Environ. Sci. Technol. 1999, 33, 3755-3760
Quantifying Methane Oxidation from Landfills Using Stable Isotope Analysis of Downwind Plumes J . P . C H A N T O N , * ,† C. M. RUTKOWSKI,† AND B. MOSHER‡ Department of Oceanography, Florida State University, Tallahassee, Florida 32306, and University of New Hampshire, CSRC/Morse Hall, 39 College Road, Durham, New Hampshire 03824
Landfills are major contributors to the atmospheric CH4 budget. A major uncertainty in estimating CH4 flux from landfills is determining the attenuation of CH4 emission by methanotrophic bacteria in the aerobic outer portions of the cover soil. These bacteria intercept the gas as it migrates toward the atmosphere. To estimate cover soil oxidation, we made seasonal measurements of the difference in the δ13C of CH4 within the anoxic zone and CH4 released from landfills and captured downwind at two landfills in the Northeastern United States. Anoxic zone CH4 at the Nashua Landfill averaged -54.6 ( 0.7‰, n ) 205, and displayed no significant seasonal pattern. Methane was in excess over ambient air concentrations in the downwind plume ranging from 2.13 to 3.41 ppmv. The δ13C of excess CH4, as determined by mass balance with correction for ambient air CH4, varied from -49.35 to -54.28‰. We used these values to calculate soil CH4 oxidation, which ranged from 0 to 23.6%. Oxidation was greatest in the summer and in the fall, with an annual value of 12 ( 8%. At a second landfill, plume CH4 ranged from 1.96 to 2.92 ppmv with excess δ13C values of -52.17 to -58.06‰. Oxidation at this landfill ranged from 0 to 14%.
Introduction Methane, a greenhouse gas with important atmospheric chemistry, has increased in the atmosphere by a factor of 2 in the past century. Due to this systematically varying CH4 concentration, a significant amount of research has been undertaken to determine CH4 sources and sinks. Until the trend slowed recently, the rate of CH4 increase in the atmosphere had been approximately 1% per year (1-4). The trend may be again increasing (5). While there exists a large degree of uncertainty in source strengths, the bulk of atmospheric CH4 sources are anthropogenic and emissions could be mitigated. Through sound environmental policy making, based upon solid scientific observation, we suggest that there is greater potential to reduce CH4 input to the atmosphere as compared to carbon dioxide and nitrous oxide inputs. Sources of CH4 to the atmosphere are both natural and anthropogenic and include wetlands, rice agriculture, coal and gas mining, landfills, termites, and ruminants. Toward this end, it is key to better define the source strength * Corresponding author phone: (850)644-7493; fax: (850)644-2581; e-mail:
[email protected]. † Florida State University. ‡ University of New Hampshire. 10.1021/es9904033 CCC: $18.00 Published on Web 09/23/1999
1999 American Chemical Society
estimates to properly direct mitigation efforts. The imbalance between sources and sinks in the global CH4 budget is less than 6% of the total global source (1) or perhaps even approaching balance (3, 6), so a small decrease in source strength could result in stabilization of atmospheric CH4 or even better, a reduction in the atmospheric burden (4, 7, 8). As CH4 is a more potent greenhouse agent than CO2, lowering the atmospheric CH4 concentration may be a very realistic and worthwhile goal. The relatively short residence time of CH4 in the atmosphere (7-10 yr) relative to CO2 and N2O means that the effects of mitigation efforts would be rapidly observed. Landfills are major contributors to the atmospheric CH4 budget; estimates range from 9 to 70 Tg yr-1 (9). Landfills in the United Statets account for 20-40% of the global landfill contribution (9-11). Estimates of this kind are determined by statistical manipulation of data utilizing per capita refuse production and the CH4 production potential of waste (1214). Actual field measurements of CH4 emission from landfills are fairly rare, and little work has been performed to reconcile the national and global estimates of CH4 emission with this limited database (15). One of the major uncertainties in estimating CH4 emissions from landfills by the statistical approach described above is factoring in the attenuation of the CH4 production by the quantity of CH4 oxidized. This attenuation is driven by methanotrophic bacteria in the aerobic outer portions of the landfill soil that intercept the produced gas as it migrates toward the