JOURNAL OF THE AMERICAN WATER RESOURCES ASSOCIATION
JUNE
AMERICAN WATER RESOURCES ASSOCIATION
2004
OCCURRENCE OF TOTAL DISSOLVED PHOSPHORUS IN
UNCONSOLIDATED AQUIFERS AND AQUITARDS IN IOWA1
Michael R. Burkart, William W. Simpkins, Amy J. Morrow, and J. Michael Gannon2
INTRODUCTION
ABSTRACT: Seven sets of ground water samples from 103 observation wells were analyzed for total dissolved phosphorus (TDP) in
four areas and five materials including loess and loess derived alluvium in the Deep Loess Hills of western Iowa, outwash and
fractured till adjacent to Clear Lake in north central Iowa, fractured till in central Iowa, and a sand and gravel aquifer in northwest Iowa. Land use in ground water recharge zones in all four
areas is dominated by crop or animal production or both. Concentrations of TDP exceeding the minimum laboratory detection limit
of 20 µg/l as P were found in all areas and in all materials sampled.
Samples from the outwash deposits associated with Clear Lake contained significantly larger concentrations than all other areas and
materials with a median of 160 µg/l. Water from fractured till in
three areas produced the smallest range of concentrations with a
median of 40 µg/l. The mean value of TDP in all sample sets
exceeded 50 µg/l, an important ecological threshold that causes
increased productivity in lakes and perennial streams and one
being considered as a surface water nutrient standard by regulatory agencies. These results clearly show that ground water in essentially all near-surface aquifers and aquitards discharging to Iowa’s
streams and lakes is capable of sustaining P concentrations of 50 to
100 µg/l in streams, lakes, and reservoirs. Consequently, even if
point discharges and sediment sources of P are substantially
reduced, ground-water discharge to surface water may exceed critical thresholds under most conditions.
(KEY TERMS: ground water hydrology; nonpoint source pollution;
water quality; phosphorus; agriculture; till; loess; alluvium.)
Increased interest in surface water impairment by
nutrients has been generated by state and federal
efforts to understand and mitigate nonpoint source
contamination. Many surface waters in Iowa and
other Midwestern states are impaired by excess nutrients, primarily nitrogen and phosphorus (P). There
has been clear evidence for decades that P from a
variety of sources is discharged to surface water bodies by runoff. Furthermore, it is well understood that
phosphorus in runoff constitutes most of the total P
(TP) loads found in streams. Stream baseflow has also
been shown to carry substantial loads of P from point
sources such as wastewater. Consequently, more data
on P concentrations in ground water from wells will
be required to distinguish P in baseflow derived from
ground water and P derived directly from point discharges to streams. There is increasing discussion
among state and federal regulators about establishing
P standards for streams that are based on concentrations associated with aquatic ecosystem health. These
concentrations are much smaller than those found in
runoff. Such discussions raise the question of whether
these standards can be met during periods when
ground water is the only source of discharge. It is
hypothesized that P concentrations in ground water of
most of the materials comprising aquifers and
aquitards adjacent to Iowa streams constitute sources
of nutrient enrichment to streams, lakes, and reservoirs.
Burkart, Michael R., William W. Simpkins, Amy J. Morrow, and J. Michael Gannon, 2004. Occurrence of Total Dissolved Phosphorus in Unconsolidated
Aquifers and Aquitards in Iowa. Journal of the American Water Resources
Association (JAWRA) 40(3):827-834.
1Paper No. 03118 of the Journal of the American Water Resources Association (JAWRA) (Copyright © 2004). Discussions are open until
December 1, 2004.
2Respectively, Hydrologist, National Soil Tilth Laboratory, USDA-ARS, 1250 Pammel Drive, Ames, Iowa 50011; Professor, Iowa State University, Department of Geological and Atmospheric Sciences, Ames, Iowa 50011; Chemist, National Soil Tilth Laboratory, USDA-ARS, 1250
Pammel Drive, Ames, Iowa 50011; and Geologist, Iowa Geological Survey, 109 Trowbridge Hall, Iowa City, Iowa 52241-1319 (E-Mail/Burkart:
burkart@nstl.gov).
