MEASUREMENT AND CHARACTERIZATION OF
LANDFILL GAS SURFACE EMISSIONS
AT LANDFILLS WITH SOIL COVERS
Jeffrey L. Pierce
SCS Energy
Long Beach, California
Alex Stege
SCS Engineers
Phoenix, Arizona
ABSTRACT
BACKGROUND
USEPA’s New Source Performance Standards for Municipal
Solid Waste Landfills (NSPS) prescribes a landfill surface
emissions monitoring methodology which relies on
identification of discrete exceedances of a 500 ppm methane
standard. The South Coast Air Quality Management District
(SCAQMD), which covers the three-county Los Angeles
region, requires application of a similar methodology.
SCAQMD also requires use of an integrated sample
monitoring methodology. The integrated sample monitoring
methodology involves collecting composite samples of gas in
bags along the surface of the landfill. The total organic
carbon content of any composite sample (as methane) cannot
exceed a limit of 50 ppm.
Anaerobic decomposition of organic material in municipal
solid waste landfills is known to produce substantial
quantities of landfill gas. Peak landfill gas production occurs
shortly after organic waste is buried. Landfill gas production
at a landfill with 5 million tons of waste in place will
typically range from one million standard cubic feet/day
(mmscfd) to five mmscfd. Landfill gas production varies
with climate (wet versus dry) and with waste age. Landfill
gas will continue to be produced long after waste is buried.
The landfill gas production half-life (number of years after
waste placement until landfill gas production drops to half of
its initial rate) ranges from 7 years to 30 years in wet versus
dry climates.
The paper will report on an analysis of landfill gas surface
emissions monitoring data collected at 25 southern California
landfills. The paper will also report on an analysis of the
concentration of 13 air toxics found in the integrated samples
from the surface of 13 landfills, air toxics concentrations in
ambient air samples upwind of the landfills (background
levels), and compare the background concentrations of air
toxics to landfill surface concentrations. The concentrations
of air toxics in raw landfill gas will be compared to the
concentrations found in the landfill surface samples to draw
inferences on the ability of soil covers to attenuate air toxics.
Landfill gas primarily consists of methane (55% to 60%) and
carbon dioxide (40% to 45%). When observed at landfill gas
extraction wells, flare stations and monitoring probes, the
landfill gas is diluted by air to various degrees. As a result,
the principal gases in landfill gas are normally considered to
be methane, carbon dioxide, nitrogen and oxygen. Landfill
gas also contains non-methane organic compounds
(NMOCs). The NMOC concentration in landfill gas is
typically in the range of 300 to 700 ppmv. The NMOCs
range from ethane, with two carbon atoms and the lowest
molecular weight, to much higher molecular weight organic
compounds (such as 1,1,1-trichlorothane).
The source of the data used in this study is quarterly
monitoring reports filed with SCAQMD by landfill owners.
Landfill gas has recently become the focus of increasing
scrutiny and control. Methane is 21 times more potent as a
greenhouse gas than carbon dioxide on a weight basis.
Methane emissions to the atmosphere are not currently
regulated, but it is expected that they will be regulated as a
component of international greenhouse gas control efforts.
The primary thrust of landfill gas emissions control under
USEPA’s New Source Performance Standards for Municipal
Solid Waste Landfills (NSPS-MSWL) is NMOC control. A
large fraction of the NMOCs found in landfill gas are volatile
organic compounds (VOCs). When VOCs combine with
nitrogen oxide in the atmosphere they will form ozone. A
much smaller fraction of the NMOCs are hazardous air
pollutants (HAPs). VOCs and HAPs emitted from the refuse
mass (through the landfill cover) are inventoried by landfill
owners during their preparation of Title V permit
applications. VOC and HAP emissions from the refuse mass
are also a factor in permitting new landfills or expanded
landfills, if a landfill is subject to New Source Review (NSR)
and/or Prevention of Significant Deterioration (PSD)
evaluations.
Regulators have taken the simplistic view that methane,
VOCs, and HAPs emissions at a landfill can be calculated
based solely on landfill gas generation, or from landfill gas
generation minus landfill gas collection/destruction (when a
flare is present). There are two flaws with this approach.
First, the possibility of the attenuation of emissions as landfill
gas passes through the cover is not considered. Second, there
is no reliable way to project landfill gas generation.
Regulators do at least acknowledge the first flaw. The
International Panel for Climate Change (IPCC) guidelines for
national greenhouse gas inventories1 sets forth a methodology
for estimating methane emissions from landfills. The
methodology incorporates an “oxidation factor” which:
“accounts for the CH4 that is oxidized in
the upper layers of the waste mass and in
cover material, where oxygen is present.
Because the default methodology relies on
an estimate of CH4 generation, it is
important to recognize that oxidation may
reduce the quantity of CH4 generated that
is ultimately emitted. A number of
researchers
are investigating and
quantifying the effects of CH4 oxidation in
waste disposal sites. However, as yet there
is no internationally accepted factor that
can be applied to account for CH4
oxidation. The CH4 oxidation factor in the
equation has therefore been set equal to 0,
pending the availability of new data. A
better understanding of the factors
influencing CH4 oxidization, and more
accurate quantification of it, may allow for
a revised oxidation factor (or default
values) in future editions of the IPCC
Guidelines. It is important that the
oxidation factor be applied after
subtraction of CH4 recovered, as this CH4,
is generally pulled from well below the
surface of the SWDS [Solid Waste
Disposal Site], before oxidation can
occur.”
USEPA has a methodology for estimating landfill gas
emissions. While USEPA acknowledges that a landfill’s
cover may play a role in attenuating landfill gas emissions,
USEPA is less sanguine than IPCC. When discussing the
ability of USEPA’s landfill air emissions model to estimate
uncontrolled landfill gas emissions, USEPA’s AP-422 states:
“It should be noted that the model above
was designed to estimate LFG generation
and not LFG emissions to the atmosphere.
Other fates may exist for the gas generated
in a landfill, including capture and
subsequent microbial degradation within
the landfill’s surface layer. Currently,
there are no data that adequately address
this fate. It is generally accepted that the
bulk of the gas generated will be emitted
through cracks or other openings in the
landfill surface.”