atmosphere. For a discussion of the process of CH4 oxidation, see King (16). Values for the percentage of produced CH4 that is oxidized as it passes through this aerobic layer of the soil range considerably. For example, oxidation values of 80% have been reported based on soil δ13C-CH4 and radon profiles (17). Czepiel et al. (18) estimated an oxidation value of 10% based upon soil incubations and modeling. Liptay et al. (19) reported warm season values of 20-32% CH4 oxidation and estimated that the annual value should be similar to Czepiel’s figure of 10%. Liptay et al. (19) utilized a stable isotopic technique that involved placing chambers directly on the landfill soil cap. However, this surface is very heterogeneous with fissures and cracks often observed in landfill cover soils. Through these conduits, it is possible that CH4 can escape from the landfill with less exposure to the gauntlet of methanotrophic bacteria present in the oxidation zone of the cover soil (Figure 1). Stable isotopes are useful for determining CH4 oxidation because, as it occurs, the remaining CH4 becomes 13C enriched due to preferential utilization of the lighter 12C isotope by bacteria. Carbon isotopic composition is expressed in the δ notation, which is defined as follows:
δ13C ‰ ) ((Rsample/Rstandard) - 1) × 1000
(1)
where Rsample is the 13C/12C ratio of the sample and Rstandard is the 13C/12C ratio of a standard marine carbonate (PDB, 0‰). Typical biogenic CH4 is typified by values below -50‰. Following oxidation, CH4 may exhibit 13C enriched values of -30 to -50‰. Typical organic matter is 13C enriched relative to CH4 with a δ13C of -25‰. There were two objectives of this study. The first objective was to measure the anoxic vent gas of a landfill over time for comparison to anoxic gas values in the literature and to verify that temporal variations in the δ13C of produced CH4 were small. This is important because landfill gas released from vents and cracks is little altered in its δ13C value relative to anoxic zone gases (20), so a knowledge of seasonal variation VOL. 33, NO. 21, 1999 / ENVIRONMENTAL SCIENCE & TECHNOLOGY
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FIGURE 1. Diagram showing the main means of CH4 escape from landfills: (i) escape through fissures and vents, which is measured through downwind plume sampling; (ii) transport through the soil cap, which is measured utilizing the chamber technique and the downwind plume sampling method. Methane is produced in the landfill interior, which is warm year-round due to the composting of organic matter. Methane oxidation, however, occurs in the outer rind of the landfill where O2 penetrates. Because CH4 oxidation occurs in the outer skin of the landfill, it is sensitive to environmental temperature variations that influence rates of enzymatic processes in microbial communities, while the CH4 production rate, which occurs in a warm insulated environment, is not. Values are for the δ13C of CH4 in ‰. The diminishing vertical arrows on the right-hand side of the figure indicate the attenuation of the methane flux from the landfill by CH4 oxidation as the gas passes from the anoxic zone through the gauntlet of methanotropfic bacteria in the oxic soil layer. will reveal how frequently these gases should be sampled in future studies. We hypothesized that little seasonality should occur because the environment within the landfill is heated from the composting of organic material to a temporally uniform temperature. The second objective of this work was to determine the portion of CH4 oxidized as it advected from the landfill to the atmosphere. This is important because the extent of CH4 oxidation is a major uncertainty in estimating the extent of landfill CH4 emissions. The extent of CH4 oxidation was determined by collecting air downwind from several landfills and determining the isotopic composition of excess CH4 within the plume relative to the isotopic composition of CH4 within the anoxic zone of the landfill. A comparison between these two distinct parcels of CH4, the anoxic zone and the emitted CH4, combined with a knowledge of the isotopic discrimination of bacterial oxidizers (R, defined below) allows us to calculate the fraction of CH4 oxidized as it escapes the landfill. As opposed to measurements made with chambers, samples of plume CH4 integrate the entire landfill including CH4 escaping vents and fissures in addition to CH4 passing through the landfill cover soil (21).