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BURKART, SIMPKINS, MORROW, AND GANNON
The purpose of this paper is to determine if the
concentrations of P in various ground water settings
in agricultural areas of Iowa are sufficient to sustain
nutrient enriched surface waters. A synoptic sample
of ground water from a variety of aquifers and
aquitards in the proximity of impaired water bodies
was used to test this hypothesis in Iowa, a state with
the most intensive and extensive agricultural land
use in the United States. This and similar analyses
will be important to the discussion of the thresholds
to be established for surface water contamination in
intensive agricultural regions with similar geologic
materials.
A national analysis of orthophosphate (Nolan and
Stoner, 2000) showed no significant difference among
shallow wells under agricultural areas, which had
mean values of 10 µg/l and under urban land areas
with mean values of 20 µg/l. The mean values of 10
µg/l were found in deeper aquifers that were assumed
to have less direct influence of surface sources of P.
However, samples from eastern Iowa aquifers (Savoca
et al., 2000) yielded significantly different concentrations under urban and agricultural land use. These
researchers reported median values for TDP of 33 µg/l
in agricultural areas compared to15 µg/l in urban settings. Water samples from Midwestern aquifers,
including five from Iowa were found to have median
values of 20 µg/l TDP (Kolpin et al., 1996). Other samples from aquifers in Iowa yielded values with a median of 10 µg/l TDP in a heavily pumped sand and
gravel aquifer adjacent to the Mississippi River (Lucy
et al., 1995) and municipal wells from a variety of
aquifers produced water with median values of 100
µg/l orthophosphate (Schaap and Linhart, 1998).
Until recently, P transported through ground water
to surface water has not been considered a critical
transport pathway for agricultural nonpoint sources
of P (Sims et al., 1998). However, evidence exists for
both matrix and preferential flow in drainage water
and dissolved P from surface sources transported
through soil (Baker et al., 1975; Beauchemin et al.,
1998; Sims et al., 1998; Stamm et al., 1998; Dils and
Heathwaite, 1999; Hooda et al., 1999; and Simard et
al., 2000). Soil leaching studies have also shown the
potential for P transport through the unsaturated
zone of as much as 232 µg/l TDP (Haygarth et al.,
1998). Field investigations show that the introduction
of large sources of P can increase the concentrations
in ground water more than 10-fold within a few years
(Novak et al., 2000). Long term use of inorganic fertilizer and concentration of manure P provide potential
loads in soils (Sims et al., 1998) that could exceed the
P sorptive capacity (PSC) of soil and enhance leaching. Leaching is of particular concern in organic
and/or sandy soils (Sharpley and Rekolainen, 1997)
and in soils with small PSC (Beauchemin et al., 1998).
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There are few reports in North America where the
PSC has been exceeded under agricultural systems
(Novak et al., 2000), although studies of septic system
plumes (Robertson et al., 1991; Harman et al., 1996;
Robertson et al., 1998) may provide analogs for
manure disposal systems. Excessive manure disposal
has produced a “chemical time bomb” for leaching in
Europe (Behrendt and Boekhold, 1993).
Analysis of 750 wells in Nebraska Sand Hills
aquifers found concentrations of TDP of as much as
300 µg/l in shallow wells with somewhat smaller concentrations in deeper wells (Helgesen et al., 1994).
Piezometers and seepage meters in the bed materials
of the Tualatin River, Oregon had 60 to 2,900 µg/l
TDP (Rounds et al., 1994). Concentrations in baseflow
from central and western Wisconsin streams yielded
TDP concentrations ranging from 10 to 160 µg/l
(Mason et al., 1990). These measurements established
ground water as a significant source of P to streams
in that region of Wisconsin.
For the last few decades, research on the effects of
agriculture on ground water quality in the Midwest
has focused on the occurrence of nutrients, primarily
nitrogen ions. More recently, renewed concerns about
P in streams, lakes, and riparian corridors has stimulated the reexamination of the occurrence of P in
ground water. Initial sampling for P at four Iowa
ground water study sites led to the test of an hypothesis that ground water concentrations in a variety of
aquifers and aquitards near agricultural sources of P
may provide sustained sources of P for surface water
bodies.