With regard to the second flaw, the reason why there is no
way to reliably estimate landfill gas generation is because
landfill gas generation models are calibrated based on
measured landfill gas recovery. Total landfill gas generation
cannot be measured (except possibly at landfills which are
both completely lined and capped with geomembranes). As a
result, it is necessary to make a baseless assumption on the
percentage of the landfill gas which is captured. USEPA’s
AP-42 makes such an assumption when it states:
“Gas collection systems are not 100
percent efficient in collecting landfill gas,
so emissions of CH4, and NMOC at a
landfill with a gas recovery system still
occur. To estimate controlled emissions of
CH4, NMOC, and other constituents in
landfill gas, the collection efficiency of the
system must first be estimated. Reported
collection efficiencies typically range from
60 to 85 percent, with an average of 75
percent most commonly assumed. Higher
collection efficiencies may be achieved at
some sites (i.e., those engineered to control
gas emissions). If site-specific collection
efficiencies are available (i.e., through a
comprehensive surface sampling program),
then they should be used instead of the 75
percent average.”
USEPA does not describe what they consider to be a
“comprehensive surface sampling program”; however, we do
know that USEPA has adopted a landfill surface emissions
monitoring methodology as an element of NSPS-MSWL.
One might conclude that the NSPS-MSWL methodology
would satisfy USEPA’s definition of a “comprehensive
surface sampling program.”
For the reasons outlined above, methane, VOCs and HAPs
emissions from landfills are becoming an increasing
environmental concern, and an increasing problem for
landfill owners. It appears that regulators are willing to
acknowledge that at least some attenuation of these pollutants
are occurring in the landfill cover. At this point in time,
however, it does not appear that a strong enough case has
been put forth to the regulators to consistently allow even a
token default attenuation to be applied during air emission
inventories and air emission projections. While regulators
must be and are inherently cautious in their assumptions, it is
very likely that air emissions from landfills are being
significantly overestimated. The task of permitting new
landfills and landfill expansions is more difficult when air
emissions are over estimated.
The paper which follows will (1) summarize surface
emissions monitoring data from southern California landfills;
(2) review the validity of surface emissions monitoring
methodologies employed by regulatory agencies; and (3)
employ the data to search for signs of confirmation of
widespread attenuation of landfill gas pollutants by landfill
soil covers.
surface emissions for over 20 years. Surface emissions
monitoring under NSPS-MSWL is only six years old.
NSPS-MSWL sets forth the following surface emissions
monitoring requirements:
1.
2.
3.
SCAQMD Rule 1150.1 includes the following requirements
for point source 500 ppmv monitoring (known to SCAQMD
as instantaneous surface monitoring):
1.
2.
CURRENT SURFACE EMISSIONS MONITORING
METHODOLOGY
3.
Regulators have developed two methodologies for
monitoring landfill surface emissions -- instantaneous (point
source) monitoring and integrated (composite) monitoring.
The methodology prescribed in NSPS-MSWL employs
instantaneous monitoring. SCAQMD's Rule 1150.1 requires
both instantaneous and integrated monitoring. SCAQMD has
required large landfills in Los Angeles, Orange, San
Bernardino, and Riverside Counties to monitor landfill
“After installation of a collection system,
the owner or operator is to monitor
surface concentrations of methane along
the entire perimeter of the collection area
and along a serpentine pattern spaced 30
meters apart (or a site-specific established
spacing) for each collection area on a
quarterly basis using an organic vapor
analyzer (OVA), flame ionization detector
(FID), or other acceptable portable
monitor. Steep slopes or other dangerous
areas can be excluded from monitoring.
The background concentration of methane
is to be determined by moving the probe
inlet upwind and downwind outside the
boundary of the landfill at a distance of at
least 30 meters from the perimeter wells.
Surface emission monitoring shall be
performed with the probe inlet placed
within 5 to 10 centimeters off the ground.
Monitoring is to be performed during
typical meteorological conditions.”
The entire landfill disposal area is to be
monitored. Any area of the landfill that
the Executive Officer deems as
inaccessible or dangerous for a technician
to enter may be excluded from the area to
be monitored by the landfill operator.
A portable FID shall be used to
instantaneously measure the concentration
of organic compounds (measured as
methane) at any location on the landfill.
The operator shall monitor the entire
landfill disposal area for methane using
the described portable equipment. The
sampling probe shall be placed at a
distance of 0 to 3 inches above any
location of the landfill to take the
readings. A 50,000 square-ft-grid shall be
used to develop a serpentine walk pattern.
The serpentine pattern will employ a
separation of 25 ft.
Rule 1150.1 includes the following requirements for
composite sampling (known to SCAQMD as integrated
surface sampling):


1.
2.





The number of samples, which must be
collected, depends on the area of the
landfill surface. The entire landfill
disposal area must be divided into
numbered 50,000-square-ft-grids. Any
area that the Executive Officer deems
inaccessible or dangerous for a technician
to enter may be excluded from the
sampling grids monitored by the landfill
operator.
Samples collected must meet the
following conditions:
Average wind speed must not exceed 5
miles per hour. Surface sampling must
be terminated when the average wind
speed exceeds 5 miles per hour or the
instantaneous wind speed exceeds 10
miles per hour.
Surface sampling shall be conducted
when the landfill is dry. The landfill is
considered dry when there has been no
measurable precipitation for the
preceding 72 hours prior to sampling.
An integrated surface sampler is to be
employed. It is a portable self-contained
unit with its own internal power source.
The integrated sampled consists of a
stainless steel collection probe, a
rotameter, a pump, and a 10-liter Tedlar
bag enclosed in a light-sealed cardboard
box.
The integrated surface sampler shall be
used to collect a surface sample of
approximately 8 to 10 liters from each
grid. During sampling, the probe shall
be placed 0 to 3 inches above the landfill
surface. The sampler shall be set at a
flow rate of approximately 333 cubic
centimeters per minute and the
technician shall walk through a course of
approximately 2,600 linear ft over a
continuous 25-minute period.
A
serpentine pattern separated by 25 ft
shall be walked through the 50,000square ft grid.”
The action level for remedial action
under the integrated monitoring rule is
50 ppmv.


There are two significant differences
between SCAQMD Rule 1150.1 and
NSPS-MSWL:
Rule 1150.1 requires both point source
(instantaneous)
and
composite
(integrated)
surface
emissions
monitoring. NSPS-MSWL requires only
point source monitoring; and
Rule 1150.1 requires a spacing of 25 ft
between the parallel sections of the
serpentine pattern used in instantaneous
monitoring. NSPS-MSWL allows a
spacing of 100 ft. The narrower distance
required by Rule 1150.1 makes Rule
1150.1 monitoring more thorough than
NSPS-MSWL monitoring.
Rule 1150.1 also requires that the
integrated sample from at least one grid,
and from all grids exceeding 50 ppmv,
be analyzed for the presence of what
SCAQMD considers to be “core group”
HAPs. The data collected under this
provision of Rule 1150.1 provides a
large body of near landfill surface HAPs
data.