Materials and Methods Field Sites. We attempted to determine whole landfill oxidation by measuring the CH4 plume downwind of the landfill site to capture both CH4 that escaped through the soil cap and CH4 that traveled through cracks and fissures. Samples of vent gas and plume air were collected frequently from a landfill at Nashua, NH. This site had been active since 1971 and covers 62 acres in south central New Hampshire. A gas recovery system had been installed and on-line for approximately 1 year before the samples were collected. Inactive areas of the landfill were covered with 1-2 m of sandy-clay loam surface soil. Soil characteristics have been described as 53 ( 11% sand, 18 ( 17% silt, and 29 ( 16% clay (18). Some surface fissuring, slumping, and erosion were observable at the site. The Nashua Landfill has been the site of several studies and is described more thoroughly (18, 19, 21, 22). The CH4 emission rate from this landfill varied from 61 to 78 g of CH4 m-2 d-1 prior to gas recovery (22) and fell by roughly 40% when gas recovery was initiated (23). 3756
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Samples were collected less frequently at a privately operated landfill in Massachusetts, PLF-A (57 acres). PLF-A had an active gas collection system during sample collection, and 40% of it was covered with a geomembrane. The soil cover at this landfill did not appear to be different from the soil at Nashua. The identity and location of this landfill are kept in confidence in accordance with the wishes of the landfill operator. The CH4 emission rate at PLF-A was varied between 128 and 139 g m-2d-1 (22) and fell by 75% with initiation of gas recovery (23). The plume of air 1-2 km downwind of the landfills was sampled for CH4 concentration and δ13C on 14 separate dates at Nashua, NH, and on 5 days at PLF-A. In a limited number of cases, these samples were collected during tracer flux experiments (e.g., ref 22), and mobile SF6 and CH4 instrumentation was used to locate the plume. In the other cases, the prevailing wind direction was carefully determined at the landfill site, and the sampler was positioned in the plume downwind. Extensive previous measurements at these two sites (22) have shown that there are no interfering CH4 sources nearby either site. Samples were collected in aluminum flasks that were pressurized to approximately 20 psi using a batterypowered metal bellows pump. Storage tests in these containers show that sample integrity is good for several months (24). The measured CH4 δ13C value was corrected for background, or ambient, CH4 through mass balance to obtain the δ13C of excess ([CH4]xs) using the following equation:
[δCH4]xs ) (([CH4](meas)[δCH4](meas)) - ([CH4]amb)[δCH4]amb))/ ([CH4](meas) - [CH4]amb) (2) where [CH4] (meas) and [δCH4](meas) represent the concentration and δ13C values of CH4 in the downwind plume and [CH4]amb and [δCH4]amb represent the concentrations and δ13C values of background air measured upwind of the landfill, 1.84 ( 0.02 ppmv, -46.38 ( 0.15‰, n ) 5 samples, and 15 determinations. This value compares well with a series of measurements made on background air collected at the Harvard forest in Massachusetts. Mean values for CH4 concentration and isotopic composition of 1.85 ( 0.02 ppmv and -46.47 ( 0.29‰ were obtained from analysis of 33 samples over a 1-year period (25). These values represent the concentration and isotopic composition of CH4 in air at ground level in the region. They do not represent clean unpolluted air from the troposphere. Gas Analysis. Methane concentrations were determined for both anoxic vent samples and emitted plume samples on a Shimadzu 14A gas chromatograph with a flame ionization detector, a 1-mL sampling loop, and a 2 m, 0.32 cm diameter stainless steel column packed with Porapak Q. Stable isotopic samples were determined using a Finnegan Mat Delta S gas chromatograph/combustion isotope ratio mass spectrometer (GCC-IRMS) following published methods (26, 27). For emitted plume air samples, a cryogenic focusing device was used on the front end of the gas chromatograph. The cryofocusing process was conducted in two steps. In the first step, the CH4 was trapped from 10 mL of air on a packed 0.32 cm diameter, 10 cm long column of Porapak Q in an ethanolliquid N2 slush. After 3 min, the slush was removed, the Porapak Q column was warmed, and the CH4 was focused onto the head of the analytical column that was held in liquid N2. The analytical column was Poraplot Q. After an additional 3 min, the analytical column was warmed, and the CH4 passed through the Poraplot Q column into the combustion column. On the 960 °C combustion column, the CH4 was converted to CO2 and then entered the mass spectrometer. The standard deviation of replicate analyses is generally about 0.15‰.
