Environ. Sci. Technol. 2011, 45, 313–319
Landfill Methane Oxidation Across Climate Types in the U.S. J E F F R E Y C H A N T O N , * ,† T A R E K A B I C H O U , ‡ CLAIRE LANGFORD,† GARY HATER,§ ROGER GREEN,§ DOUG GOLDSMITH,| AND NATHAN SWAN⊥ Earth, Ocean and Atmospheric Science, Florida State University, Tallahassee, Florida, United States, Department of Civil and Environmental Engineering, FAMU-FSU College of Engineering, 2525 Pottsdamer Street, Tallahassee, Florida, United States, Waste Management, 2956 Montana Ave. Cincinnati, Ohio, 45211, United States, Alternative Natural Technologies, Inc., 1847 Whittaker Hollow Road, Blacksburg, Virginia 24060-1076, United States, and Cygnus Environmental Group, 1944 Roanoke Ave., Louisville, Kentucky 41205, United States
Received June 4, 2010. Revised manuscript received October 26, 2010. Accepted November 14, 2010.
Methane oxidation in landfill covers was determined by stable isotope analyses over 37 seasonal sampling events at 20 landfills with intermediate covers over four years. Values were calculated two ways: by assuming no isotopic fractionation during gas transport, which produces a conservative or minimum estimate, and by assuming limited isotopic fractionation with gas transport producing a higher estimate. Thus bracketed, the best assessment of mean oxidation within the soil covers from chamber captured emitted CH4 was 37.5 ( 3.5%. The fraction of CH4 oxidized refers to the fraction of CH4 delivered to the base of the cover that was oxidized to CO2 and partitioned to microbial biomass instead of being emitted to the atmosphere as CH4 expressed as a percentage. Air samples were also collected at the surface of the landfill, and represent CH4 from soil, from leaking infrastructure, and from cover defects. A similar assessment of this data set yields 36.1 ( 7.2% oxidation. Landfills in five climate types were investigated. The fraction oxidized in arid sites was significantly greater than oxidation in mediterranean sites, or cool and warm continental sites. Sub tropical sites had significantly lower CH4 oxidation than the other types of sites. This relationship may be explained by the observed inverse relationship between cover loading and fractional CH4 oxidation.
1. Introduction Landfills are responsible for 1-2% of global overall greenhouse gas emissions (1) and are the second largest anthropogenic CH4 source in the United States, following ruminants (2). Landfills are point sources of CH4 to the atmosphere and as such they make good targets for mitigation. At landfills without gas collection systems, which are generally smaller and older facilities, the CH4 generated transits the soil cover where it maybe be partly oxidized by CH4-oxidizing bacteria * Corresponding author e-mail:
[email protected]. † Earth, Ocean and Atmospheric Science, Florida State University. ‡ Department of Civil and Environmental Engineering, FAMUFSU College of Engineering. § Waste Management. | Alternative Natural Technologies, Inc. ⊥ Cygnus Environmental Group. 10.1021/es101915r
2011 American Chemical Society
Published on Web 12/06/2010
(3, 4). Soil CH4 and higher hydrocarbon oxidation can be enhanced by cover treatments that increase oxygen transmisivity, pore space, vegetative growth, and water retention (5-10). If passive vents are present at these sites they can be treated with biofilters (11, 12). At more modern and larger landfills, gas collection reduces CH4 emissions. However, some portion of the CH4 generated escapes through the cover and via leaks in the gas collection system. Lately, the approach of enhancing the response of soil CH4-oxidizing bacteria to attenuate CH4 escape has received attention (7, 13-16). Landfill cover CH4 oxidation remains an under-constrained value in landfill CH4 budgets. Regulatory agencies currently credit landfill cover CH4 oxidation with 0-10% consumption of the CH4 transiting the soil-oxidized layer (17, 18). A recent literature review of landfill cover CH4 oxidation (19) reported a mean value of 35 ( 6%. In an effort to further constrain the quantity of CH4 oxidized we undertook a field program to measure CH4 oxidation in a series of landfills across the United States. The landfills were similar in that they were dominated by intermediate cover; they all had gas extraction systems, and they were all operated by a single company. Our overall campaign strategy was to refine landfill CH4 budgets, in a three-pronged approach. At each landfill we determined CH4 emissions with the OTM10 (other test method) technique (20-24), and a gridded geospatial static chamber based approach (25, 26). Coincident with the chamber emission measurements, we applied the stable isotope approach (3) for the determination of CH4 oxidation on emitted CH4 and on CH4 in air initially present at the surface of the landfill, before the chamber tests started. This paper reports the results of the oxidation measurements.