SURFACE EMISSIONS MONITORING AS AN
INDICATOR OR LANDFILL GAS CAPTURE
The authors previously reported the results of a study3, which
evaluated the relationship between expected landfill gas
capture and measured surface emissions at 25 Southern
California landfills. The study was based on a review of
three years of SCAQMD quarterly monitoring reports (1994,
1995 and 1996). It focused only on total organic carbon
emissions (virtually all of which are methane), and did not
consider the concentrations of air toxics. The authors
concluded that instantaneous monitoring was a poor indicator
of overall landfill gas collection system performance, and that
integrated monitoring might be an acceptable indicator of
overall landfill gas collection system performance.
For each of the 25 landfills, a projection of landfill gas
generation was made using USEPA’s Landfill Air Emissions
Estimation Model and AP-42’s “old” default, dry-climate
modeling coefficients (Lo = 125 m3/Mg and k = 0.02).
Actual landfill gas recovery was divided by projected landfill
gas generation to establish estimated percentage landfill gas
recovery for each site. The average number of 500 ppmv
exceedances per quarter per average was determined for each
landfill based on 12 quarters of monitoring. Figure No. 1
plots the average number of 500 ppmv exceedances per
quarter against estimated percentage landfill gas recovery.
The intuitive relationship (decreasing number of exceedances
with increasing landfill gas capture) is not apparent. Two
landfills show landfill gas recoveries in excess of 100%. In
at least these two cases, the USEPA model is clearly under
projecting landfill gas generation.
The predictive capability of integrated monitoring was put to
a similar test. Only nine of the 25 landfills reported surface
emissions data for every grid for each sampling event. An
average surface emissions concentration for a landfill could
only be computed if data from the entire surface of the
landfill was available. Many landfills report only the
concentration of surface emissions for grids exceeding 50
ppmv in their formal filings to SCAQMD. The average
surface methane concentration for 12 quarters for the nine
landfills that had complete data was then determined and was
then plotted against percentage landfill gas recovery. Figure
No. 2 is a reproduction of that plot. Figure No. 2 does show
the intuitively expected trend -- average surface methane
concentration decreased with estimated increases in landfill
gas capture. As will be seen later in this paper, atmospheric
methane in Los Angeles is about 3 ppmv. For this reason, a
value of 3 ppmv represents zero landfill gas emissions.
A shortcoming common to both the instantaneous and the
integrated surface emissions monitoring methodologies is that
they do not measure the rate of surface emissions. The
methodologies do not consider the overall gas flow at the
surface of the landfill (outward or inward) and/or the effect of
atmospheric air dilution that occurs within 2 inches of the
surface. Some regulators view the number of exceedances of
the 500 ppmv standard as a surrogate for the quantity of
emissions. Obviously, it is a poor surrogate -- both in theory
and in practice. It is the authors’ opinion that instantaneous
monitoring is more an indicator of the need to make localized
cover repair than it is an indicator of overall air emissions.
The integrated monitoring methodology is a better indicator
of overall air emissions. It does attempt to establish an
average concentration of pollutants across the landfill
surface, and higher average integrated readings generally
indicate higher rates of air emissions. However, without
knowing the rate of gas flow through the cover, it is not
possible to calculate pollutant mass flow rate. One thing can
be said with some certainty, if the average integrated
concentration of the landfill surface is close to atmospheric,
then the mass flow of emissions from the landfill must be
close to zero.
Tables 1, 2, 3, 4 and 5 provide summaries of the data which
provides the basis for Figure Nos. 1 and 2.
DATABASE USED IN AIR TOXICS
INVESTIGATION
As mentioned above, all landfills regulated under Rule
1150.1 must file quarterly monitoring reports. Most landfills
undertake three monitoring events per quarter. Analyses are
performed on the raw landfill gas at the flare station; on
integrated surface gas samples from grids having more than
50 ppmv of methane or from random grids when no grid
exceeds 50 ppmv; and on the ambient air from permanent
stations “upwind” and “downwind” from the landfill.
SCAQMD does not input the data in these reports into a
database. SCAQMD reviews the reports and files them in a
library. In order to analyze the data, it was necessary to
arrange for access to the data and to then construct a
database. Access to the reports is available only through a
formal Public Records Access Request. SCS filed such a
request and extracted the desired information from the
reports through on-site review. The period covered by the
review was 1998 through mid-2000. Some quarterly reports
were missing from the library. All available information was
input into the database. Table No. 5 provides background
data on the 13 landfills.
Characteristics of Raw Landfill Gas
Table No. 6 summarizes the HAPs concentrations in the raw
landfill gas for the 13 landfills. The data is based on samples
collected at the landfill flare stations. The characteristics of
the landfill gas at these 13 sites are similar to what was
reported by Pierce and Stege4 for a group of 25 southern
California landfills for a different study period (1995 through
1998).
Characteristics of Landfill Surface Gas
Table No. 7 summarizes the average concentration of thirteen
HAPs and methane seen in the near landfill surface of the 13
landfills. All of the landfills have landfill gas collection
systems which provide fairly comprehensive coverage of the
refuse mass. Two methane values are shown on Table No. 7.
The first is the methane concentration from the integrated
samples (a few grids) and the second is the average methane
concentration over the surface of the landfill (all grids). The
former can sometimes be much higher than the latter since
grids over 50 ppmv receive preference for HAPs analysis. In
some cases, the latter is true because the landfill operator can
select any grid for HAPs analysis when there are no grids
reporting over 50 ppmv. He may select grids which have
methane concentrations below the average methane value for
the landfill.
Ambient Air Quality
Upon initial review of the data on Table No. 7, one is struck
by the very low, near surface concentrations of HAPs on the
surface of all of the landfills. The Los Angeles region is
densely populated and is highly industrialized. For these
reasons, the ambient air concentrations of background HAPs
concentrations may become an important consideration in the
interpretation of the surface emissions data. Table No. 8
summarizes “upwind” ambient air HAPs data for the 13
landfills. While “upwind” may not have always been
“upwind” during each sampling event, the “upwind” samples
were generally upwind and do provide a reasonable
approximation of the average “background” HAPs
concentrations in the vicinity of the landfill.
Bear in mind that the air toxics concentrations listed on Table
No. 7 are concentrations within two inches of the landfill
surface. The concentrations are very low, even prior to
movement away from the landfill surface, and ambient air
dilution.