SAMPLING AND ANALYTICAL METHODS
In May 2002, 103 observation wells were sampled
and TDP was selected for analysis to assure consistent comparison among sites. Areas selected for sampling were in four on-going research projects where
ground water is in direct hydraulic connection to a
surface water body. These studies involve the dominant materials comprising shallow unconsolidated
aquifers and aquitards in Iowa and much of the
glaciated Midwest. The materials include aquitards
comprised of loess, fractured till, and fine grained
alluvial materials, and sand and gravel aquifers.
Selection of TDP eliminated the need for field acidification required by some other analytes for which
delays of even minutes can precipitate P, particularly
in the presence of oxidized forms of Fe. Analysis for
TP from unfiltered samples was discounted because
preliminary analyses showed concentrations
attributable to formation sediments from wells
completed in loess and till. Dissolved phosphorus is
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OCCURRENCE OF TOTAL DISSOLVED PHOSPHORUS IN UNCONSOLIDATED AQUIFERS AND AQUITARDS IN IOWA
most representative of P transported through ground
water below the water table, although colloidal P
could move through small fractures and interstices as
well. Samples were filtered in the field using in-line,
disposable 0.45 µm filters to achieve a uniform degree
of separation between TP and TDP (Eaton et al.,
1995). Analysis of deionized water passed through
these filters yielded no P.
The automated analytical method used for TDP
provides a detection limit of 20 µg/l. Water samples
were stored at 4˚C until digestion of 8 ml with 0.5 ml
of a 1:1 (vol/vol) solution of 7.2 N sulfuric acid and 1.4
M ammonia persulfate in an autoclave at 121˚C and
103 to 138 kPa for 30 minutes. Samples, calibration
standards, and reference materials were treated
equally. The digestion converts all forms of P to PO4-3
(USEPA, 1983) which was assayed by flow injection
analysis (Lachat, 2002). The PO4-3 complexes with
ammonium molybdate and antimony potassium tartrate under acidic conditions and is then reduced with
ascorbic acid. The resulting color absorbs light at
880 nm which is proportional to the concentration of
PO4-3 in the sample (Lachat, 2002).
recommended rates can produce a steady accumulation of P in the soil because P removed by harvesting
is only 16 kg/ha for soybean and 20 kg/ha for corn as
grain (Kellogg et al., 2000). In the Clear Lake area 70
percent of the surface soils in the watershed had high,
from 21 to 31 mg/kg, and very high or less than 31
mg/kg, Bray-P analyses (Iowa State University,
1999a; Klatt, 2002). Iowa State University does not
recommend further P fertilization on fields of this
type. The addition of manure P provides an additional
source for P leaching. Manure varies substantially
among livestock types, production systems and management systems, but in liquid form may contain 0.4
to 3.2 g/l P (Iowa State University, 1999b). Natural
sources of P may also exist in surficial materials in
these areas. Hallberg (1980) showed concentrations of
TP of 364 to 550 mg/kg in pre-Illinoian fractured till.
Paleosol material in southern Iowa showed values
between 104 and 196 mg/kg (Woida and Thompson,
1993).
FIELD AREAS
Four study areas were selected for sampling where
observation wells had been installed in a range of geologic materials and in settings where exchange
between ground and surface water was known. The
areas (Figure 1, Table 1) are underlain by loess, fractured till, fine-grained alluvium, and sand and gravel
which represent the dominant materials in Iowa that
store and transmit ground water under water table
conditions. Multiple phosphorus sources are associated with agricultural land use in ground water
recharge zones in all four areas. Crop or animal production or both dominate the land uses, although
some urban areas exist near the Clear Lake and Rock
Valley sites. Substrata of soils between 76 and 107 cm
below land surface in all areas have been classified as
having low values of Bray-Test P for crop production
(Iowa Department of Agriculture and Land Stewardship, 1996). The Bray test (Bray and Kurtz, 1945)
measures acid soluble forms of P. This depth of soil
sample is standard for determining substrata values
of available P. Classification of soils as low P means
these soils have less than 7.5 mg/kg or about 30 kg/ha
of Bray-test P. Annual P fertilizer recommendations
from Iowa State University (1999a) for these soils are
high, ranging from 19 kg/ha for soybean to 24 kg/ha
for corn. These fertilizer recommendations are double
for low surface soil P tests. Application of P at the low
JOURNAL OF THE AMERICAN WATER RESOURCES ASSOCIATION
Figure 1. Location of Areas Sampled for
Phosphorus in Ground Water.