TABLE 1. Seasonal Anoxic Zone Dataa av δ CH4 from anoxic vent
date
Nashua Landfill, New Hamshire Aug 20, 1996 -54.00 ( 0.08 Feb 19, 1997 -54.12 ( 0.11 Feb 27, 1997 -54.41 ( 0.09 Mar 7, 1997 -54.23 ( 0.15 Apr 9, 1997 -53.81 ( 0.27 Sep 18, 1997 -54.54 ( 0.13 Oct 3, 1997 -54.64 ( 0.20 Oct 17, 1997 -54.73 ( 0.14 Dec 11, 1997 -54.40 ( 0.11 Jan 22, 1998 -54.73 ( 0.19 Apr 9, 1998 -56.66 ( 0.10 May 2, 1998 -55.22 ( 1.40 May 8, 1998 -54.16 ( 1.80 PLF-A, Massachusetts -56.77 ( 0.04 -56.49 ( 1.81 -57.15 ( 0.08 -56.28 ( 1.22
mean a
landfill site
δ13C of CH4
ref
Indiana Mainz, Germany Gerolshein, Germany Kuchino, Russia Olinda, CA Illinois (1) Illinois (2) Illinois (4) West-B, USA East-M, USA East-N, USA East-J, USA East-G, USA Germany (D1) Germany (D2) Germany (D3) Netherlands (NL1) Wayland, MA Springfield, MA PLF-A, Massachusetts PLF-B, Massachusetts Nashua, NH Rochester, NH LF-C, Massachusetts Tallahassee, FL Nashua, NH PLF-A, Massachusetts
-50.2 ( 1.8 (3) -60.4 ( 1.6 (23) -59.0 ( 3.0 (14) -61.0 ( 3.0 (3) -61.3 ( 2.0 (6) -53.8 ( 0.4 (2) -57.0 (1) -54.6 (1) -59.4 ( 0.8 (2) -53.6 (1) -51.3 ( 0.7 (8) -54.9 ( 0.4 (2) -52.5 ( 4.2 (2) -60.3 ( 2.3 (37) -60.0 ( 1.2 (12) -58.8 ( 1.0 (20) -57.4 ( 1.7 (35) -55.2 ( 2.2 (14) -53.0 ( 1.4 (4) -56.0 ( 3.4 (13) -57.9 (1) -56.3 ( 2.0 (4) -56.2 ( 1.7 (6) -58.4 ( 0.6 (2) -55.2 ( 0.2 (30) -54.6 ( 0.72 (205) -56.7 ( 0.37 (49)
41 42 29 43 28 44 44 44 44 44 44 44 44 17 17 17 17 19 19 19 19 19 19 19 20 this study this study
average
-56.5 ( 3.1
n 15 14 2 4 12 16 16 16 16 16 4 16 58
-54.59 ( 0.72
mean Feb 19, 1997 Mar 3, 1997 Mar 7, 1997 Apr 9, 1997
TABLE 2. Isotopic Signature of Anoxic Zone Methane from Landfill Sitesa
6 3 4 36
-56.67 ( 0.37
Average, standard deviation to 1σ, and n are reported.
Stable isotopic samples for the anoxic vent gases were determined using direct injection on the GCC-IRMS. Samples were diluted to 1% CH4 by addition with nitrogen. Samples were then analyzed by injecting 0.1-0.5 mL of sample into the GCC-IRMS inlet system.