PRIOR RESEARCH EFFORTS ON METHANE AND
NMOC ATTENUATION IN SOIL
A number of investigations have been undertaken which have
demonstrated the ability of soil to attenuate methane and
NMOCs. It was not the purpose of this paper to conduct a
literature review; however, the authors feel that a brief
summary of two of the more significant investigations would
be useful as an aid in interpreting the data reviewed herein.
Kjeldsen, et al.
Kjeldsen, et al. (1997)5 conducted laboratory batch
experiments on methane, benzene, toluene, trichloroethylene
(TCE) and 1,1,1-trichloroethane (TCA). Hand augered soil
samples were taken at different distances from the landfill,
and were placed in glass bottles along with various mixtures
of methane and NMOCs. The headspace concentration of
methane and NMOCs were analyzed over time. Soil samples
from locations close to the landfill (20 m to 100 m distant)
were employed because it was believed that the soils were
pre-acclimated to landfill gas. The tests were conducted at
two temperature (10oC and 25oC). Control runs were made in
parallel with the test runs. Control runs used soil which was
treated to arrest microbial activity. The purpose of the
control runs was to segregate the impact of nonmicrobial
processes (primary abiotic degradation and sorption) from
microbial activity.
Kjeldsen, et al. drew the following conclusions from the
batch tests:





The control runs showed little or no degradation of
methane, indicating that microbial activity was
responsible for the observed degradation;
Degradation seemed to follow zero order reaction
kinetics and was 3 to 4 times slower at 10oC than at
25oC. Head space oxygen was ample during all
test runs and oxygen deficiency was not a factor;
Longer initial lag phases and lower degradation
rates were seen in the soil samples further from the
landfill, particularly for the furtherest sample
where almost no underground landfill gas
migration had been observed. This again suggests
microbial activity, and also suggests the need for a
period of time to cultivate the microorganisms
suited for methane and NMOC oxidation;
High degradation rates for the two aromatic
hydrocarbons (toluene and benzene) were
observed. Slow degradation of TCE and TCA was
observed.
The degradation of TCE and TCA proceeded only
when methane was present. This suggests cometabolic degradation of TCE and TCA with
methane.
Kjeldsen, et al., possibly mindful of the potential criticism
that their experiments were based only on carefully
controlled small-scale laboratory experimentation, also
reported on their field investigations. Four flux chambers
and four sets of soil sampling probes were installed at
locations up to 100 m away from the landfill. Each set of
sampling probes were multi-depth probes and were installed
to a depth of 100 cm. As expected, the methane flux rates
observed in the flux chambers were inversely related to their
distance from the landfill (varying from >300 liters
CH4/m2/day to < 0.3 liters CH4/m2/day). The flux rates for
the proximate (near) flux chambers are extremely high -considering that they are located on soil adjacent to the
landfill and not on the landfill cover itself. Kjeldsen, et al.,
attribute this to high rates of underground landfill gas
migration. A thick clay cap had been installed at the landfill,
but a landfill gas collection system had not been installed.
The nested soil sampling probes show that methane and
carbon dioxide concentrations increase with depth and that
oxygen concentrations decrease with depth. One could
attribute these observations to either simple diffusion (as air
mixes with landfill gas) or to oxidation of methane.
Kjeldsen, et al., concluded that methane oxidation was
occurring because they saw some indication of decreasing
CH4/CO2 ratios as the depth decreased and as oxygen became
more available. While not mentioned by Kjeldsen, et al., it
can also be observed from the data reported in their paper
that the N2/O2 ratio increases from the atmospheric ratio of
3.8 to a ratio of over 40 with depth -- clearly indicating that
oxygen was being increasingly consumed as depth increased.
The ability of these specific soils to support microbiotic
degradation was proven in the laboratory. The oxygen
consumption, coupled with Kjeldsen, et al’s observation on
the change in the CH4/CO2 ratio, strongly supports the view
that microbiotic degradation was the major factor in the
decrease in methane concentration (i.e., the concentration
decrease was not due solely to air diffusion). Kjeldsen, et
al.’s soil gas profiles showed oxygen levels dropping from
20% to 1% at depths between 10 cm to 90 cm below the
surface. Deeper oxygen intrusions were seen at stations
distal (further) from the landfill (i.e., stations exposed to
lower methane mass loadings).
soil cover -- apparently due to the greater impact of the
landfill gas extraction well. Negative methane fluxes of
0.001 g/m2/day to 0.010 g/m2/day were reported. The flux
boxes were also used as field incubators. They were injected
(spiked) with methane to simulate initial methane
concentrations of up to 10%. The unspiked and the spiked
tests were employed to calculate CH4 oxidation rates as a
function of initial CH4 concentration. The range was 0.001
g/m2/day to 50 g/m2/day.
Bogner’s work showed that methane is oxidized in soil
covers and showed that the rate of oxidation increases in
response to methane concentration. The conclusion is that
landfill methane must be oxidized if it is present in the cover
soil and if oxygen is present.
Conclusions From Prior Research
The above summarized research allows one to draw several
conclusions:


Bogner, et al.

Bogner, in conjunction with various authors, has published
several articles on work undertaken to study methane
oxidation by landfill soil covers. Bogner, et al. (1997) 6
reported on research undertaken at the Mallard Lake Landfill
(DuPage County, IL). Flux box and soil gas concentration
profiles were employed in this investigation. The landfill had
an active landfill gas collection system in operation during
their field tests. Bogner, et al. concluded that a net negative
flux of methane was being seen at the landfill surface and that
atmospheric methane was being pulled in through the cover
and was being oxidized in the very top layers of the soil
cover. This observation was made at flux boxes both
proximate and distal to an active landfill gas extraction well.
This led to the counter-intuitive conclusion that a landfill
could be a methane sink rather than a methane source. This
phenomenon could occur at landfills where a properly
designed and operated landfill collection system is in place.
Absolute atmospheric methane uptake was observed to be
greater at the proximate flux box . The oxygen enriched zone
at the proximate location, extended completely through the

Microbial activity in landfill covers can destroy
methane, VOC and HAPs;
At landfills without active landfill gas control
systems, this impact might be minimal. The
oxygenated zone in the cover is probably relatively
shallow at such landfills. It is difficult for air
diffusion from the atmosphere to compete with
landfill gas driven outward from the landfill by
diffusion and by convective forces from within the
landfill;
At landfills with active landfill gas collection
systems, microbial activity may, however, have a
major impact on air emissions. The negative
convective force of these systems not only reduces
landfill gas flow outward through the cover, but
these systems also draw air into the cover and
provide widespread, relatively deep oxygenated
zones. The constraint of lack of oxygen seen at
sites without active landfill gas collection systems
is largely eliminated at landfills which have
moderately aggressively to aggressively operated
landfill gas collection systems. Methane, VOCs,
and HAPs which escape the convective impact of
the landfill gas collection system will have an
opportunity to be oxidized in the aerobic zone
created by these systems. The cover can act as a
“polishing” step and/or as a supplementary control
measure;
Not all NMOCs are biodegraded at the same rate.