2. Materials and Methods Studies were conducted seasonally at twenty different landfill covers on 37 occasions. The landfills spanned five climate zones as illustrated in Figure 1: humid subtropical, humid continental-warm summer, humid continental cold-summer, mediterranean, and arid. One southern California landfill, Lancaster, was similar to the very dry Colorado site and thus these two are grouped in the arid classification. Generally the covers investigated at these landfills were intermediate covers composed of a single 0.30-1.22 m thick layer of native soils, most commonly clay. One daily cover was investigated (Table 1). All of the landfills employed gas collection systems. Three types of isotope samples were collected. The first consisted of chamber-collected CH4 which represents the CH4 flux emitted directly through the soil cover and which accumulated in the chamber over the course of the 25-min sampling. However, not all fugitive CH4 escaping from landfills follows this route, some escapes from leaking pipes and gas wells and large defects in the cover. To quantify CH4 oxidation that included these routes, we collected and analyzed samples of ambient air present at the chamber sites at the initiation of each measurement. These samples were collected from the chambers immediately upon their placement and represent a snapshot of mixed CH4 that had passed through the soil cover but also CH4 from large cracks nearby and additionally from leaking pipes and wells. The third type of isotope sample was anoxic CH4 (from inside the landfill) collected from gas wells. The chambers used in this study were constructed of polished aluminum with a size of 0.63 × 0.63 × 0.20 m. Chambers were sealed to the ground by clamping them to preinstalled collars. Methane samples were collected from each chamber sequentially over a 25 min period and emission rates are reported in Goldsmith et al. (27). VOL. 45, NO. 1, 2011 / ENVIRONMENTAL SCIENCE & TECHNOLOGY
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FIGURE 1. Sites where landfill cover methane oxidation was determined in this study. The site in southern California (Lancaster) was reclassified as arid, based upon its similarity to the Colorado site (DADS). Source of map is http://printable-maps.blogspot.com/ 2008/09/climate-maps-united-states-and-canada.html (accessed December 3, 2010). Methane oxidation was determined from the stable isotope approach. Samples were only analyzed when the flux was positive to determine the δ13C of residual CH4 following oxidation as it passed through the soil beneath the chamber. The δ13C of residual chamber CH4 was determined from the following equation: δR )
(δF × CF) - (δI × CI) CF - CI
(2)
where δR is the δ13C value of the residual CH4 emitted from the landfill, δI and δF are the initial and final δ13C values of CH4 measured at the initiation and completion of the flux measurement, and CI and CF are the initial and final CH4 concentrations. Samples of landfill surface CH4 were corrected for background air ambient CH4 concentration and its isotopic value in a similar fashion (28, 29). The corrected δ13C values for landfill CH4 from chamber and surface gas represent CH4 that has been exposed to oxidation. Anoxic zone CH4 (δA), has not been exposed to methanotrophic bacteria. These samples were collected from gas wells. From the difference between the δ13C of anoxic zone CH4 and the CH4 that has been exposed to oxidation we calculate the percentage of CH4 oxidized, including the carbon isotopic fractionation factor for bacterial oxidation and correcting for isotopic fractionation during gas transport from the anoxic zone. This parameter, Rox, is a measure of the bacteria’s preference for the light isotope over the heavy isotope, given by the following: Rox ) kL /kH