Results and Discussion The δ13C of CH4 produced within the anoxic zone of landfills is uniform. In this study, CH4 from the anoxic zone at the Nashua Landfill in New Hampshire were measured on 13 separate days from 1996 through 1998. On each date, 2-58 samples were collected, and each sample was analyzed two to four times (Table 1). Samples from each day were weighted separately, and a Kruskal-Wallis one-way analysis of variance (ANOVA) on ranks was run comparing the four different seasons with spring ) March 20-June 20, summer ) June 21-September 21, fall ) September 21-December 20, and winter ) December 21-March 19. No separation for different years was made. Results from the statistical analysis indicate that there are no statistically significant differences in the data when treated as different seasonal groupings, p < 0.05. The δ13C of CH4 for four dates at the PLF-A landfill (Massachusetts) is also assumed to display uniformity with respect to seasonal changes and had an average value of ) -56.67 ( 0.37 ‰, n ) 49. A review of published isotopic values for anoxic zone CH4 is presented in Table 2. Landfills in four different countries are listed that have CH4 δ13C values ranging from -50‰ to -61‰ with an average δ13C value of -56.5 ( 3.1‰. Isotopic samples from the anoxic zone of the Leon County Landfill in Florida ranged from only -54‰ to -56‰ over a period of 13 months and also showed no seasonal variations (20). Bergamaschi et al. (17) also found little seasonal variation in biogenic CH4 δ13C values in European landfills in a similar study. The δ13C of -54.54 ( 0.72‰ for Nashua is within the range of reported anoxic vent CH4 (Table 2). Methane in the anoxic portion of landfills escapes to the atmosphere by flowing through the cover soil cap, through cracks or fissures in the soil cap, and through vents. Several studies on landfills have attempted to determine CH4 flux and oxidation by use of chambers placed over the cover soil, often in conjunction with other techniques such as stable
a
Each landfill is weighted equally in the mean presented below. Landfills PLF-A and Nashua are presented twice for comparison, but the mean of the two values was used in preparing the grand mean. The number of samples from each landfill is given in parentheses.
isotopes (11, 15, 19-21, 28-30). Yet, it has been hypothesized, based on δ13C CH4 balances of landfill emissions, that direct CH4 emissions through cracks, fissures, and vents constitute the primary pathways for gas migration from the landfill system at two European landfills (17). However, at the Nashua Landfill and at PLF-A, CH4 emission rates made with chamber techniques compared extremely well with whole landfill tracer flux measurement techniques (21, 22). Since the chamber measurements have been employed primarily to determine gas flux across the landfill cover soil, the excellent agreement between these techniques argues against Bergamaschi’s hypothesis. The differences could be explained by different physical characteristics between sites such as cover soil type or excessive waste settlement that may provide preferred gas pathways in the European sites. Conditions within landfills are quite variable, and the Nashua and PLF-A sites were very well managed (31). At Nashua, CH4 emission from a large concrete riser pipe was found to be only 1-2% of the total flux from the landfill (22), indicating that the bulk of the flux came through the soil cover. Chanton and Liptay (20) placed a chamber directly over a fissure on a clay area of Leon County Landfill, Tallahassee, FL. Methane escaping from the fissure resembled that of the vent pipes that were directly connected to the anoxic zone of the landfill. The chamber capturing gas from the fissure had fairly uniform δ13C values for 14 months ranging from -53‰ to -55‰, similar to vent δ13C of -55.2 ( 0.2‰. Chambers placed over more uniform soil released CH4 with isotopic values that were greatly 13C enriched, consistent with more oxidation (20). At Nashua, CH4 concentrations within the air plume ranged from 2.13 to 3.41 ppmv (Table 3). Excess δ13C of CH4 within the air plume at the site ranged from -49.35‰ to -54.23‰. Excess CH4 δ13C values for each date were VOL. 33, NO. 21, 1999 / ENVIRONMENTAL SCIENCE & TECHNOLOGY