The degradation of toluene and benzene (both


aromatic compounds) is much faster than for TCA
and TCE;
The rate of biodegradation is sensitive to
temperature. Changes in total destruction of
pollutants may be observed with season in regions
where there is a sharp difference between winter
and summer temperature. To some extent, this will
be mitigated by the heat generated by the landfill;
and
Rainfall may have a temporary impact on
biodegradation -- if it reduces inward air flux
through the cover. An offsetting impact to this
phenomena, if it occurs, may be improved landfill
gas capture by the landfill gas collection system.
The above conclusions are sweeping, far reaching, and of
great importance. They are, however, based on very limited
testing.
Several conclusions can be drawn from Table No. 9. The
lowest “dilution” factors were seen for 1,1,1-trichloroethane
(TCA) and 1,2-dichloroethane. At worst, these “dilution”
factors represent dilution alone (i.e., no microbiotic
degradation). It is interesting to note that TCA was shown by
Kjeldsen et al. to be a relatively slowly degrading compound.
Any “dilution” factors above 850 imply that other
mechanisms are operable in reducing the near surface HAPs
concentrations.
Benzene, toluene and vinyl chloride show the highest rate of
degradation. They show an order of magnitude reduction
greater than the above two compounds. One would expect
fairly high degradation rates for benzene and toluene based
on the work of Kjeldsen et al. The highest degradation rate
was seen for methane.
The relative degradability of the compounds appears to be as
follows:
RELATIONSHIP BETWEEN RAW LANDFILL GAS
AND SURFACE GAS FOUND IN THIS STUDY


If an inert substance was present in the raw landfill gas,
which was analyzed in both the raw landfill gas and the
landfill surface gas, and this substance was not in the
atmosphere (or was a constant value), then it would be
possible to directly calculate the impact of air dilution on the
surface gas. It would be possible to segregate air dilution
from other mechanisms. Unfortunately, such a tracer
substance is not available in the available data.
It is therefore necessary to adopt another tactical approach.
Kjeldsen et al.’s finding that the rate of biodegradation of
individual compounds can greatly vary offers an alternative
approach. The hypothesis is that the observed “dilution”
factors of individual organic compounds (as observed
between the raw landfill gas and the surface gas) will vary if
a mechanism other than dilution was operable. One might
take this a step further, and infer a relative degradability
between the compounds based on this approach.
Table No. 9 summarizes the average “dilution” factors seen
at the 13 landfills for 13 compounds plus methane. The
methodology was to take, on a compound-by-compound
basis, the compound’s landfill surface concentration data,
then subtract the corresponding ambient air concentration
data, and then divide by the corresponding raw landfill gas
concentrations.

Most degradable: Methane, toluene, benzene, and
vinyl chloride;
Moderately degradable: perchloroethylene, 1,1dichlorethane, trichloroethene, dichloromethane,
and xylene.
Least degradable: 1,2-dichloroethane, 1,1,1trichloroethane (TCA), 1,1-dichloroethylene,
chlorobenzene, and dichlorobenzene.
CONCLUSIONS
Exceedances of USEPA's and SCAQMD's 500 ppm emission
standard commonly occur at southern California landfills.
The number of 500 ppm exceedances do not appear to
correlate well with overall landfill gas capture. For this
reason, the 500 ppm surface emissions monitoring
methodology is a poor measure of overall landfill gas
capture.
A correlation between the results of the integrated sample
monitoring methodology and overall landfill gas capture
appears to exist, suggesting that this tool could be used to
measure landfill gas collection system performance.
A comprehensive review of surface emissions, ambient air
and raw landfill gas analytical data from 13 Southern
California landfills was undertaken. A database was created
which includes analytical determinations for 13 HAPs plus
methane at these 13 landfills. There are over 13,000 HAPs
entries, in addition to 16,000 methane entries, in this
database. An analysis of this data implies that factors other
than air dilution are operable in reducing landfill gas air
pollutants. The most likely factor, based on very sitespecific, small area studies, and/or bench-scale studies by
other researchers, is microbial degradation of the compounds.
The findings herein suggest that microbial degradation of
methane and HAPs in landfill soil covers is a common
phenomenon, and confirms the widespread applicability of
observations by other researchers.
The above conclusion is offered with two caveats. Most of
the landfills addressed in this investigation do not have final
covers. Performance may have been better had all of the
landfills had better cover. The landfills were all in a dry
climate and the landfill gas collection systems at these
landfills were, in general, being aggressively operated (i.e.,
high levels of air infiltration were being maintained).
Landfills in other climates, and with a different philosophy of
landfill gas collection system operation, may experience less
dramatic performance in pollutant attenuation.
REFERENCES
(1)
(2)
(3)
(4)
(5)
(6)
”Revised 1996 IPCC Guidelines for National
Greenhouse Gas Inventories; Chapter 6; Waste.”
September, 1996. Intergovernmental Panel on
Climate Change (IPCC).
USEPA. 1997. “Compilation of Air Pollutant
Emission Factors; AP-42; Volume 1; Stationary
Sources.”
Pierce and Stege. October, 1997. “Landfill Surface
Emissions Monitoring Under NSPS: An
Adequate Measure of Landfill Gas Collection
System Performance?” SWANA’s 35th Annual
International Solid Waste Exposition.
Pierce and Stege. March, 1999. “Destruction of
Landfill Gas Toxics in Conventional Enclosed
Flares.”
Peter Kjeldsen, Anne Dalager and Kim Broholm.
December, 1997. “Attenuation of Methane and
Non-Methane Organic Compounds in Landfill
Gas Affected Soils.” Journal of the Air & Waste
Management Association.
Bogner, Spokas and Burton. September, 1997.
“Kinetics of Methane Oxidation in a Landfill
Cover Soil: Temporal Variations, A WholeLandfill Oxidation Experiment, and Modeling of
Net CH4 Emissions.” Environmental Science and
Technology.
FIGURE 1.
500 PPMV EXCEEDANCES VERSUS ESTIMATED LFG RECOVERY
Average Number of Exceedances per Acre
per Quarter
0.60
0.50
0.40
0.30
0.20
0.10
0.00
0%
20%
40%
60%
80%
100%
120%
Estimated Percent LFG Recovery
FIGURE 2.