The Deep Loess Research Station (DLRS)
The Deep Loess Research Station (DLRS) consists
of four small (30 to 40 ha) watersheds which are
underlain by 20 m to 60 m of loess (Prior, 1991) in two
units with similar hydraulic properties (Kramer et
al., 1999; Tomer and Burkart, 2003). Divides and
hill slopes are composed of the Peoria Loess of Wisconsinan age underlain by loess in the Loveland Formation of Illinoian age (Bettis, 1990). Valleys contain
alluvium of the Holocene DeForest Formation that is
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BURKART, SIMPKINS, MORROW, AND GANNON
TABLE 1. Summary of Total Dissolved Phosphorus in Ground Water From Selected Sites in Iowa.
Well
Depths
(m)
Number
of Wells
Minimum
10-7 to 10-5
4.5 to 20.7
13
ND*
0,640
125
050
2.1 to 2.8
Alluvium
10-8 to 10-4
3.3 to 4.50
26
ND*
1,030
077
030
1.6 to 3.2
Fractured Till
10-8 to 10-5
4.2 to 26.5
17
ND*
0,440
094
050
2.0 to 3.6
10-4
1.5 to 4.50
17
20
0,570
212
160
0.5 to 1.6
10-8 to 10-5
1.5 to 4.50
06
20
0,210
090
045
1.0 to 1.5
Walnut Creek Fractured Till
10-8 to 10-5
1.7 to 7.60
08
20
0,440
074
020
1.0 to 2.2
Rock Valley Alluvial Aquifer
10-4 to 10-3
7.5 to 150
16
20
0,260
093
080
3.6 to 6.1
Site and Material
Deep Loess Station
Loess
Clear Lake
Outwash
Fractured Till
TDP Concentrations
(µg/l)
Maximum
Mean
Median
Water
Table
Depth
(m)
Hydraulic
Conductivity
(m/s)
*ND is less than 20 µg/l (detection limit).
stratified and reworked loess. These materials are
underlain by pre-Illinoian weathered tills that form a
lower, no-flow boundary to local ground water flow
systems. More complete descriptions of the geology
and hydrology of this study area including analysis of
hydraulic conductivity values are presented in Tomer
and Burkart (2003) and Kramer et al. (1999). Fifty-six
wells, installed to study the general hydrology and
water quality associated with agricultural watersheds, were sampled for this study (Table 1). Wells
were installed using a hollow stem auger and till
wells were completed by reaming the screened interval with a flared Shelby tube. Screens of either 0.3 or
0.61 m were used with a gravel pack to 0.3 m above
the screen, and all wells were grouted to the surface.
the early 1970s to about 190 µg/l in 2000 (Downing
and Kopaska, 2001). It will become hypereutrophic, at
increased concentration of 4 µg/l/yr, with concentrations of 240 µg/l by 2040. Twenty-three wells in the
Clear Lake project were sampled (Table 1). Wells were
installed using a hollow stem auger with outwash
wells having a 0.61 m screen surrounded by a 0.91 m
long silica sand pack. Till wells were installed by
reaming the screened interval with a flared Shelby
tube and installing a 0.61 m screen and no sand pack.
Six outwash wells had hydraulic heads indicating
ground water recharge from the lake and 11 showed
ground water discharge to the lake.
Walnut Creek
Clear Lake
Walnut Creek drains a 5,600 ha watershed that is
underlain by late Wisconsinan fractured till (Alden
Member of the Dows Formation) and pre-Illinoian
fractured till of the Wolf Creek Formation. Low redox
potentials, dissolved Fe, and methane production
characterize most of the unweathered materials below
4.5 m in the watershed (Parkin and Simpkins, 1995).