(3)
where kL and kH refer to the rate constants of the light (12CH4) and heavy (13CH4) isotopes. The fraction of CH4 (fox) oxidized in upward transit through the landfill cover soil is then given by (3, 30) fox )
(δR - δA) 1000 × (Rox - Rtrans)
(4)
where δR is calculated using eq 2 and δA is the carbon isotopic content of anoxic CH4 sampled from gas wells), and Rox and 314
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Rtrans are the isotope fractionation factors associated with CH4 oxidation and with transport of CH4, respectively. The fractionation factor (Rox) was determined from the measured soil temperature (T, °C) using the regression equation for Rox with temperature (31). The parameter Rtrans has in most previous studies been assumed to be 1, which assumes that CH4 transport is dominated by advection, a process that does not cause isotopic fractionation (32, 33). This assumption seemed reasonable for landfills which did not have gas collection systems and that were used in many previous studies (3, 4, 26, 33, 34). However, more modern larger facilities including all of those we measured in this study, do have gas collection. Recent laboratory experiments and field studies have shown that if diffusion is important in gas transport, ignoring it can result in underestimates of CH4 oxidation by as much as a factor of 2 (11, 30, 35). Thus the oxidation values reported when Rtrans has been assumed to be ) 1 represent lower limits of CH4 oxidation. Gas collection systems may reduce the importance of advection in the transport of CH4 across the soil cover and diffusion may play a greater role. For this reason, we made two series of calculations for each data set, the chamber collected isotope sample data set and the surface air collected sample data set (Tables 1 and 2). In the first series of calculations, we set Rtrans ) 1. The fraction of CH4 oxidized (%) values calculated in this way clearly represent lower limit estimates of CH4 oxidation as argued previously (34-36). In a second series of calculations we allowed Rtrans to exceed one. In our second set of calculations, we increased Rtrans until any single value for fraction oxidized for that landfill exceeded 100% oxidation. In other words we choose the largest possible value of Rtrans that gave a relative oxidation less than or equal to 100% for all samples. We rounded Rtrans to three decimals places, and to be conservative, we rounded down. Generally Rtrans varied from 1.000 to 1.018 across the landfills. The value of Rtrans ) 1.0178 was determined by De Visscher et al. (30), close to the theoretical value of 1.0195. In some cases, we were unable to increase Rtrans above 1 without violating our conditions that the fraction oxidized of the samples could not exceed 100%.
TABLE 1. Fraction of Methane Oxidized (%) Determined on Chamber-Captured Emitted CH4. Mean, Standard Error and Median Values for Each Landfill Assuming No Isotopic Fractionation with Gas Transport (rtrans = 1) and Assuming Isotopic Fractionation with Gas Transport (rtrans >1)a site (n)
month-year