AVERAGE METHANE CONCENTRATION OF INTEGRATED SAMPLES
VERSUS LFG RECOVERY
Average Methane Concentration of
Integrated Samples (ppmv)
25
20
15
10
5
0
0%
20%
40%
60%
80%
Estimated Percent LFG Recovery
100%
120%
TABLE 1
LANDFILL GAS GENERATION PROJECTIONS
VERSUS ACTUAL LANDFILL GAS RECOVERY
TWENTY-FIVE SOUTHERN CALIFORNIA LANDFILLS
Site
Identifier
1
2
3
4
5
6
7
8
9
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
Waste
In-Place
Average
Waste
(106 tons) Age (yrs)
13.91
14.5
7.99
8.9
11.08
3.1
16.00
26.0
18.51
5.2
16.69
10.0
15.80
23.2
18.63
17.5
27.07
5.1
6.49
12.5
6.41
12.9
14.99
16.7
2.09
7.2
3.40
4.7
10.82
7.9
29.92
29.3
31.34
16.7
20.35
14.0
7.99
14.0
9.00
23.0
2.22
9.0
3.30
21.0
3.00
8.2
2.10
19.0
1.05
18.0
Years
Since
Closure
4
Active
Active
15
Active
1
12
10
1
Active
Active
Active
Active
Active
1
11
7
Active
Active
14
Active
11
Active
11
13
LFG Generation
Old AP-42 New AP-42
(cfm)
3,213
1,666
2,635
3,084
4,514
4,181
3,059
4,028
7,454
1,989
1,400
4,858
516
1,104
3,430
5,138
6,894
4,875
1,800
1,763
573
669
725
440
223
(cfm)
2,570
1,333
2,108
2,467
3,611
3,345
2,447
3,222
5,963
1,591
1,120
3,886
413
883
2,744
4,110
5,515
3,900
1,440
1,410
458
535
580
352
178
Actual LFG
Recovery
Percent LFG Recovery
(cfm)
Old AP-42 New AP-42
1,883
59%
73%
1,124
67%
84%
1,000
38%
47%
1,855
60%
75%
1,525
34%
42%
4,540
109%
136%
2,550
83%
104%
3,860
96%
120%
7,750
104%
130%
1,150
58%
72%
1,314
94%
117%
3,037
63%
78%
0
0%
0%
300
27%
34%
2,411
70%
88%
2,400
47%
58%
4,130
60%
75%
3,396
70%
87%
1,400
78%
97%
1,700
96%
121%
80
14%
17%
400
60%
75%
183
25%
32%
70
16%
20%
215
97%
121%
TABLE 2
SUMMARY OF LANDFILL CHARACTERISTICS
TWENTY-FIVE SOUTHERN CALIFORNIA LANDFILLS
Site
Identifier
1
2
3
4
5
6
7
8
9
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
Waste
In-Place
Landfill
(106 tons) Acreage
13.91
115
7.99
80
11.08
148
16.00
145
18.51
177
16.69
146
15.80
90
18.63
178
27.07
162
6.49
94
6.41
127
14.99
190
2.09
43
3.40
50
10.82
100
29.92
145
31.34
300
20.35
170
7.99
136
9.00
70
2.22
80
3.30
60
3.00
30
2.10
30
1.05
50
Years
Since
Landfill
Agency
Closure
4
Active
Active
15
Active
1
12
10
1
Active
Active
Active
Active
Active
1
11
7
Active
Active
14
Active
11
Active
11
13
Type
Canyon/Mound
Canyon
Canyon
Canyon/Mound
Canyon/Mound
Canyon
Canyon
Mound
Mound
Canyon/Mound
Mound
Mound
Canyon/Mound
Canyon
Mound
Mound
Canyon
Canyon
Canyon
Mound
Mound
Mound
Canyon
Canyon
Mound
Monitoring
Pub. - Consult.
Pub. - Consult.
Pub. - Consult.
Pub. - Consult.
Pub. - Consult.
Pub. - Self
Pub. - Self
Priv.- Self
Priv.- Self
Pub. - Consult.
Pub. - Consult.
Pub. - Consult.
Pub. - Consult.
Priv.- Consult.
Pub. - Self
Priv.- Self
Priv.- Self
Priv.- Self
Priv.- Self
Priv.- Consult.
Priv.- Self
Pub. - Self
Pub. - Self
Pub. - Self
Pub. - Self
System
Coverage
Percent
LFG
(%)
Recovery
65%
59%
73%
67%
75%
38%
65%
60%
65%
34%
100%
109%
90%
83%
100%
96%
100%
104%
65%
58%
75%
94%
100%
63%
0%
0%
50%
27%
90%
70%
60%
47%
90%
60%
95%
70%
85%
78%
95%
96%
35%
14%
90%
60%
40%
25%
15%
16%
75%
97%
TABLE 3
SUMMARY OF INSTANTANEOUS EMISSIONS MONITORING
TWENTY-FIVE SOUTHERN CALIFORNIA LANDFILLS
Site
Identifier
17
16
9
18
8
5
6
4
7
12
1
3
15
20
19
2
10
11
14
22
23
21
24
13
25
Waste
In-Place
6
Landfill
Percent
LFG
(10 tons) Acreage Recovery
31.34
300
60%
29.92
145
47%
27.07
162
104%
20.35
170
70%
18.63
178
96%
18.51
177
34%
16.69
146
109%
16.00
145
60%
15.80
90
83%
14.99
190
63%
13.91
115
59%
11.08
148
38%
10.82
100
70%
9.00
70
96%
7.99
136
78%
7.99
80
67%
6.49
94
58%
6.41
127
94%
3.40
50
27%
3.30
60
60%
3.00
30
25%
2.22
80
14%
2.10
30
16%
2.09
43
0%
1.05
50
97%
500 ppmv
Exceedances
500 ppmv
Exceedances
Per Quarter
500 ppmv
Exceedances
Per Quarter
Per Quarter
2
43
34
11
20
9
46
6
46
49
20
36
18
2
3
21
25
23
14
0
0
0
0
13
0
Per Acre
0.01
0.30
0.21
0.06
0.11
0.05
0.31
0.04
0.51
0.26
0.17
0.24
0.18
0.02
0.03
0.26
0.26
0.18
0.27
0.00
0.00
0.00
0.00
0.31
0.00
Per 106 Tons
0.07
1.44
1.26
0.52
1.06
0.48
2.73
0.36
2.89
3.24
1.44
3.26
1.62
0.17
0.43
2.59
3.83
3.58
3.97
0.04
0.00
0.00
0.00
6.41
0.00
TABLE 4
SUMMARY OF INTEGRATED EMISSIONS MONITORING
NINE SOUTHERN CALIFORNIA LANDFILLS
Site
Identifier
6
7
8
9
10
11
12
16
18
Waste
In-Place
Percent
LFG
(106 tons)
Recovery
16.69
109%
15.80
83%
18.63
96%
27.07
104%
6.49
58%
6.41
94%
14.99
63%
29.92
47%
20.35
70%
Methane
Concentration
500 ppmv
Exceedances
Per Quarter
(ppmv)
6.84
10.33
7.66
11.15
17.68
10.17
21.55
16.85
7.77
Per Acre
0.31
0.51
0.11
0.21
0.26
0.18
0.26
0.30
0.06
TABLE 5.