Much of the watershed has poor natural drainage
that is augmented by extensive network of subsurface
tile drains. More complete descriptions of the geology
and hydrology of this study area including analysis of
hydraulic conductivity values are presented in Eidem
et al. (1999). Land use is typical of central Iowa with
more than 85 percent in a corn/soybean rotation.
Eight wells completed in the late Wisconsin till at
three nests in upland positions in the watershed were
sampled for this study (Table 1). One nest of wells is
Clear Lake covers 1,467 ha making it the third
largest of 34 natural lakes of glacial origin in Iowa,
and it is managed for recreation and fishing. It lies in
the Algona-Altamont moraine complex of the Des
Moines Lobe. The surficial materials are of late Wisconsinan age. More complete descriptions of the geology and hydrology of this study area including analysis
of hydraulic conductivity values are presented in
Simpkins et al. (2001). The lake’s maximum depth is
5.8 m with an average depth of 2.9 m. It drains an
area of 4,888 ha with slopes of 0 to 25 percent. Land
use in the watershed is dominated by corn, soybean,
and alfalfa crops with limited animal production. The
lake was formerly oligotrophic to mesotrophic, but has
increased in TP concentration from about 60 µg/l in
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An alluvial aquifer that serves the City of Rock
Valley’s well field consists of 9 to 15 m of stratified
sand and gravel in the Holocene DeForest Formation
overlain by finer grained recent floodplain deposits in
which soils have formed. The aquifer fills the floor of
a valley that is 850 to 2,100 m wide and incised into
pre-Illinoian fractured till. Rogg Creek flows through
the valley across the aquifer, losing discharge in the
vicinity of the wells sampled for this study. More complete descriptions of the geology and hydrology of this
study area including analysis of hydraulic conductivity values are presented in Thompson (1987). The land
use immediately surrounding the wells is approximately 50 percent urban and 50 percent rural. Essentially all of the rural land is devoted to row crop
production or row crop recently placed in the Conservation Reserve Program (CRP). Irrigated corn fields
and a cattle feedlot are located in the valley immediately upgradient of the sampled wells. Sixteen wells,
installed as part of a wellhead protection study by the
Iowa Geological Survey, were sampled for this study
(Table 1). A hollow stem auger was used to install
nests of wells at 7.5 m and 15 m deep. The formation
was allowed to collapse around 1.5 m long screens and
the annulus grouted from 0.3 m above the screen to
the land surface.
from Walnut Creek produced the smallest concentrations.
The range of TDP varied among the areas and
among aquitards and aquifers in the same area,
although the differences are dominantly in the maxima. The differences in maxima may be related to type
and duration of P sources, although it may be difficult
to distinguish between large sources or long term,
moderate sources. The only significant difference
occurs between Clear Lake and the other sites, which
raises questions about the role of ground water residence time verses sources in P transport and sorption.
Some of the ground water containing P near Clear
Lake is about 20 years old, based on 3H analyses
(Simpkins et al., 2001). Ground water flow paths may
introduce bias in sampling nonpoint source pollution.
Larger P concentrations may be more frequent where
ground water flow converges toward a single surface
feature, such as Clear Lake. In addition, sampling
ground water at the very end of the flow line at the
edge of the lake, as in this study, reflects all the
upgradient contributions of P in the watershed. These
and other explanations for the distribution of P concentrations in these areas are the subject of ongoing
research.
No significant differences with depth were
observed at any of the sites or for any materials,
although the Rock Valley aquifer samples were all
from the same depth. The lack of stratification or
other discernable difference related to well depth supports the conclusion that P concentrations are relatively uniformly distributed throughout these shallow
ground-water systems. Substantiation of such interpretations in many other areas will be critical to
understanding trends as they develop in vulnerable
areas.
RESULTS
DISCUSSION
Concentrations of TDP exceeding the laboratory
detection limit of 20 µg/l were found in all areas and
in all materials. Concentrations summarized in Table
1 included an assumed concentration of half the
detection limit (10 µg/l) for censored values (nondetects). A limited number of samples yielded concentrations less than the detection limit, but only from
the DLRS aquitards. Samples from the DLRS
aquitards also produced the largest concentration
(1,030 µg/l) of all samples. The samples from the outwash aquifer near Clear Lake contained significantly
larger concentrations (p < 0.05) than all other areas
and materials with a mean of 212 µg/l. Fractured till
Specific standards for ground water P have not
been established, but policy standards for P in
streams and lakes are currently being debated.