mean %
std err
mean %
std err
rtrans
anoxic gas
Arid DADS (6)
Oct-07
Rtrans ) 1 57.2 3.6
Rtrans > 1 87.2 5
1.008
δ13C % -61.4
Mediterranean Altamont (8) Kirby Canyon (4) Tricities (17) Kirby Canyon (8) Tricities (48) Redwood (2)
Oct-09 Jan-08 Feb-08 Jun-08 Aug-08 Jun-09
25.7 20.8 20.1 61 27.4 28.1
9.1 12.7 6.3 14.8 3.7 5.4
34.7 38.4 31.5 61 27.4 73.3
12.3 23.6 8.1 14.8 3.7 20.5
1.006 1.013 1.014 1.000 1.000 1.009
-58.0 -57.7 -58.3 -56.8 -58.0 -58.0
Humid Continental-Cool Summer Old Spruce (6) New Spruce (5) Metro (11) Spruce (56) Emerald Pk (18) Metro (20) Suburban (21) Suburban (18)
Aug-07 Aug-07 May-08 Jul-08 Oct-08 Oct-08 Sep-09 Dec-09
29.7 93.2 15.6 65.4 18.8 25.5 26.6 1.1
12.6 3.5 5.1 3.4 6.2 4.4 5.5 0.8
42 93.2 28.9 65.4 18.8 43 26.6 2.4
17.7 3.5 9.3 3.4 6.2 6.9 5.5 1.7
1.005 1.000 1.001 1.000 1.000 1.010 1.000 1.078
-56.9 -56.9 -56.3 -55.8 -59.4 -56.1 -58.9 -59.5
Humid Continental-Warm Summer OL 7 (32) OL 8 (27) (daily cover) OL 7 (16)) OL 5 (9) OL 7 (11) OL 8 (5) Central (8) OL 5 (17) OL 7 (11) OL 7 (17) OL 7 (19) Maplewood (21) Atlantic (37) Maplewood (33)
Jul-06 Jul-06 Mar-07 Jul-07 Jul-07 Jul-07 Aug-07 Aug-07 Aug-07 Dec-07 Jan-08 Apr-08 Aug-08 Sep-08
21.7 69.9 13.4 40.7 73.9 51.3 78.4 56.3 62 19.1 33.7 15.9 35.4 47
4.6 5 3.4 8 11 8.5 12 8.3 8.2 3.4 7.6 4.5 5.4 6.1
21.7 69.9 31.7 53.3 73.9 68.4 78.4 56.3 62 32.8 33.7 18.1 35.4 47
4.6 5 8.3 9.8 11 11.1 12 8.3 8.2 5.7 7.6 5.2 5.4 6.1
1.000 1.000 1.016 1.004 1.000 1.004 1.000 1.000 1.000 1.013 1.000 1.003 1.000 1.000
-57.7 -57.7 -57.4 -56.4 -57.6 -57.6 -57.4 -56.4 -56.4 -57.6 -57.6 -56.9 -56.3 -56.6
Humid Subtropical Springhill S (48) Springhill S (13) Atascocita (19) Atascocita (51) Clearview (12) Clearview (15) Pine Ridge (12) Pine Ridge (20)
5&6/2006 Apr-07 Feb-08 Nov-08 Apr-09 Nov-09 Dec-09 Apr-09
23.2 12.3 19 11.2 19.7 6 5.9 26.1
4.3 5.1 2.7 1.9 5.1 2.5 4.2 3.5
25.6 20.2 39.6 18.3 24.4 13.1 10.7 36.1
4.7 8.3 6.6 3.1 6.4 6.1 7.8 5.1
1.005 1.008 1.013 1.009 1.005 1.014 1.012 1.006
-56.6 -56.6 -57.9 -55.6 -55.9 -56.5 -56.2 -56.6
a Number of determinations at each landfill shown in parentheses, first column. Over 700 samples were analyzed. Mean Rtrans ) 1.007.
Isotopic ratios were determined by standard methods (3, 33). A replicate was analyzed for most samples, yielding a standard deviation of approximately 0.15%. Values are reported in the “δ” scale in % relative to the standard, VPDB (Vienna Pee Dee Belemnite). It should be noted that the isotopic technique provides the lower limits of CH4 oxidation. The stable isotope approach measures the δ13C of residual CH4 following oxidation. Emitted CH4 that is captured by any technique is weighted toward the isotopic signature of the nonoxidized fluxes or the fluxes oxidized to a lesser extent. For example, areas where oxidation is near 100% or 100% will not be reflected in captured gas because under these conditions there is simply no CH4 left to be emitted. Quantitative oxidization (100%) imparts no signal to residual CH4 as the CH4 is all
consumed. Because of this, even values that we have corrected for diffusion are biased to the low side. In actuality, one can overcome this bias toward less oxidized sources of CH4 by sampling at a finer or smaller scale than down wind plume sampling. That is what the air sampling approach in this paper was designed to do. By sampling at a smaller scale, the more oxidized samples are collected with less interference from oxidized sources.
