).
Mercado et al. (1988) investigated the feasibility of the in-situ denitrification
scheme in Israel. Besides the necessary pumping and injection wells, the
experimental system included a substrate solution preparation and storage system,
and an automatic substrate feeding and injection system. The organic substrate used
in all the experiments was an aqueous solution of sucrose mixed with small
amounts of phosphoric acid, to supply the phosphorus required for bacterial
synthesis. The sucrose was chosen because of its low cost and ease of handing.
Substrate introduction into the injection wells was carried out in pulses twice a day.
The reasons for substrate injections in pulses were:
(1)
Batch introduction of substrate by pulses followed by continuous
recharge of dilution water results in immediate transport of the
substrate solution from the screen zone of the injection wells into the
aquifer, thus minimizing biomass production over the screen and
consequently minimizing the clogging of injection wells.
(2)
The success of the in-situ nitrate removal process depends to a large
extent on minimizing biomass production, in order to prevent
eventual aquifer clogging. Intermittent substrate injection by pulses
was expected to result in conditions of nitrate consumption for
endogenous respiration rather than for biomass synthesis.
Results indicated a slight nitrate reduction of about 10% accompanied by
almost 100% consumption of sucrose. However, the main reason for the relatively
low nitrate removal is the fact that only one injection well was effectively
incorporated in the Daisy scheme, with the result that only one "leaf" of the Daisy
functioned properly. Therefore, denitrified water around the injection well was
99
highly diluted by indigenous nitrate contaminated groundwater converging into the
pumping well. Another explanation is the local hydrological confining conditions
(impermeable clay). Groundwater oxygen levels at 6 mg 0 2 /L were not sufficient for
the decomposition of the substrate loads injected in surplus of that consumed by
denitrification. Thus, the main explanation for the low sucrose recovery (although
only 10% was used for denitrification), is that sucrose is a readily fermentable
substrate.
If one were to use this technique for Cape Cod, one would have to first prove
its effectiveness in unconfined situations, with the entire system functioning
properly, and with perhaps a more denitrification-specific organic substrate, such as
ethanol or acetate. Also, the controllability of the Daisy system can be considerably
improved by inserting monitoring wells between injection and pumping wells in
which chemical parameters would be monitored regularly. This system would best
serve drinking water withdrawal wells.
4.6.1(b) Peat
Peat has been found to be an effective medium for the treatment of municipal
waste water. Peat has several characteristics that make it a desirable material to use
in waste water filter beds. It retains its permeability well, even when worked with
machinery. It provides a good substrate for vegetation. It has a very high cation
exchange capacity and probably could be supplemented with Al, Fe, or Ca to enhance
its P-adsorption capacity. Acidic peat appears to be particularly effective in bacteria
removal. When it loses its waste water treatment capabilities through clogging,
saturation with P, and/or decomposition, the surface layer of peat on a filter bed can
be removed, replaced with fresh peat, and the old peat applied to the land, where it
will ultimately decay and become incorporated into the soil. The grass cuttings
overlying the system can also be disposed of this way. Research has indicated that a
100
peat filter can be utilized in the treatment of septic system effluent. The peat filter is
very similar to a sand filter and compares favorably in the absence of a cost analysis.
Costs would be highly dependent upon local availability of sphagnum peat and/or
filter sand. One disadvantage of the sand filter would be the need for a separate
disinfection unit. The disadvantages of the peat filter seem to be the effluent color
(yellow) and low pH.
Brooks et al. (1984), measured N0 3--N effluent from full-size sphagnum peat
filter fields (figure 34) < 4.5 mg/L, well below the drinking water standard of 10
mg/L, but right at the Massachusetts planning goal. After five years of operation,
system one from figure 34 continued to function at a satisfactory level with virtually
no maintenance other than cropping the grass during the growing season and
having the septic tank pumped after four years of operation. The total nitrogen
concentration was reduced 83%. System two, which was less labor intensive than
system one, had a total nitrogen concentration reduction of 76%. Contrary to the
laboratory study of Rock et al. (1984), Brooks et al. observed an averaged effluent
dissolved oxygen concentration of 6.9 mg 0 2/L, suggesting that the filter field was
aerobic. This suggests that denitrification probably occurred in anaerobic
microenvironments. In addition, the acidic, aerated peat offers a favorable
environment for the growth of fungi that utilizes the nutrients provided by the
applied waste water. A number of fungi can use organic nitrogen, ammonia
compounds, and N0 3 --N directly. The reduction in nitrogen components may be
due, in part, to the activity of these fungi (Brooks and Zibilske, 1983).
The following tables summarize the construction details and treatment
efficiencies for this system:
101
SECTION VIEW
_
90 cm
I
4.8 m-
crushed stone
dist ribution lateral
75 cm
PEAT
15 cmI
10 cmi
-.
.
19m
\- liner
\underdrain-- sand
___
PLAN VIEW
SYSTEM I
F-------19m
r
rTn
4.8 M co
influent
port
SYSTEM 11
distribution
effluent
network
port
12.2 mn
4.8 mn
IJ
influent
sump/pump
distribution
network
effluent
sump/pump
Figure 34: Schematic Diagram of System One and Two Peat Filter Beds (From Brooks et al., 1984)
102
F
E
~0
0
Cn
N
I\O
S04
I-
C1
.