Threshold concentrations of 50, 76, and 100 µg/l TP
are emerging as important thresholds. The U.S. Environmental Protection Agency (USEPA) proposed that
TP not exceed 50 µg/l for water entering lakes and
reservoirs and 100 µg/l for streams not discharging to
lakes and reservoirs (USEPA, 1986). More recent
ambient water quality criteria (USEPA, 2000) establishes a threshold for TP of 76 µg/l in surface waters
in the Cornbelt and Northern Great Plains Ecoregion,
which includes Iowa. Aquatic ecological criteria
provide TP thresholds of 50 to 100 µg/l that may be
located down gradient of a pasture and another is
located down gradient of livestock pens that are periodically washed with water during cleaning. Wells
were installed with a hollow stem auger and completed with a 0.61 m screen with no sand pack after reaming the screened interval with a flared Shelby tube.
Alluvial Aquifer
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BURKART, SIMPKINS, MORROW, AND GANNON
useful to evaluate the effect of ground water discharge
to streams. Sustained TP loads needed to support
increased productivity in streams (Litke, 1999) will
require ground water discharge of P in Iowa and other
parts of the humid Midwest where ground water discharge dominates stream discharge during most periods. Chlorophyll concentrations of 10 µg/l are
suggested as the lower boundary for a proposed trophic state in temperate streams (Dodds et al., 1998) and
eutrophy in lakes (Smith, 2003). An empirical equation resulting from global analysis of temperate
streams (Van Niewenhuyse and Jones, 1996) was
used to determine that 50 µg/l TP correlated well with
concentrations of 10 µg/l chlorophyll in streams draining generally more than 6,000 km2 and 100 µg/l TP
for streams draining 50 km2. Another criterion for
estimating ecological thresholds of P uses the Redfield
ratio that showed algae to have an ideal elemental
composition of nitrogen that is 15 or 16 times that of
P (Redfield, 1958). Using this ratio, P concentrations
exceeding 65 µg/l will be sufficient to support algal
growth in many water bodies when nitrate-N concentrations exceed 10 mg/l, the drinking water maximum
contaminant level.
A substantial fraction of the TDP concentrations
analyzed for this study exceed values proposed as ecological or policy thresholds. The minimum threshold
proposed by USEPA for this region and the minimum
value to produce critical quantities of chlorophyll is 50
µg/l. This threshold is exceeded by the mean values at
all sites and in all materials (Figure 2). This threshold is exceeded by 50 percent of the samples from at
least one material in each site except the late Wisconsinan till at Walnut Creek. More than 25 percent of
all samples exceed 100 µg/l (Figure 2), the maximum
threshold being considered by the USEPA (2000).
Figure 2. Distribution of Total Dissolved Phosphorus Among
Selected Sites and Materials in Iowa. Stream threshold
values shown as dotted lines. Letters indicate
significant groupings (p < 0.05) of sample sets.
It is clear that ground water in these aquifers and
aquitards can discharge the P needed to produce
nutrient enriched streams. These and similar materials are capable of discharging sustained P concentrations needed to support increased productivity in
perennial streams and lakes. Furthermore, because
the reported concentrations are dissolved, substantial
portions, if not all, are readily available to support
algal growth. Consequently, it will be difficult for
many surface water bodies to meet standards for TDP
if they are established using thresholds that cause
nutrient enrichment.
It is likely that P will continue to enter surface
water from aquifers and aquitards for decades, particularly since it is present throughout shallow ground
water systems such as those studied here. Of particular interest for additional investigations should be
areas with:
CONCLUSIONS AND RECOMMENDATIONS
FOR FUTURE RESEARCH
High concentrations of TDP are likely to be present
in ground water in all near surface aquifers and
aquitards in areas of Iowa with intense agriculture.