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TABLE 3. Air Samples from Nashua and PLF-A Landfills from 1996 to 1998a date
soil temp (°C)
r
concn of methane in plume (ppmv)
av δ CH4 measured in plume
av excess δ CH4 from plume
significant difference
oxidation %
n
-51.57 ( 0.38 -51.36 ( 0.70 -53.54 ( 1.59 -52.51 ( 0.26 -52.06 ( 0.56 -52.52 ( 0.27 -52.43 ( 0.25 -54.23 ( 1.23 -53.39 ( 0.44 -53.09 ( 0.39 -54.28 ( 0.75 -49.35 ( 1.28 -49.69 ( 0.68 -50.23 ( 3.71
yes, p < 0.05 yes, p < 0.05 yes, p < 0.05 yes, p < 0.05 yes, p < 0.05 yes, p < 0.05 yes, p < 0.05 no yes, p < 0.05 yes, p < 0.05 no yes, p < 0.05 yes, p < 0.05 yes, p < 0.05
12.5 ( 0.4 13.6 ( 0.8 6.0 ( 1.4 6.6 ( 0.3 8.0 ( 0.6 6.4 ( 0.3 6.9 ( 0.3 0 3.7 ( 0.5 4.7 ( 0.3 0 21.6 ( 1.3 18.3 ( 0.7 23.6 ( 3.7
8 8 12 12 12 12 18 15 15 15 15 12 12 6
-53.99 ( 0.40 -56.18 ( 0.33 -52.17 ( 0.01 -57.06 ( 0.73 -58.06 ( 1.45
yes, p < 0.05 no yes, p < 0.05 no no
10.5 ( 0.4 0 14.2 ( 0.0 0 0
3 4 6 9 12
Hampshireb
Aug 22, 1996 Aug 24, 1996 Feb 25, 1997 Feb 27, 1997 Mar 1, 1997 Mar 4, 1997 Apr 15, 1997 Apr 17, 1997 Apr 19, 1997 Apr 22, 1997 Apr 24, 1997 Sep 18, 1997 Oct 3, 1997 Oct 17, 1997
22 22 9 5 4 3 5 6 4 5 6 20 15 13
1.0235 1.0235 1.0288 1.0307 1.0311 1.0315 1.0307 1.0304 1.0311 1.0307 1.0304 1.0240 1.0266 1.0269
Nashua Landfill, New 2.67 ( 0.20 -47.92 ( 0.29 2.86 ( 0.14 -48.11 ( 0.34 2.46 ( 0.36 -47.77 ( 0.86 2.66 ( 0.15 -48.21 ( 0.07 2.53 ( 0.15 -47.88 ( 0.44 2.52 ( 0.06 -49.98 ( 0.03 3.04 ( 0.45 -48.52 ( 0.04 2.18 ( 0.47 -46.21 ( 2.2 2.69 ( 0.25 -48.27 ( 0.14 3.41 ( 0.44 -49.24 ( 0.11 2.85 ( 0.45 -48.43 ( 0.01 2.13 ( 0.10 -46.57 ( 0.06 2.39 ( 0.13 -46.96 ( 0.23 2.19 ( 0.13 -46.71 ( 0.18
Aug 21, 1996 Aug 29, 1996 Feb 26, 1997 Mar 3, 1997 Mar 5, 1997
21 20 5 5 3
1.0240 1.0241 1.0307 1.0307 1.0316
2.49 ( 0.20 2.92 ( 0.10 2.11 ( 0.13 1.96 ( 0.03 2.03 ( 0.01
PLF-A, Massachusettsc -48.27 ( 0.34 -49.94 ( 0.11 -46.86 ( 0.04 -47.48 ( 0.10 -46.99 ( 0.06
a Methane plume concentrations are reported in the fourth column. Average measured δ13C of CH is reported in the fifth column. Excess δ13C 4 for CH4 is reported in the sixth column. Statistical significance was determined by nonparametric, one-way ANOVA and is reported in the seventh column. Oxidation percentage, fo % ) [(δE - δA)/((Rox - Rtrans) × 1000] × 100 (symbols described in text) are reported in the eigth column. The number of analysis on samples is reported in the ninth column. The number of samples collected in the downwind plume varied from three to six. b Anoxic δ CH4 for 1996-1997 is -54.6 ( 0.72, n ) 205. c Anoxic δ CH4 for 1996-1997 is -56.7 ( 0.37, n ) 49
statistically compared by one-way nonparametric KruskalWallis ANOVA. Twelve out of 14 days were significantly 13C enriched relative to the anoxic vent values, p < 0.05. The anoxic δ13C of CH4 within the landfill represents the CH4 before it passes through the gauntlet of oxidizing bacteria in the soil cap. For two dates in April 1997 there was no significant difference between emitted CH4 and anoxic vent CH4, suggesting that no significant oxidation took place at those times. Oxidation percentage was determined by the following equation, which describes isotopic fractionation in an open system (19):
fo % ) [(δE - δA)/((Rox - Rtrans) × 1000)] × 100 (3) where fo % ) the percentage of CH4 oxidized in transit through the cover soil cap. The term R is defined as the ratio of the rate constants for CH4 containing the two isotopes assuming first-order kinetics:
R ) kL/kH
(4)