E
I
e
I I
0
~2
n
s
-J
E
0
ci~
zoIC
c~J
-Jx
o
z
5
a-
n
-J
z
E
'C
p0
C,
0
(\j
(W)
Hid3G1
Figure 36: Long Point (Vertical) Denitrification Wall with Chemical Levels Up- and Downgradient
of the Wall (From Robertson and Cherry, 1995)
108
Table 9: Summary of Construction Details for Experimental Peat Filter Bed
Variable
System One
System Two
Date Installed
Type of Distribution
Bed Area ( m2)
Peat Depth Below
Distribution Lines
T otal Peat Depth (cm)
Soil Type
November 1978
Gravity
91
75 cm
August 1981
Dosed
59
75 cm
90
Shallow Silt Loam, Fissured
Bedrock
90
Silt Clay
Lined
Underdrain Pipe
1.5
Lined
Underdrain to Tank
1.5
Grass
No Cover
Lining
Effluent Collection
Design Hydraulic Loading
(cm/d)
Bed Cover
Source: Brooks et al. (1984)
Table 10: Treatment of Septic Tank Effluent by Sphagnum Peat Filter Beds
(Note: Averages reported, range in parentheses)
System I
System II
Septic Tank Effluent
36.3 (2-111)
58.8 (44.8-72.8)
Peat Effluent
2.4 (0-17)
10.4 (8.1-15.8)
%Reduction
93%
82%
Septic Tank Effluent
0.10 (<0.1-0.18)
0.07 (0.01-0.23)
Peat Effluent
4.20 (1-11.5)
4.40 (<0.01-11.1)
Septic Tank Effluent
48.7 (12.3-100.5)
68.6 (53.9-84.2)
Peat Effluent
8.1 (0.0-10.1)
16.7 (10.5-25.2)
%Reduction
83%
76%
Parameter
NH3-N (mg/L)
N03--N (mg/L)
Total N (mg/L)
Source: Brooks et al. (1984)
The average peat effluent N0 3 --N concentration is sufficient from a drinking water
standard perspective. However, it is near to the Massachusetts planning goal.
An additional advantage of this system is that during winter months,
satisfactory treatment continued. Prolonged cold weather had no adverse effect on
103
the sphagnum peat bed operation. These beds were found to function equally well,
with and without snow cover, and at low temperatures. It has been proposed that
the heat generated by microbial activity, coupled with the insulating properties of
peat, prevented complications related to freezing that occurred in some
conventional septic fields during the winter. Also, Brooks et al. (1984) observed
excellent reductions in coliforms so that additional disinfection is not warranted.
Another possible advantage of the peat system is that the peat itself could
serve as the carbon source (Jaouich, 1975). Reddy et al. (1980) demonstrated that
flooded organic soil can reduce N0 3 -N levels via denitrification. These results
suggest that denitrification can become a viable mechanism for nitrogen removal in
peat systems without the addition of an external carbon source. Of the six peat
systems in place on Cape Cod, three are known to be working well hydraulically
(Barnstable County Department of Health and The Environment, 1996).
4.6.1(c) Reactive Porous Media Barriers
Robertson and Cherry (1995) designed a new alternative septic system
utilizing reactive anaerobic porous media barriers for passive in-situ attenuation of
nitrate. The reactive material consists of nitrate-reactive solid organic carbon
(sawdust) which promotes nitrate attenuation by heterotrophic denitrification. The
difference between this configuration and the peat system (which also provides
passive in-situ attenuation of septic system nitrogen) is that attenuation occurs by
nitrate denitrification and requires anaerobic conditions. The denitrification barrier
provides this condition by sediment saturation, excluding atmospheric oxygen
(oxygen flux from the atmosphere to the subsurface is greatly reduced when a zone
of saturation is encountered), and by incorporation of the necessary organic carbon
as solid phase material. The barrier can be constructed in two configurations: as a
subsurface layer (horizontal) installed below the weeping tile field at the time of
104
construction, or it can be retrofitted to existing septic systems as a vertical wall
intercepting the nitrate plume at a downgradient location. Treatment is passive so
that once installed, no energy use or maintenance is required for a long period of
time.
For horizontal construction, denitrification is achieved by installing an
engineered sequence of porous media layers into an excavation located below the
tile bed (figure 35). This can be used at any water table depth provided the barrier
porous media is appropriately sized to remain saturated when positioned above the
water table. The preferred design for the denitrification layer is a porous media
matrix material of coarse silt or fine sand. This is advantageous for Cape Cod which
has sandy soil. The denitrification layer is placed at the bottom of the excavation.
The denitrification layer is installed at a sufficient depth below the septic system
weeping tiles so that effluent ammonium (NH4 +) is oxidized to nitrate during
percolation through the unsaturated sediments lying above the barrier layer. To
ensure nitrification, a nitrification layer constructed of coarser grained sand is placed
above the denitrification layer.