Results in this study may be typical of many similar
agricultural systems in the Midwest. It is unlikely,
however, that the results presented here can be used
to define natural background concentrations of P
because of the long term source of agricultural P in all
areas sampled. Detailed analysis of sources and
transport processes in each area will be needed to
determine if the material characteristics, flow system,
or sources play a greater role in the observed occurrence of P in ground water.
JAWRA
1. Large and persistent sources of high soil concentrations of P, intensive animal production, and
substantial annual P fertilizer applications, particularly in aquifer recharge areas.
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JOURNAL OF THE AMERICAN WATER RESOURCES ASSOCIATION
OCCURRENCE OF TOTAL DISSOLVED PHOSPHORUS IN UNCONSOLIDATED AQUIFERS AND AQUITARDS IN IOWA
2. Intrinsic material characteristics that support
leaching and transport of P including low soil sorption
capacities (sandy and/or high organic matter soils)
and unconsolidated materials and bedrock with
extensive fractures or other macropores.
Haygarth, P.M., L. Hepworth, and S.C. Jarvis, 1998. Forms of Phosphorus Transfer in Hydrologic Pathways From Soil Under
Grazed Grassland. Eur. Jour. Soil Sci. 49:65-72.
Harman, J., W.D. Robertson, J.A. Cherry, and L. Zanini, 1996.
Impacts on a Sand Aquifer From an Old Septic System: Nitrate
and Pphosphate. Ground Water 34(6):1105-1114.
Helgesen, J.O. R.B. Zelt, and J.K. Stamer, 1994. Nitrogen and Phosphorus in Water as Related to Environmental Setting in Nebraska. Water Resour. Bull. 30(5):809-822.
Hooda, P.S., M. Moynagh, I.F. Svoboda, A.C. Edwards, H.A. Anderson, and G. Sym, 1999. Phosphorus Loss in Drainflow From
Intensively Managed Grassland Soils. Jour. Environ. Qual.
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Soil Properties and Interpretations Database: Documentation.
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Resources Conservation Service, and Iowa Department of Agriculture and Land Stewardship. Ames, Iowa. 28 pp. Available at
http://icss.agron.iastate.edu/ispaid60man.pdf. Accessed in February, 2003.
Iowa State University, 1999a. General Guide for Crop Nutrient
Recommendations in Iowa. PM 1688, Iowa State University
Extension. Ames, Iowa, 19 pp.
Iowa State University, 1999b. Managing Manure Nutrients for
Crop Production. PM 11811, Iowa State University Extension.
Ames, Iowa, 10 pp.
Kellogg, R.L., C.H. Lander, C.M. Moffitt, and N. Gollehon, 2000.
Manure Nutrients Relative to the Capacity of Cropland to
Assimilate Nutrients: Spatial and Temporal Trends for the
United States. U.S. Department of Agriculture, Natural
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Available
at
h t t p : / / w w w. n r c s . u s d a . g o v / t e c h n i c a l / l a n d / p u b s /
manntr.html. Accessed in February, 2003.
Klatt, J.G., 2002. Agronomic and Environmental Soil Phosphorus
Testing in Relation to Phosphorus Loss From Manured Field
Plots and an Impaired Watershed. M.S. Thesis, Iowa State University, Ames, Iowa, 68 pp.
Kolpin, D.W., K.E. Zichelle, and E.M. Thurman, 1996. Water Quality Data for Nutrients Pesticides, and Volatile Organic Compounds in Near-Surface Aquifers of the Midcontinental United
States 1992-1994. Open File Report No. 96-435, U.S. Geological
Survey, Iowa City, Iowa, 47 pp.
Kramer, L.A., M.R. Burkart, D.W. Meek, R.J. Jaquis, and D.E.
James, 1999. Field-Scale Watershed Evaluations on Deep-Loess
Soils: II. Hydrologic Responses to Different Agricultural Land
Management Systems. Jour. Soil and Water Conservation
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3. Hydrologic vulnerability such as shallow
aquifers and aquitards, recharge areas, artificial subsurface drainage, high water table, and short ground
water residence time
4. Ground water with low redox potentials where
Fe is mobile allowing P to remain in solution.
The presence of P in the ground water settings
studied for this paper may stimulate an interest in
more routine analysis and interpretations of this
nutrient in ground water monitoring and research
programs.
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