BACKGROUND INFORMATION ON THE THIRTEEN LANDFILLS
COVERED BY THE AIR TOXICS INVESTIGATION
Approximate
Waste Tonnage
Average Waste
Age (Years)
Status
Annual
Rainfall
Landfill No. 1
16,000,000
15
Closed without final cover
16”
Landfill No. 2
24,000,000
19
Active
16”
Landfill No. 3
45,700,000
13
Closed with final cover
16”
Landfill No. 4
31,000,000
13
Active
16”
Landfill No. 5
7,900,000
17
Closed with final cover
16”
Landfill No. 6
13,000,000
10
Active
16”
Landfill No. 7
24,000,000
17
Active
14”
Landfill No. 8
2,500,000
9
Active
14”
Landfill No. 9
15,000,000
16
Active
16”
Landfill No. 10
7,000,000
14
Active
16”
Landfill No. 11
7,000,000
15
Active
14”
Landfill No. 12
7,000,000
6
Active
11”
Landfill No. 13
20,000,000
16
Active
17”
Landfill
TABLE 6.
CHARACTERISTICS OF RAW LANDFILL GAS1
Compounds in LFG
Benzene
Chlorobenzene
Dichlorobenzene
1,1-Dichloroethane
1,2-Dichloroethane
1,1-Dichloroethylene
Dichloromethane
Perchloroethylene
Toluene
1,1,1-Trichloroethane
Trichloroethene
Vinyl Chloride
Xylene
Methane (ppm)
Average2
Maximum2
Minimum2
3,639
597
1,842
1,684
250
169
6,618
1,974
33,896
183
903
1,710
20,228
326,699
11,787
4,102
5,682
5,100
1,547
989
26,650
5,160
68,809
412
2,431
13,455
46,380
438,850
678
74
308
353
23
33
193
702
6,169
50
295
131
7,166
223,000
NOTES:
1) All values are in ppb, except methane, which is in ppm. Based on LFG samples
collected during 245 sampling events. Detection limit values were used for
compounds reported to be present below the detection limit (ND), unless all samples
from a collection event were ND for that compound.
2) Reported values are the average, minimum, and maximum of the average values for
each of 13 landfills.
TABLE 7.
CHARACTERISTICS OF LANDFILL SURFACE GAS
Compounds in LFG
Landfill #1 Landfill #2 Landfill #3 Landfill #4 Landfill #5 Landfill #6 Landfill #7
Benzene
Chlorobenzene
Dichlorobenzene
1,1-Dichloroethane
1,2-Dichloroethane
1,1-Dichloroethylene
Dichloromethane
Perchloroethylene
Toluene
1,1,1-Trichloroethane
Trichloroethene
Vinyl Chloride
Xylene
Methane (ppm)
1.05
0.10
0.97
0.05
0.20
0.03
1.02
0.32
3.09
0.18
0.23
0.05
2.35
6.36
1.03
0.06
0.66
0.05
0.33
0.04
0.50
0.18
2.48
0.18
0.06
0.06
1.70
3.18
0.45
3.45
1.40
0.36
1.41
0.27
0.41
1.16
6.95
0.14
0.38
0.71
5.10
5.22
2.23
0.42
4.61
0.44
0.18
0.10
1.97
1.36
18.88
0.27
0.40
0.10
20.84
20.93
2.05
0.55
3.68
0.23
0.14
0.10
0.49
0.30
12.42
0.44
0.14
0.10
16.77
4.18
0.95
0.18
1.10
0.27
0.15
0.10
1.82
0.24
7.45
1.19
0.20
0.18
4.69
9.71
0.68
0.11
1.11
0.25
0.15
0.10
0.93
2.04
2.19
0.49
1.56
0.18
1.01
5.50
Methane - average of field
measured grids (ppm)
5.26
2.91
6.71
13.30
6.33
6.60
6.08
NOTES:
All values are in ppb, except methane, which is in ppm. Results shown are average concentrations of compounds from
integrated surface samples collected from each site.
Analysis includes a total of 326 surface samples analyzed in laboratories, and field-measured methane readings from 15,330
grids on 137 days of sampling. Detection limit values were used for compounds reported to be present below the detection
limit. Values shown in bold include at least 50% of samples with compounds below the detection limits.
TABLE 7.
CHARACTERISTICS OF LANDFILL SURFACE GAS (cont.)
Compounds in LFG
Average of
Landfill #8 Landfill #9 Landfill #10 Landfill #11 Landfill #12 Landfill #13 13 Sites
Benzene
Chlorobenzene
Dichlorobenzene
1,1-Dichloroethane
1,2-Dichloroethane
1,1-Dichloroethylene
Dichloromethane
Perchloroethylene
Toluene
1,1,1-Trichloroethane
Trichloroethene
Vinyl Chloride
Xylene
Methane (ppm)
0.83
1.17
1.10
0.30
0.30
0.10
0.94
0.61
4.79
0.22
0.26
0.10
2.82
3.29
0.71
0.12
1.10
0.28
0.16
0.10
0.56
0.33
3.79
0.18
0.18
0.10
1.79
3.70
0.45
0.11
1.10
0.28
0.16
0.10
0.46
0.24
2.77
0.18
0.16
0.10
1.31
4.76
0.67
0.13
1.11
0.26
0.15
0.10
0.69
0.40
3.30
0.18
0.18
0.10
1.64
5.18
0.71
0.13
1.26
0.98
0.23
0.13
12.49
1.00
7.38
0.64
0.60
0.28
3.71
26.35
0.88
0.06
0.67
0.05
0.28
0.04
0.52
0.16
1.81
0.20
0.11
0.05
1.18
4.63
0.98
0.51
1.53
0.29
0.30
0.10
1.75
0.64
5.95
0.35
0.34
0.16
4.99
7.92
Methane - average of field
measured grids (ppm)
5.91
6.34
6.13
5.58
7.36
4.54
6.39
NOTES: All values are in ppb, except methane, which is in ppm. Results shown are average concentrations of compounds from
Integrated Surface samples collected from each site.