The usefulness of the barriers has been demonstrated. If it is assumed that
the barrier organic carbon is consumed only by heterotrophic denitrification,
Robertson and Cherry (1995) demonstrated that a denitrification layer of modest
thickness has the potential to last a very long time. They considered a single family
residence generating 1000 L/day of effluent with average NH4+-N content of 40
mg/L. Potential annual nitrate loading would be 15 kg. A one meter thick barrier
layer, 100 m 2 in area and containing 2% by weight organic carbon, would contain
3600 kg of organic carbon. By equation {5}, the carbon mass would be sufficient for
200 years of denitrification. Even if only 10% of the organic carbon was available for
denitrification, the layer would still last for the typical design life of a septic system,
about 20 years.
105
Figure 35 shows chemical profiles (mg/L) after one year of operation at the
Killarney (Canada) site. Effluent NH 4 +-N (99 mg/L) was almost entirely
transformed by oxidation to nitrate in the sand layer overlying the denitrification
barrier. The resulting N0 3--N level of 125 mg/L occurring at the top of the barrier
was then attenuated to a very low level (1.2 mg/L). Similar chemical profiles were
observed below the Borden tile field.
The denitrification wall can also be placed in a vertical position (figure 36),
used to retrofit existing septic systems by placement at a downgradient location if the
nitrate plume is migrating horizontally at a shallow depth. The wall is installed
below the water table perpendicular to the plume flowpath, in a vertical position.
The reactive material is placed into an excavation constructed with the aid of
dewatering wells. Because the barrier is placed below the water table and is not
intended to impede groundwater flow, sand and sawdust (80% sand, 20% sawdust) is
the preferred matrix material for the vertical wall configuration. Again, this is
advantageous for Cape Cod.
Figure 36 shows concentrations up- and downgradient of the barrier wall after
about one year of operation. Consistent chloride (Cl-) levels (51-52 mg/L)
demonstrate that undiluted septic plume water was migrating through the bottom
half of the barrier. Very high N0 3 -N values present upgradient of the wall (57-62
mg/L) were attenuated to much lower values (2-25 mg/L) downgradient.
4.6.1(d) Bioremediation via Autotrophic, Hydrogen-Oxidizing, Denitrifiers
on Cape Cod
Bioremediaton has received considerable interest as a potential tool for insitu treatment of contaminated groundwater. The approach involves identifying a
microbial process that will (1) remove or transform a given contaminant, preferably
resulting in innocuous products; and (2) either stimulate the indigenous microbial
106
KILLARNEY
LAYER
C?
0
SANOY
TOP SCIL
WEEPNG
a
I,
50
NH
:cc
CO
0
-N
50
NC
iCOO
3
50
-N
SO
00
0
30
00C
.-.
60
0
50
0
t0O
20C
GRAEL
0.2
-SANC
0.4
ILY
SILT+
SAWDUST
0.6
SILT + COMPOST
0.8
SILT+RYES ED
-
1.0
-
.
BEDROCK
BORDEN
0
LAYER
-
E
SILT
7/93
/'
12
0.2
-SANO-
0
*
(rT1LE
.
.GRAVEL.
*-a
3/8
PLY
0.4
Ig'.
F
50
100 0
NH 4-N
a
25
+--2N03-N
5
25
500o
25
50
SO 4
0
25
DOC
25
k
PIEZO.
0.8
LYSIMETER
SAWDUST
-
1.4 L-
jSSAND
.
-
SILT +
/93
SILT a PEAT
Figure 35: Killarney and Borden (Horizontal) Denitrification Layers Showing
Chemical Profiles
(mg/L) After One Year of Operation (From Robertson and Cherry, 1995)
107
*
SAND
0.6
-
C=-.
-
CL
TOP SOIL
50
population to induce the process of interest or add nonnative microorganisms that
are capable of catalyzing the process. The bioremediation approach usually implies
that additional constituents (limiting nutrients and/or exotic organisms) must be
introduced into the aquifer over and above the original contaminants already
)
present. Because denitrification produces an innocuous end product (N2
denitrification may be well suited as a mechanism for in-situ bioremediation of
nitrate contamination (Smith and Ceazan, 1991).
The ideal electron donor to use for bioremediation by denitrification in
groundwater systems would (1) be innocuous, (2) be a substrate that the indigenous
population of denitrifiers could utilize, (3) be a substrate that could fuel oxygen
respiration if it were first necessary to consume background oxygen, and (4) have a
very limited utility for any nontargeted group of microorganisms. Hydrogen (H2)
appears to fit all of these requirements.