3. Results and Discussion 3.1. Fraction of CH4 Oxidized (%). Soil flux-chamber derived CH4 oxidation estimates for the 20 landfills are tabulated (Table 1). In sum, 701 determinations were made over the course of 37 seasonal sampling events. Mean values for landfill CH4 oxidation efficiency calculated from chamberVOL. 45, NO. 1, 2011 / ENVIRONMENTAL SCIENCE & TECHNOLOGY
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TABLE 2. Fraction of Methane Oxidized (%) Determined on CH4 in Landfill Surface Aira rtrans ) 1 site (n)
rtrans > 1
δ13%C
mean %
std err
mean %
std err
rtrans
anoxic gas
Arid DADS (6) Lancaster (6)
Oct-07 Jan-08
52 19.2
3.1 2.4
75 49.4
5.5 6.1
1.007 1.017
-61.4 -61.6
Mediterranean Altamont (8) Kirby Can (4) Tricities (17) Kirby Can (8) Tricities (48)
Oct-09 Jan-08 Feb-08 Jun-08 Jun-08
18.5 32.1 11.5 68.6 32.9
6.5 3 2.8 7.2 4.1
34.6 80.4 35.7 68.6 32.9
12.3 12.6 7.9 7.2 4.1
1.010 1.017 1.017 1.000 1.000
-58.0 -57.7 -58.3 -56.8 -58.0
Humid Continental-Cool Summer Old Spruce (6) New Spruce (5) Metro (11) Spruce (56) Emerald Pk (12) Metro (11) Suburban (18) Suburban (12)
Aug-07 Aug-07 May-08 Jul-08 Oct-08 Oct-08 Sep-09 Dec-09
25.6 95.1 6.7 46.8 23.5 19 20.6 3.7
11.3 2.5 2.1 3.1 5.2 1 4.1 0.3
39.5 95.1 24.4 46.8 26.4 42.5 28.7 7.9
17.2 2.5 9.6 3.1 6.1 7.6 5.9 1.6
1.006 1.000 1.017 1.000 1.002 1.014 1.005 1.018
-56.9 -56.9 -56.3 -55.8 -59.4 -56.1 -58.9 -59.5
Humid Continental-Warm Summer OL 7 (33) OL 8 (27) daily cover OL 7 (16) OL 5 (9) OL 7 (11) OL 8 (5) Central (8) OL 5 (17) OL 7 (11) OL 7 (17) OL 7 (19) Maplewood (15) Atlantic (37) Maplewood (33)
Jul-06 Jul-06 Mar-07 Jul-07 Jul-07 Jul-07 Aug-07 Aug-07 Aug-07 Dec-07 Jan-08 Apr-08 Aug-08 Sep-08
20.4 69.7 15.8 24.9 52.7 35.9 82.4 43.3 36.5 16.6 22.2 14.8 34.4 41.4
3.8 4.3 1.6 5.6 12.5 1.8 6 6 6.5 9.6 3.5 2.3 4.2 4.7
20.4 69.7 45.9 57 52.7 82.4 82.4 43.3 38.6 39.9 35.9 46.2 34.4 41.4
3.8 4.3 4.7 10 12.5 5 6 6 7.1 5.8 5.8 6.5 4.2 4.7
1.000 1.000 1.018 1.009 1.004 1.009 1.000 1.000 1.001 1.018 1.010 1.017 1.000 1.000
-57.7 -57.7 -57.4 -56.4 -57.6 -57.6 -57.4 -56.4 -56.4 -57.6 -57.6 -56.9 -56.3 -56.6
Humid Subtropical Springhill S (13) Springhill S (47) Atascocita (19) Atascocita (48) Clearview (12) Clearview (14) Pine Ridge (10) Pine Ridge (20)
Apr-07 5&6/2006 Feb-08 Nov-08 Apr-09 Nov-09 Dec-09 Apr-09
12.6 11.7 14.5 4.5 6.4 6.2 5.1 21.4
2.5 1.5 1.6 0.6 1.5 0.7 18 5.3
45 28 45.2 21.4 22 23.7 16 23.1
8.2 3.7 5.4 3.1 5.1 3.1 5.2 5.5
1.015 1.016 1.017 1.018 1.018 1.018 1.018 1.001
-56.6 -56.6 -57.9 -55.6 -55.9 -56.5 -56.2 -56.6
a Mean, standard error and median values for each landfill assuming no isotopic fractionation with gas transport (Rtrans ) 1) and assuming isotopic fractionation with gas transport (Rtrans > 1). Number of determinations at each landfill shown in parentheses next in the first column. 670 samples were analyzed. Mean Rtrans ) 1.009.