where kL and kH refer to the rate constants of the light and heavy isotope, respectively. δE represents the δ13C value of emitted CH4, δA represents the δ13C value of anoxic zone CH4, Rox represents the isotopic fractionation factor for bacterial oxidation, and Rtrans represents the isotopic fractionation factor associated with gas transport. Liptay et al. (19) and Bergamaschi et al. (17) have argued that gas transport across the soil cap is dominated by advection. Therefore we assume R trans ) 1 and that the δ13C value of CH4 within the anoxic zone is what enters the oxidation zone. Chanton and Whiting (32) have demonstrated that, in advective transport, isotopic fractionation is minimized. We further assume that the δ13C value of residual CH4 due to oxidative microbial processes within the soil is what is emitted to the atmosphere and captured in our chambers. Detailed measurements by Bergamaschi et al. (17) were consistent with this assumption; the δ13C value of CH4 3758
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captured in surface-accumulating chambers was similar to the surface-most soil-air sample (the sample most close to the soil-air interface). To the extent that we have underestimated the importance of molecular diffusion in the transport of CH4 from the landfill, we have underestimated oxidation. This is because diffusion transports the lighter isotope of CH4 preferentially (32), canceling some of the effect 13C enrichment caused by CH -oxidizing bacteria. However, 4 this diffusion effect is often self-compensating in that the buildup of the heavy isotope within a reservoir (in this case the soil) often compensates for the preferential diffusion of the lighter isotope from the reservoir. In other words, the steeper concentrations gradients in the heavy isotope that result from its smaller diffusion coefficient compensate for the smaller diffusion coefficient (32). The term Rox represents the preference of bacteria to consume CH4 containing the lighter isotope, leaving the remaining pool enriched in 13C. Various values have been reported for Rox and range from 1.008 to 1.031 (17, 33). Bergamaschi et al. (17) derived an R value of 1.008 ( 0.004 in situ from CH4 and radon profiles. Liptay et al. (19) used closed-system incubations to arrive at a value of 1.022 ( 0.008 for 25 °C. As this factor was determined for Nashua soils, we used this same mean value in our calculation. This value was then adjusted for a temperature dependence equal to 0.000435/°C (20). Landfill soil temperature at the 5-cm interval was estimated from air temperature by use of a model that assumed a damping of the diurnal air temperature amplitude (Table 3; 34). For each date, 3-6 samples of downwind plume air were collected. Each of these samples was then run two or three times to determine CH4 concentrations and isotopic values. The final value reported in Table 3 represents the average and standard deviation, to 1σ, for all samples collected on the specific date. Each emitted CH4 sample value was run through the calculation to obtain the oxidation percentage. The percentage reported for each date represents the mean and standard deviation of the oxidation percentages calculated from the samples collected each day. The mean isotopic composition of anoxic zone CH4 (Table 1) was used in the
calculation as there was no significant variation in this parameter between sampling dates. Some uncertainty in the calculated oxidation percentage arises due to the uncertainty of the background air CH4 concentration and isotopic composition, 1.84 ( 0.02 ppmv and -46.38 ( 0.15‰. The percent oxidation was only slightly sensitive to the uncertainty in the isotopic value. Changing the background isotope value by (0.15‰ only altered the percent oxidation by 1% for the most part and occasionally 2% (for example, 15% oxidation is known to (1%, 14-16%). The percent oxidation calculation was more sensitive to the uncertainty in the concentration determination ((0.02 ppmv), which affected it by 2-5% (for example, if