Smith and Ceazan (1991) discovered preliminary evidence from the Otis Cape
Cod site that indicated that denitrification was stimulated by the presence of
hydrogen and subsequently hypothesized that hydrogen would be a reasonable
choice to stimulate denitrification during bioremediation (figure 37). When H 2 was
added, nitrate concentrations of more than 1 mM (mmol/L) (62 mg NO 3 ~/L) were
completely depleted in only 24 hours, representing more than a hundredfold
increase in the rate of nitrate consumption above the endogenous rate. Smith and
Ceazan isolated and characterized the microorganisms responsible for the
denitrification and all were capable of autotrophic growth as denitrifiers, using H2
and N03- for energy requirements and fixing carbon dioxide for carbon (no organic
carbon was present in the medium). The reaction products, water (the oxidation
product of H2) and nitrogen (the reduction product of nitrate), are present in pristine
groundwater and hydrogen is not toxic in the low, solubility-limited concentrations
that would be added to groundwater. The oxidation of hydrogen will be the
109
1.2
I
i
I
I
0
E
z
ENDOGENOUS
0.8
0
z
Lj
0
z
0 0.4
-H
0
2
Li
0.0
0
24
48
HU RS
Figure 37: Time Course of Nitrate Concentration (mM) in Sediment Slurries (From Smith and
Ceazan, 1991)
110
terminal electron-accepting process in a microbial food chain (Lovley et al., 1990).
Thus, the addition of hydrogen would be much more oriented to the target
microbial process (denitrification) than would the addition of a general electron
donor, such as glucose. Some of the hydrogen-oxidizing denitrifiers (Paracoccus
denitrificans) can also oxidize hydrogen aerobically. This means that, given the
common situation where oxygen and nitrate were both present, these organisms
could utilize the added hydrogen to first deplete the in-situ oxygen, a prerequisite for
denitrification, and then switch to denitrification to consume the nitrate that was
present (Smith and Ceazan, 1991).
4.6.1(e) Sulfur/Limestone Denitrification
In Chapter 2, I focused on heterotrophic denitrification. I have not yet
described autotrophic (inorganic energy source) denitrification. Under anoxic
conditions, nitrate is converted into nitrogen gas. Elemental sulfur, which is used
as an electron donor, is converted into sulfate. Including the production of biomass,
the reaction proceeds as follows:
2
55 S + 50 NO 3 + 38 H2O + 20 CO2 + 4 NH4 --> 4 C 5H70 2N + 25 N 2 + 55 SO4 - +64 H+ {8}
In this process, both sulfur and limestone are present in the form of granules (0.2-0.6
cm) on which the bacteria settle and adhere. Limestone is used to maintain the
optimal pH range and as an inorganic carbon source for the bacteria. This process
has been used at a demonstration plant in The Netherlands since December 1986
and is designed for denitrification of water containing no more than 75 mg N0 3 /L
(Kruithof et al., 1986). One could use crushed sea shells, which are readily available
on Cape Cod, with an elemental sulfur source to denitrify groundwater. An
inorganic carbon source (bicarbonate, for example) must be present in the water to
enable denitrification and since sulfate is produced in the treatment process, use of
111
the denitrification on sulfur is restricted to treatment areas where the sulfate
concentration is low (no more than 25 mg S0 4 2 -/L, Kruithof et al., 1986). As shown
in Table 4, sulfate levels are quite low and bicarbonate is present. However, the
feasibility of using this biochemical process has not been attempted in-situ.
Nonetheless, this process seems economically advantageous and should not be
abandoned.
4.8 Conclusion
In Chapter 4, I reviewed alternative septic systems and in-situ remedial
schemes which all use biological denitrification as a means of attenuating nitrate in
septic system effluent. However, not all of these are feasible , from either an
economic and/or engineering perspective, for use on Cape Cod. In Chapter 5, I will
propose plans for action; namely, what should be done for Cape Cod's
contamination?
112
Chapter 5
What Should Be Done For Cape Cod's Nitrate Contamination?
5.1 Introduction
The residents of Cape Cod face a problem of nitrate contamination of their
groundwater (their primary source of drinking water) and their coastal and aquatic
environments. Certain aspects of Cape Cod aggravate the problem. The
hydrogeology of Cape Cod allows for the mobility of nitrate with little attenuation
besides low levels of natural, not anthropogenic, biological denitrification. There is
very little subsurface organic carbon on Cape Cod, with much of it probably resistant
to biodegradation, and no significant natural replenishment of this resource. Thus,
restoration of nitrate contaminated aquifers on Cape Cod via biological
denitrification would probably require an injection of a carbon source to remove the
carbon limitation on denitrification and cause sufficiently anaerobic conditions for
denitrification. Site specific economic, engineering, and geohydrologic factors
control the feasibility and utility of aquifer restoration by this method. The injection
of organic compounds into the aquifer constitutes the introduction of either
synthetic or altered natural products which itself could pose a serious
contamination problem if performed improperly or incorrectly.
Urbanization is growing rapidly on Cape Cod. This not only heightens the
demand for high-quality, low-nitrate groundwater for drinking, but also augments
the loading of nitrogen-rich domestic sewage to the aquifer providing drinking
water to the local population. Ocean disposal of this sewage is prohibited by the
Massachusetts Ocean Sanctuaries Act. Even if sewers and waste water treatment
113
plants were installed and constructed at huge public expense and dramatically
reduced the loading of nitrogenous wastes to the aquifer, the problem of nitrates in
groundwater would continue for years since the rate of groundwater movement is
so slow that the bulk of septic nitrogen entering coastal waters lags behind urban
development by nearly a decade (Valiela et al., 1992). In addition, if a plan is
implemented to dramatically reduce the nitrate levels in drinking water and it is
successful from a drinking water perspective, this does not guarantee success from
an environmental (i.e., coastal waters) perspective. Adverse effects from nitrate to
surface waters are known to occur at concentrations .(and, coincidentally, lower
housing/building density) substantially lower than much lower than concentrations
(and housing density) in drinking water which lead to adverse health effects.