derived CH4 varied from 34 to 41% across the different landfills. The lower value reported 34% ( 3.8% (n ) 701 total samples, and 37 measurement periods) was the mean when diffusive fractionation was not accounted for, for example, Rtrans )1 in eq 4. Allowing for diffusive fractionation (Rtrans >1) yielded a mean value of 41 ( 4.7%. The distribution of the oxidation values was normal (Sigma Stat, see also ref 37) indicating that arithmetic means are an acceptable manner for reporting the data’s central tendency. Median values varied from 26 to 35%, respectively. Mean and median values were obtained by weighting each landfill measurement date equally. It may be argued that the value determined for fractional oxidation assuming diffusive fractionation (41 ( 4.7%) is the best estimate. The landfills studied all had active vacuum extraction gas collection systems that would reduce overpressure within the landfill and thus reduce advection, increasing the relative importance of diffusive gas transport. 316
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However, to be conservative, we will assume that these values represent the upper limit values for the purposes of this discussion. Pairing these “maximum values” with minimum values obtained from assuming no diffusive fractionation, allows us to “bracket” the values for CH4 oxidation. We determined the midway point between these two estimates, and we represent this value as our best estimate of soil cover CH4 oxidation. The midpoint of the chamber-flux derived mean values is 37.5 ( 3.5. It is important to point out that while the maximum theoretical value determined for diffusive fractionation during diffusion in air is 1.0195 (30) the maximum value determined experimentally (30) was 1.0178. The values for Rtrans used in our calculations are listed in Table 1. The average Rtrans value used was 1.007, emphasizing our conservative treatment of the data, and further indicating that advection and diffusion are both in play.
TABLE 3. Compiled Data Means, Medians and Number of Samples for Fraction Methane Oxidized Expressed as a Percenta
mean % ox n (sample events) median % ox std err total n samples
soil flux CH4A
soil flux CH4B
surface CH4A
surface CH4B
34.0 37 26.1 3.8 701
41.7 37 35.4 4.7 701
28.9 37 21.4 3.8 670
43.3 37 39.9 3.4 670
a A ) assuming no isotopic fractionation with gas transport (Rtrans )1), and B, assuming isotopic fractionation with gas transport (Rtrans > 1).