the uncertainty was 5%, then a value of 13% is known to 8 or 18%). Of course, as the CH4 concentration in the plume increased, the calculation of oxidation became less sensitive to the uncertainty in the ambient concentration and isotopic value. Confidence in the data is warranted, however, because “Keeling” type plots (where the sample δ13C is plotted against the reciprocal of the CH4 concentration) yielded intercept values similar to the values obtained from our mass balance approach. Finally, one additional note of caution associated with our approach must be offered. When methane is quantitatively oxidized in the soil, none of it escapes the landfill to impart an isotopic signal to the methane in the plume. This would again make our estimate of methane oxidation a lower limit. On the basis of our calculations, oxidation percentage for Nashua range from 0%, for the two dates already discussed, to 23.6% (Table 3). When the data are grouped by season, summer and fall have the highest oxidation with summer at 15.9 ( 4.5% and fall at 21.0 ( 2.7%. Spring has 3.8 ( 2.9%, and winter has 6.8 ( 0.9%. Landfill PLF-A also showed oxidation variation for the five dates sampled (Table 3). At PLF-A, CH4 concentration within the air plume ranged from 1.96 to 2.92 ppmv. Three of the dates had 0% oxidation while the other two dates had 10.5 ( 0.4% and 14.2 ( 0.0%. Methane oxidation within soils is controlled by several factors including temperature, pH, and moisture content (35-40). Temperature appears to be the driving factor for the observed seasonal changes in this study. Oxidation occurs at the outer surface of the landfill. Colder temperatures reduce the microbial activity causing less oxidation at this outer rind. In contrast, composting keeps the interior warm, in turn allowing for CH4 production and for the δ13C of anoxic zone CH4 to remain constant. Czepiel et al. (18) likewise determined that temperature was the primary influencing factor for an oxidation experiment at the Nashua Landfill during the fall of 1994. Although the data sets for both landfills are limited, it is clear that seasonal variations do occur in the oxidation of CH4 escaping the landfill. Weighing each season equally, a value of 11.9 ( 8.0% is determined for annual oxidation at the Nashua Landfill. Our value of 12% is equivalent to the annual landfill oxidation rate determined for Nashua by Czepiel et al. (18), who modeled, from soil incubation data, oxidation rate based on soil factors such as moisture, temperature, and in situ CH4 mixing ratios to arrive at an annual value of 10%. This value was confirmed in a chamber isotope study (19). Czepiel et al. (21) found that the chamber method for determining fluxes yielded results that were similar to the SF6 plume method. It would appear, at least in this study, that the chamber and plume methods for evaluating CH4 oxidation with 13C yield similar results. This study is the third approach used to evaluate CH4 oxidation at the Nashua Landfill. The studies have converged on a values of 10-12% for this site. It appears that chamber capture methods and plume capture methods yield similar results
both in terms of emission estimates (21, 22) and in terms of estimating CH4 oxidation with 13C techniques (18, 19, this study).
Acknowledgments This study was supported by the Southeastern Regional Center of the National Center for Global Environmental Change within the U.S. Department of Energy under Cooperative Agreement DE-FC03-90 ER61010 and by the Landfill Methane Program of the Global Change Division of the U.S. Environmental Protection Agency. We would like to thank the staff and operators of the landfills for their cooperation in this study. We thank Candace Schwartz for assistance in the laboratory.
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Received for review April 9, 1999. Revised manuscript received August 20, 1999. Accepted August 24, 1999. ES9904033