I have presented thus far the sources, fate and transport, contamination, and
effects of nitrate in the subsurface environment and a discussion of remediation
technology that has been developed to deal with nitrate in groundwater. In this
chapter, I present my opinion of what I feel should be done on Cape Cod with
respect to nitrate contamination. This is not a proposed policy or suggestions for
those who put forward and implement policy for the residents of Cape Cod. It is
what I feel, in my opinion of the scientific and engineering aspects of the
circumstances of nitrate contamination on Cape Cod, would be beneficial.
5.2 Immediate Action for Domestic Sewage Disposal
In the near future, certain actions taken by the residents of Cape Cod would be
beneficial to reducing the loading of nitrate to groundwater and to receiving coastal
waters. As mentioned earlier, there is rapid urbanization on Cape Cod, especially
near the coast. Homes far and near to the coast may load similar (e.g., chemically,
physically) sewage to groundwater but their proximity to receiving coastal waters is
dissimilar and this would impact the quality of those waters differently. Sewage
114
from homes near the coast would immediately and, most likely, adversely affect
groundwater and receiving waters. Large clusters of homes and high-sewage
producers (e.g., apartment buildings, schools, hospitals, large businesses, hotels, etc.)
impact groundwater differently than single-family homes and their nitrate loading
to groundwater must be addressed differently as well. Thus, one needs to address
these circumstances distinctly.
5.2.1 Homes Near the Coast
Sewage from homes near the coast of Cape Cod (note the higher degree of
housing density along the coast in figure 13) flows directly into receiving waters.
Although there is little attenuation of nitrate in the subsurface environment on
Cape Cod, what little denitrification occurs as sewage travels in groundwater to
coastal receiving waters does not occur in sewage from homes directly on the coast.
Thus, the problem of nitrate contamination from domestic sewage in this case is
different. The primary focus is the immediate disposal of sewage to surface waters,
not groundwater. One should also be cognizant of the reason why many people
decide to relocate to Cape Cod. Most residents are attracted to Cape Cod by its scenic
beauty. Thus, most homeowners, especially along the coastline, would prefer a
denitrifying technology that would minimize the amount of above-ground
machinery, other obstructions, and other aspects of the technology would could be
viewed as aesthetically unpleasant.
Organic carbon injection into the subsurface for homes near the coast is both
impractical and environmentally unsound. It is impractical because the time of
travel of the domestic sewage to the receiving waters is far shorter than the
characteristic time for denitrification. Thus, the injection of organic substrate for
denitrifying bacteria would be for naught; the bacteria would not have enough time
to significantly attenuate nitrate in the domestic sewage before flowing into coastal
115
waters. It would be injudicious to inject any chemical into the subsurface that
would have no positive impact on water quality and could adversely affect both the
subsurface and surface waters. Thus, injection of organic substrate is not valid from
an engineering perspective for homes near or along the coast and would probably be
deemed as ocean dumping under the Massachusetts Ocean Sanctuaries Act.
If there are no proprietary restrictions, the reactive porous media barriers
described in 4.6.1(c) could serve well in this capacity.- They are compact and require
little maintenance while providing a long service life. The Daisy (4.6.1(a)) and
sulfur/limestone (4.6.1(e)) systems are not a viable option since they are designed for
use near drinking water/withdrawal wells. The use of peat, as mentioned in
4.6.1(b), would not necessarily function well for homes along the coast. From an
engineering perspective, in the immediate vicinity of the peat, observations of the
effluent from the peat system has shown that it can have a low pH and a yellowish
color (Brooks et al., 1984). The short travel time of this effluent to the receiving
waters could result in surface waters being impacted by this yellowish, acidic
effluent. From the viewpoint of a homeowner, the peat could be a source of odors
and an eyesore (much different the rest of their lawn). As mentioned in 4.6.1(b),
once the peat layer loses its waste water treatment capabilities, the surface layer is
removed and replaced by fresh peat, which is not readily available on Cape Cod.
This would dramatically increase the cost of this remedial technology.
Currently, the RUCK system appears to be the alternative septic system
technology most suited for use nearest the coast if there are at least ten feet from
ground surface to the aquifer. It is a well-established denitrifying septic system.