FIGURE 2. The % oxidation as a function of climate type in 5 regions across the U.S The upper panel is based upon the δ13C of CH4 that passed across the soil cover and that was captured within a chamber. The lower panel is derived from the δ13C of CH4 collected at the surface of the landfill. The values shown are the midpoint between values determined assuming no isotopic fractionation with gas transport (rtrans ) 1) and by assuming isotopic fractionation with gas transport (rtrans > 1). Error bars indicate the difference between these two determinations. The number of measurements in the upper panel was 1, 6, 8, 14, and 8. The number of measurements in the lower panel was 2, 5, 8, 14, and 8. These values represent the fraction of CH4 oxidized upon passing through the soil. Goldsmith et al. (27) compared chamber derived CH4 emissions from this same data set to CH4 emissions measured with OTM-10, a laser-based approach that measures CH4 emissions from soil plus infrastructure leakage. They reported that chamber fluxes were 66% of those determined with the laser-based approach. If we assume that 66% of the CH4 emissions were via the soil and oxidized to 37.5%, and 34% of the emissions were via pipes and wells and not oxidized at all, then we derive a value of 0.66 × 37.5% ) 25%, using the mean value. 3.2. Oxidation by Climate Type. Five climate types were investigated. At arid sites there was very little CH4 emission so few measurements were obtained, however this type of site appeared to yield the highest fractional oxidation values (Figure 2A). At warmer sites emissions were observed with greater frequency so more oxidation measurements are reported. Humid subtropical sites gave the lowest fraction oxidized values. Applying a 1-way ANOVA test to the data of Figure 2A we found that the fraction oxidized in arid sites was significantly greater than oxidation in mediterranean sites, or cool and warm continental sites (P ) 0.036). Subtropical sites had significantly lower CH4 oxidation than the other two types of sites (aid and (mediterranean plus cool and warm continental). The middle three classifications were not different. 3.3. Oxidation Based on Surface Air Collections. For CH4 collected at the surface of the landfill, fractional oxidation values are reported (Table 2). In sum 670 samples were collected in 37 seasonal sampling events at 20 different landfills. Mean values varied from 29 to 43% without and with allowing for diffusive fractionation. Values for the two approaches, chamber and surface CH4, and without and with
allowing for diffusive fractionation during gas transport are compiled in Table 3. For the case of no fractionation with gas transport (A, Table 3), the chamber captured CH4 yields a higher fraction oxidized than the surface air collected samples as expected, since the surface air samples contain a contribution from less oxidized sources such as pipes and wells. However for case B, (fractionation with transport) the surface air samples exhibit greater oxidation than do the chamber samples by 1.6%. This difference falls with the standard error of the measurements which was 3-5%. This is somewhat surprising, and it could be that the air samples mostly reflect gas transport through the soil. For the air samples, the midpoint between values determined assuming no isotopic fractionation (which represented a conservative or minimum estimate), and those values calculated allowing limited fractionation with gas transport was 36.1 ( 7.2%. The CH4 collected in these air samples represents gas from all pathways so there is no need for the 0.66 factor correction applied to the chamber collected data. These data show the same pattern as the chamber data when graphed by regions (Figure 2B). In this case, however, the 1 way ANOVA found no significant differences. The lack of a significant difference in this data set is because the effects of leaking cover defects and gas collection wells and pipes were included and one would expect differences based on climate to affect the soil only, not these other sources of emission. The average Rtrans used in the calculations when Rtran was allowed to be >1 was 1.009, again emphasizing the conservative nature of the data set. Overall, our results are consistent with a recent literature review which reported a mean value of 35 ( 6% for the fraction of CH4 oxidized (19). It would seem reasonable that a revision of the 10% default oxidation value is needed. Our data, taking the most conservative approaches, indicate that landfill CH4 oxidation is at minimum, 25%, for landfills with gas collection systems. These systems reduce CH4 loading to the cover and result in higher oxidation efficiency. 3.4. Oxidation Variation with CH4 Loading to the Cover. The chamber-measured surface emissions representing the flux of CH4 through the soil cover exclusive of emissions from larger cover defects, leaking wells and piping (from 27, Fout) and the chamber-isotope based fractional oxidation (fox ) % oxidized/100) were combined to estimate the CH4 loading (Fin) into the soil cover from the waste below. For fox we used the average of the results of Rtrans ) 1 and Rtrans > 1 for chamber based oxidation (Table 1). Fin ) Fout /(1 - fox)
(5)
The rate of CH4 oxidation (Rox g CH4 m-2d-1) within the soil cover is Rox ) Fin - Fout VOL. 45, NO. 1, 2011 / ENVIRONMENTAL SCIENCE & TECHNOLOGY
(6)
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TABLE 4. δ13C % Values of Anoxic Landfill CH4 by Region
arid mediterranean humid cont cool humid cont warm humid sub trop
FIGURE 3. Methane oxidation rate (y-axis) as a function of methane loading to the bottom of the landfill soil cover (x-axis). The lines on the graph from left to right represent 100%, 50%, 25%, and 10% oxidation.