Analysis includes a total of 326 surface samples analyzed in laboratories, and field-measured methane readings from 15,330
grids on 137 days of sampling. Detection limit values were used for compounds reported to be present below the detection
limit. Values shown in bold include at least 50% of samples with compounds below the detection limits. Average values are in
bold if a majority of the sites included mostly non-detect values for the corresponding compound.
TABLE 8.
AMBIENT AIR QUALITY
Compounds in LFG
Benzene
Chlorobenzene
Dichlorobenzene
1,1-Dichloroethane
1,2-Dichloroethane
1,1-Dichloroethylene
Dichloromethane
Perchloroethylene
Toluene
1,1,1-Trichloroethane
Trichloroethene
Vinyl Chloride
Xylene
Methane (ppm)
NOTES:
Landfill #1 Landfill #2 Landfill #3 Landfill #4 Landfill #5 Landfill #6 Landfill #7
1.22
0.08
0.70
0.04
0.20
0.03
0.70
0.30
3.31
0.15
0.05
0.04
2.43
3.38
1.06
0.08
0.49
0.05
0.31
0.03
0.45
0.21
1.80
0.10
0.05
0.05
1.11
2.51
1.62
0.40
0.65
0.18
0.18
0.18
1.57
0.23
2.07
0.41
0.48
1.53
1.75
2.39
0.70
0.10
1.10
0.25
0.15
0.10
0.67
0.32
2.88
0.19
0.22
0.10
1.85
3.07
0.52
0.10
1.10
0.20
0.13
0.10
0.58
0.20
2.21
0.16
0.12
0.10
1.47
2.60
0.74
0.10
1.10
0.22
0.14
0.10
0.18
0.30
5.96
0.16
0.19
0.10
6.64
2.56
0.59
0.10
1.10
0.19
0.13
0.10
0.28
0.22
1.88
0.17
0.14
0.10
1.37
2.49
All values are in ppb, except methane, which is in ppm. Results shown are average concentrations of compounds from ambient
air samples collected from locations upwind of each site. Analysis includes a total of 181 ambient air samples. Detection limit
values were used for compounds reported to be present below the detection limit. Values shown in bold include at least 50%
of samples with compounds below the detection limits.
TABLE 8.
AMBIENT AIR QUALITY (cont.)
Compounds in LFG
Benzene
Chlorobenzene
Dichlorobenzene
1,1-Dichloroethane
1,2-Dichloroethane
1,1-Dichloroethylene
Dichloromethane
Perchloroethylene
Toluene
1,1,1-Trichloroethane
Trichloroethene
Vinyl Chloride
Xylene
Methane (ppm)
Average of
Landfill #8 Landfill #9 Landfill #10 Landfill #11 Landfill #12 Landfill #13 13 Sites
0.46
0.12
1.32
0.27
0.16
0.12
0.28
0.25
3.09
0.16
0.23
0.10
3.04
2.31
0.49
0.10
1.10
0.21
0.14
0.10
0.47
0.38
2.54
0.18
0.17
0.10
1.55
2.40
0.46
0.10
1.10
0.21
0.14
0.10
0.37
2.89
2.33
0.16
0.20
0.10
1.70
2.62
0.53
0.11
1.10
0.21
0.14
0.10
0.41
0.44
2.80
0.17
0.22
0.10
1.72
2.51
0.47
0.39
1.37
0.38
0.38
0.39
3.88
0.69
4.40
0.42
0.46
0.38
4.61
2.71
0.89
0.06
0.60
0.05
0.30
0.04
0.40
0.12
1.28
0.13
0.06
0.06
0.80
3.57
0.75
0.14
0.99
0.19
0.19
0.11
0.79
0.51
2.81
0.20
0.20
0.22
2.31
2.70
NOTES: All values are in ppb, except methane, which is in ppm. Results shown are average concentrations of compounds from ambient
air samples collected from locations upwind of each site. Analysis includes a total of 181 ambient air samples. Detection limit
values were used for compounds reported to be present below the detection limit. Values shown in bold include at least 50% of
samples with compounds below the detection limits. Average values are in bold if a majority of the sites included mostly nondetect values for the corresponding compound.
TABLE 9.
RELATIONSHIP BETWEEN RAW LANDFILL GAS
AND SURFACE GAS1
A
B
C
D
E
F
Compounds in LFG
Average
Integrated
Surface
Emissions2
Average
Ambient Air
Quality2
Average of
Integrated
Minus
Ambient3
Average of
Raw LFG2
Dilution:
Ratio of
LFG to
Surface4
Benzene
Chlorobenzene
Dichlorobenzene
1,1-Dichloroethane
1,2-Dichloroethane
1,1-Dichloroethylene
Dichloromethane
Perchloroethylene
Toluene
1,1,1-Trichloroethane
Trichloroethene
Vinyl Chloride
Xylene
Methane (ppm)
0.98
0.51
1.53
0.29
0.30
0.10
1.75
0.64
5.95
0.35
0.34
0.16
4.99
7.92
0.75
0.14
0.99
0.19
0.19
0.11
0.79
0.51
2.81
0.20
0.20
0.22
2.31
2.70
0.43
0.51
1.53
0.29
0.30
0.10
1.16
0.49
3.25
0.21
0.28
0.16
3.19
5.22
3,639
597
1,842
1,684
250
169
6,618
1,974
33,896
183
903
1,710
20,228
326,699
8,371
1,177
1,206
5,762
846
1,672
5,714
4,056
10,430
852
3,250
10,468
6,350
62,588
Methane - average of field
measured grids (ppm)
6.39
2.70
3.69
326,699
88,630
NOTES:
1) All values are in ppb, except methane, which is in ppm.
2) Reported values are taken from Tables 2 - 5, and are the average of the 13 sites' average values for integrated
surface emissions, ambient air quality, and raw LFG. Values shown in bold indicate that a majority of sites'
averages included at least 50% non-detect (ND) values.
3) Equals the average of each of 13 sites' individually-calculated values for average integrated surface emissions
minus average ambient air quality. For this reason, Column D values do not necessarily equal Column B
values minus Column C values. For compounds with at least 50% ND values in ambient air samples
(indicated in bold in Table 4), the ambient air value for that site was not subtracted from the integrated surface
emission value. For compounds with average ambient air values exceeding the average integrated surface
values, a value of 0 was entered.
4) Equals the "average of raw LFG" value shown in this table divided by the "average of integrated minus
ambient" value shown in this table.