Although its cost can be exorbitant, the system provides an internal organic
substrate with few maintenance requirements, a long design life, and only one
above ground equipment piece, with a totally passive remedial scheme. Also, to
retrofit an existing septic system (i.e., redo the piping of the home and only add a
116
new RUCK tank, not an entire operating system, septic tank and RUCK tank), this
alternative septic system technology is the most economical. The FAST system has
an air blower above ground which can be loud and is approximately the same cost of
the RUCK system. The Ekofinn BioclereTM, also approximately the same cost,
requires routine operation and maintenance, and also has mechanical equipment
(below ground) which could be a noisy nuisance. The SBR is too expensive for
single homes along the coast; however, for larger septic systems along the coast, it
seems to be as good an alternative septic system as a larger RUCK system and should
not be abandoned.
5.2.2 Large Cluster of Homes
A large cluster of homes (multiple dwellings or buildings linked to a
common sewage treatment process) and high-volume sewage producers (hotels,
restaurants, apartment buildings, hospitals, etc.) face a unique problem: how to treat
such large volumes of sewage. This requires that these producers install a system
capable of handling such large volumes economically and efficiently.
Peat filters could operate well for these high-volume producers. Failure of
high-volume septic systems in winter could greatly and adversely impact
groundwater. Alternative systems selected for these high-volume producers should
be able to operate well throughout the year in order to prevent large volumes of
untreated or incompletely treated sewage from entering the aquifer. Peat filters
function well in cold climates. The studies of peat filters in Minnesota and Maine
show that this system is capable of maintaining a significant level of denitrification
throughout the year (Brooks et al., 1984). Although peat is not readily available on
Cape Cod and could prove to be expensive for the single homeowner, it is more
economically feasible for such high-volume producers. In addition to the peat filter,
the sequencing batch reactors (SBRs: Cromaglass@ and Amphidrome® systems,
117
4.5.1(e)) display optimal cost effectiveness when used for flows slightly above the
single family use (Barnstable County Department of Health and The Environment,
1995). Since cost information is not available for the peat system, I cannot compare
the two; however, it appears that both systems are capable of handling high-volume
sewage producers.
5.2.3 Other Domestic Sewage
Although alternative systems have been available for more than ten years,
none has achieved widespread usage because the evidence for the occurrence of
large-scale contaminant plumes from septic systems was not well-documented, with
the result that regulators did not discourage the use of conventional septic systems.
Thus, there had been little incentive to use the more expensive alternative designs
and to accumulate the field performance data necessary to allow accurate assessment
of their effectiveness. Now, however, non-coastal homeowners on Cape Cod
should use alternative septic system technologies or take other steps to curtail any
further degradation of the groundwater aquifer. The alternative systems are
expensive and out of reach economically for many homeowners on Cape Cod. So,
homeowners should be given incentives to use alternative technologies (including
those mentioned in 4.5 and 4.6).
5.3 Large-Scale Contamination Remedial Action
The major source of nitrate contamination on Cape Cod is from septic
systems. The few large sewage plumes on Cape Cod require a different remedial
plan than the areally extensive point sources. Ex-situ reactors, such as the
118
sulfur/limestone remedial scheme presented in 4.6.1(e), have offered regulators a
viable means of remediating large volumes of nitrate-contaminated water.
Nonetheless, there are advantages to in-situ remediation. One advantage of
in-situ remediation of contaminated groundwater is that in-situ systems are largely
passive in that after installation, in situ reactors are intended to function with little
or no maintenance for long periods. This contrasts with the energy and
maintenance-intensive character of pump-and treat, ex-situ systems. Another
advantage is that in situ denitrification can perform both denitrification and
secondary treatment: for example, filtration, degradation of organic residuals and
re-aeration, within the aquifer. Furthermore, underground processes are
independent of any seasonal temperature variations, thus maintaining the
efficiency of any such system. This is of critical importance on Cape Cod where
there can be substantial seasonal variations in temperature.
Thus, in-situ remedial
schemes, such as a series of Daisy system wells, reactive porous media barriers
placed vertically downgradient of a nitrate-rich sewage plume, an in-situ version of
autotrophic (e.g., sulfur/limestone or hydrogen oxidation) denitrification schemes,
and/or other process would best serve to remediate large-scale nitrate
contamination areas. Unfortunately, Mercado et al. (1988) did not have a properly
functioning Daisy system for their work. A system similar or exactly like the one
used in Israel should be installed on Cape Cod and its performance monitored.
Policymakers, cognizant of any adverse impact either on drinking water or the
environment, should devise schedules for pilot testing and implementation for this
and other large-scale in-situ treatment processes.
119
5.4 Conclusion: Future Work and Long-Term Action for Domestic Sewage
Disposal
Much work needs to be initiated and completed in order to better understand
denitrification and how we can use it as a means of addressing nitrate-contaminated
groundwater on Cape Cod. The following is a brief list of work that I feel would
further the possible use of denitrification as a means of addressing nitrate
contaminated groundwater on Cape Cod:
(1) In-Situ Denitrification Via Organic Carbon Substrate Injection: On Cape Cod, the
availability of an oxidizable source of organic carbon seems to be the limiting factor
for heterotrophic denitrification (Smith and Duff, 1988). So, even if denitrifying
bacteria and nutrients are present in sufficient quantities to support denitrification,
insufficient organic carbon below a depth of 2-3 m (soil root zone) prohibits
denitrification in the unsaturated zone (Starr and Gillham, 1989). One possible insitu biological treatment method is enhancement of native microbial communities.