FIGURE 4. % Methane oxidation (y-axis) as a function of methane loading to the bottom of the landfill soil cover (x-axis). Where Fin represent the CH4 loading to the bottom of the cover (g CH4 m-2d-1), Fout represents emission to the atmosphere from the cover surface (g CH4 m-2d-1), Rox (g CH4 m-2d-1) represents the CH4 oxidation rate, and fox represents the fraction of CH4 oxidized in the cover. Methane oxidation rates as a function of CH4 loading to the bottom of the landfill cover varied from a maximum of 32 g CH4 m-2d-1(Figure 3). These field measured rates are on the low end of the range of rates observed in a compilation of laboratory column studies of landfill soils which varied from 22 to 230 g CH4 m-2d-1 (7). A second literature survey (19) reported field oxidation rates varying from 0.65 to 216 g CH4 m-2d-1 with an average of 31 ( 54 g CH4 m-2d-1. The lines on Figure 3 from left to right represent 100%, 50%, 25%, and 10% oxidation. The trend is that at lower loading rates, the fractional oxidation is greater, and it decreases as loading to the cover increases. In only one case is the fraction oxidized below the 10% line. These data are plotted as fraction oxidized (in %) versus cover loading in Figure 4, where it is seen that at low loading rates % oxidation ranges considerably but exhibits higher values at lower loading rates. The top of the fraction oxidized range decreases as loading increases. These results may explain the differences in fractional oxidation observed at the different climate types (Figure 2A). Methane emission rates were lowest at the arid sites, intermediate at the mediterranean and continental sites and greatest at the humid subtropical sites (27). The arid sites apparently had less CH4 loading to the covers while the subtropical sites had the greatest cover loading. Our results indicate that an effective way to increase the fraction of CH4 oxidized by a landfill cover is to reduce the quantity of CH4 entering it. In the early life of a landfill when CH4 production rates are high, this can be achieved with an effective gas collection system, which reduces the pressure and concentration of CH4 at the base of the cover and thereby 318
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n
δ13C
σ
2 5 8 14 8
-61.5 -57.8 -57.5 -57.1 -56.5
0.1 0.6 1.5 0.6 0.7
reduces the CH4 entering the cover. Reducing advection of CH4 through covers will allow more oxygen penetration downward into the cover, to aid oxidation. It should be noted that while fraction oxidized increases with decreasing rates of CH4 loading to the bottom of the cover, overall oxidation rates in g m-2d-1 generally increase with CH4 loading (Figure 3). However if a landfill operators primary goal is to reduce fugitive CH4 emissions, our results indicate that reducing loading to the cover is a doubly effective approach because the oxidized fraction increases. 3.5. Anoxic CH4 δ 13C Signature. In Table 1 and 2 we listed the values of anoxic CH4 determined in these studies at the different landfills. Average values for landfill gas by region are compiled in Table 4. As for the chamberdetermined fractional oxidation values by regions, we report significant differences in the δ13C value of anoxic gas by regions. Again arid sites and subtropical sites were different from each other and from the mediterranean landfills, and humid continental cool and warm sites, which were the same. These differences were likely caused by differences in moisture within the landfill, which may affect CH4 production rates and CH4 production pathways. There are two main pathways of CH4 formation (38) acetate fermentation and CO2 reduction. The latter pathway produces more 13C depleted CH4 relative to the former. It would seem that drier climates favor CO2 reduction.
Acknowledgments Waste Management Inc and the Environmental Research and Education Foundation funded this work. We thank Tom Pike for assistance in the field.
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