Enhancement traditionally has referred to the injection of nutrients and oxygen to
accelerate biodegradation of waste organics. The enhancement of native
communities has been extensively used for the removal of organic waste or heavy
metals. For nitrate removal, one would choose to limit oxygen and apply a labile
carbon source to force denitrification. Denitrification in the Cape Cod aquifer system
would be limited by available labile dissolved organic carbon (DOC). If sufficient
DOC exists at the point of nitrate entry into the aquifer, nitrate will not likely persist
in the aquifer. For example, DOC concentrations of 12 mg C/L were adequate to
reduce N0 3 --N levels from about 25 (five times the planning goal) to 0 mg/L during
a travel time of 1.5 to 3.0 years in a sandy aquifer on Cape Cod (Smith and Duff,
1988).
120
Organic carbon substrate injection could remedy this organic carbon
restriction. This has the potential for relieving areally extensive nitrate
contamination of the groundwater aquifer on Cape Cod. I have yet to find any
extensive work in the literature of small-scale systems that could be used for single
homes, with all work (known to me) focused on substrate injection in the vicinity
of drinking water/withdrawal wells. The development and testing of such systems
could provide a viable and economical solution that would address nitrate
contamination on Cape Cod. Such a system, with sufficient injection of organic
carbon substrate to prevent the depletion of carbon within the aquifer to the point at
which the aquifer can no longer support denitrification, would function well on
Cape Cod, where the existence of a shallow, unconfined aquifer (2.4.1(e) and (f)); wet
soil (2.1); and bacteria in solution, not in biofilms on the surface of solid particles
(2.3), favors in-situ denitrification.
A more targeted, denitrification-specific natural (i.e., economical) organic
substrate has yet to be found. Cellulose is the most abundant renewable resource in
the world and may offer an inexpensive alternative to the synthetic carbon
compounds commonly used in the treatment of nitrate contaminated water
(Volokita et al., 1996). Volokita et al. used cotton as the sole physical and chemical
substrate for denitrifying microorganisms in laboratory columns (ex-situ). Cotton is
an abundant crop in many countries and the relatively low price of the short fiber
product makes it a very attractive substrate for denitrification. In their study, the
treated effluent contained low concentrations of DOC and was without detectable
odor or color. Bacterial counts were on the order of 105 ml-1 . Thus, post treatment
(sand filtration and chlorination) would be required before reaching drinking
quality. However, an in-situ test was not performed. I feel that, in the manner of
the reactive porous barriers, research should be conducted on Cape Cod to
determine the feasibility of such a treatment scheme.
121
Ethanol and methanol more are targeted to denitrification than cotton,
sawdust, and other natural organic substrates. However, the injection or placement
of synthetic organic compounds in an in-situ denitrification apparatus is not as
prudent as the use of a natural product. Either a more thorough search for natural,
denitrification-specific compounds and/or the development of an altered natural
organic substance for denitrification would hasten the development of a process that
would alleviate some of the nitrate contamination of the groundwater on Cape Cod.
(2) Autotrophic (Hydrogen Oxidation) Denitrification: As discussed in 4.6.1(d),
Smith and Ceazan (1991) isolated autotrophic denitrifying microorganisms which,
in the presence of hydrogen, depleted nitrate concentrations of more than 1 mM (62
mg NO3 ~/L) in only 24 hours, representing more than a hundredfold increase in the
rate of nitrate consumption above the endogenous rate. Capitalization of this
method of denitrification would greatly reduce nitrate contamination on Cape Cod.
Hydrogen gas is cheaper and more abundant than organic substrates, non-toxic at
the levels needed to affect denitrification, and its injection into the subsurface
environment would be simpler. However, no work has been initiated in the
development of a process that would use this specific biochemical pathway in order
to denitrify groundwater. Also, the existence of these denitrifiers has not been
proven throughout the Cape Cod aquifer, only at the Otis site. If they do not exist in
other parts of Cape Cod, one would have to use alien microorganisms which are less
successful in remediation than indigenous microorganisms.
Nitrate contamination
on Cape Cod is areally extensive and requires implementable remedial technologies.
It could take numerous years for this process to be a viable option on Cape Cod but it
should not be overlooked due to the enormous benefits that would arise from its
development and implementation.
122
(3) Studying and Modeling Denitrification of Domestic Sewage from Septic Systems
on Cap
: All the work that I reported in Chapter Two concerning the study of
denitrification on Cape Cod occurred at the MMR Otis Site which is not the typical
source of nitrate to the groundwater on Cape Cod. I suggest that researchers focus
now on denitrification, its rate and potential use as a means of addressing nitrate
contamination in the immediate vicinity and downgradient of a sewage plume
from a septic system. This, not large-scale contamination plumes, is the primary
source of nitrate into the subsurface environment on Cape Cod. Subsequent
research of the phenomena of denitrification near these point sources should
include the effectiveness of alternative septic systems and other remediation
technologies in two critical areas: long term effectiveness and a side-by-side
comparison of efficiency. For example, experiments at both large and small (coastal
and non-coastal homes) septic systems should be conducted. These experiments
should monitor the performance of different remediation technologies at the site
which receive the same septic system influent. On the short term, the nitrateremoval efficiency should be monitored: i.e., which remediation technology
removes the most nitrate? In addition to short-term success, the long term
effectiveness of nitrate removal should be monitored: i.e., did the systems suffer
any serious malfunctions which required outside maintenance and/or
breakthrough of nitrate in the remediation technology effluent? Also, a cost
comparison (yearly and for long periods of time) should be an integral part of the
experiment, which would facilitate the important decision residents of Cape Cod
may need to make; namely, what is the best remediation technology available to
me?
Further study could also lead to advances in the modeling of denitrification
and, subsequently, to advances in the models used by public health officials to
predict nitrate loading to watersheds and different zones (I, II, and III) of
123
groundwater protection. These models currently do not incorporate denitrification
and, although this is a conservative method of predicting nitrate concentrations, it
may be false and result in needless expenditures. Proper land use planning, zoning,
and nitrogen management controls, all currently determined by the results of
nitrogen loading models, will save communities from investing millions of dollars
in treatment systems to remove nitrate or to locate and develop new sources of
drinking water. Further study of denitrification near these point sources could
result in the development of more accurate models to assist public officials and
homeowners in addressing nitrate contamination in the future.
(4) Environmental Control of Nitrate Contamination: As noted throughout this
thesis, there are two levels of treatment for nitrate: protection of drinking water and
protection of receiving waters. Algal growth in surface waters is limited by nitrate
concentrations and concentrations 10 to 100 times less than the drinking water
standard (M. Geist, personal communication). In order to control the
environmental impacts of nitrate contamination of receiving waters from
contaminated groundwater, researchers and policymakers need to determine if
alternative treatments implemented to protect drinking water supplies can also
protect receiving waters. They should also work together to devise plans to protect
harbors from eutrophication and remediate harbors already exhibiting signs of
eutrophication. For example, I decided to address homes near and far from the coast
distinctly. Researchers should determine the width of the zone from the coast in
which significant attenuation of nitrate in groundwater does not occur (as
mentioned in 5.2.1). Thus, policymakers (and later, homeowners) could choose
appropriate technologies for their septic systems. For example, the reactive porous
media barriers presented in 4.6.1(c) would serve well for homes along the coast in
the future. This technology has not been installed yet near a home on Cape Cod but
124
has been installed at the Waquoit Bay National Estuarine Research Reserve on Cape
Cod. One possible use of this technology is to install numerous barriers along the
coast near or on harbors which display signs of eutrophication. Researchers should
also work closely with policymakers in determining at what housing/building
density do adverse environmental (in addition to drinking water) effects occur.
(5) Protection of Drinking Water Supplies: Cape Cod residents rely on clean and
safe drinking water from the groundwater aquifer. This vital resource needs to be
protected. Prevention of aquifer contamination is usually a more effective and a
considerably cheaper strategy than is aquifer restoration. This is particularly true
where the pollutant sources, such a the septic systems on Cape Cod, are areally
extensive, the concentrations are low, and the volumes of groundwater impacted
are large. Each water supply well, including the 100 municipal wells on Cape Cod,
should be vigilantly monitored for nitrate contamination and should have a
protection plan and schedule. There is no other source of drinking water on Cape
Cod so attentive monitoring and, perhaps, implementation of a protective
technology (vertical reactive porous media barriers and/or a series of Daisy system
wells) is required.
In addition, management options should be considered as part of an overall
plan to reduce nitrogen loading for all of the wellfields. Some of these options,
depending on the land use development patterns, include use of alternative
denitrifying technology for commercial and residential uses, land acquisition near
wellfields, agricultural or conservation deed restrictions for non-protected open
space, and increasing minimum lot sizes. Because there is a correlation between
housing density and the concentration of nitrate in Cape Cod groundwater,
managing density is one way that towns can protect public water supplies from
nitrate contamination.
125
The residents of Cape Cod cannot turn to ocean/surface water disposal of
wastes since this is forbidden by the Massachusetts Ocean Sanctuaries Act. I feel that
public waste water treatment works and an extensive sewer system is inevitable on
Cape Cod. The population is growing too rapidly for the status quo of individual
septic systems to continue. In addition to this, strict zoning, perhaps even
increasing the minimum lot size for homes on Cape Cod, may be required. There
are alternatives for the residents of Cape Cod to address nitrate contamination of
groundwater and these alternatives lie with the use of denitrification. However, in
the future, the residents of Cape Cod will have to make some difficult decisions
regarding their future, everything from expansion of groundwater zones of
protection for Cape Cod, installation of a sewer and waste water treatment network,
more restrictive zoning regulations, and actions needed for the restoration and
protection of coastal waters and other critical environmental settings, just to name a
few. The residents of Cape Cod should, however, note that although denitrification
alone does not resolve these multidisciplinary issues, it can and should be used as a
means of addressing nitrate contaminated groundwater on Cape Cod.
126
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