Methane

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98

METHANE

6.1 INTRODUCTION

Methane (CH4) is a colorless, odorless gas, and the simplest of all hydrocarbon molecules. In the earth's atmosphere, CH4 is ubiquitously present at approximately 2 parts per million by volume (ppmv) and is of considerable environmental concern because it is a greenhouse gas and is increasing in concentration at a rate of about 1% per year (Khalil and Rasmussen, 1990). Even though CH4 is non-toxic, its presence in soil gas creates environmental concern because it can act as an asphyxiant and is an explosion hazard when present at concentrations between 5 and 15 percent by volume in air. Recognizing the potential hazards associated with nearsurface CH4, some municipalities have included assessment and mitigation procedures in their building codes (e.g., City of Los Angeles, 1996; City of Huntington Beach, 1997). The discovery of methane in shallow soil gas can have major financial implications for building construction projects, possibly leading to delays, denial of permits, additional costs associated with engineered mitigation measures, and litigation (e.g. Groves, 2003; Wilson and Sauerwein, 2003). Determining the source and origin of CH4 is important to the determination of environmental liability and to the selection of appropriate mitigation measures.

origin (Whiticar, 1996). Thermogenic CH4, while typically generated at depths of several thousand feet within sedimentary basins, commonly migrates upward to escape to the land surface or accumulate in shallow geologic structures bounded by low-permeability strata. The detection of nearsurface gas seeps been a geochemical prospecting method for locating possible oil and gas accumulation in the deeper subsurface for many years. Biogenic CH4 is produced under anaerobic, near-surface conditions by microbial degradation of organic matter. Such microbially-produced CH4 occurs widely in association with organic-rich sediments and materials, including marine, lake, and river sediments; marshes and swamps; glacial drift; and in landfills and sewers (Schoell, 1988; Coleman et ai, 1995). Roughly 20% of the natural gas in geologic reservoirs has a biogenic origin (Whiticar, 1996), while at least 80% of the CH4 emissions to the atmosphere are of biogenic origin (Khalil and Rasmussen, 1983, Table 4). Landfills are thought to be the single largest anthropogenic source of CH4 in the atmosphere (Chanton and liptay, 2000). There are two principal enzymatic pathways by which biogenic CH4 is produced (Schoell, 1980): (1) acetate fermentation and (2) carbon dioxide reduction. (1) acetate fermentation—CH3COOH -^ CH4 -f CO2 (2) CO2 reduction—CO2 -f 4H2 -> CH4 + 2H2O

6.2 METHANE IN THE ENVIRONMENT

Fires and explosions related to occurrences of methane (and associated hydrocarbon gases) have been attributed to several causes. Underground mine explosions are among the most familiar and deadly events (e.g. Los Angeles Times, 2001). Leaking gas well casings have also been linked to explosions (e.g. Baldassare and Laughrey, 1997). An explosion and fire in Hutchinson, Kansas, was apparently caused by leaks in an underground gas storage facility (Allison, 2001). Leaking well casings and unrecognized geologic conduits allowed gas fi-om the storage facility to migrate under pressure for approximately 12 kilometers. A fatal explosion in Quesnel, Canada, was attributed to a break in a pressurized natural gas pipeline (Quesnel-Cariboo Observer, 1997). Alternatively, the risk associated with nonpressurized sources of biogenic methane is much lower. Methane is a major constituent of natural gas. On the other hand, methane is generally a minor constituent of natural petroleum liquids, with the exception of thermally mature oils and gas condensates. While liquid petroleum products seldom contain significant amounts of CH4, accounts of high CH4 content in soil gas near petroleum spills are common. In addition, several well-documented soil gas studies have shown elevated concentrations of carbon dioxide (CO2) and CH4 in association with subsurface hydrocarbon contamination (Marrin, 1987, 1991; Kerfoot et al., 1988; Robbins et al, 1990; Deyo et a/., 1993; Ririe and Sweeney, 1995; Lundegard et ai, 1998 and 2000). In some investigations, CO2 and CH4 concentrations have been shown to correlate with the distribution of soil contamination, for both volatile and semi-volatile hydrocarbons (Marrin, 1987, 1989, 1991; Conrad et A/., 1999a; GandoyBemasconi et ai, 2004). Methane in the environment can have a variety of sources and origins (Schoell, 1988; Kaplan, 1994). Naturally occurring CH4 can be classified, based on the predominant process by which it formed, as either thermogenic (produced by abiotic processes) or biogenic (produced by biological processes). Thermogenic CH4 is produced at depth within sedimentary basins by the therm^ degradation of sedimentary organic matter, and is commonly associated with coal or accumulations of oil and natural gas. Roughly 80% of the natural gas in geologic reservoirs has a thermogenic

The fermentation pathway involves production of shortchain methylated precursors (commonly acetate but also including formate, methanol, and methylated amines) from the source organic matter. Subsequently, methanogenic bacteria disproportionate the precursor into CO2 and CH4. The CO2 reduction pathway involves reduction of CO2 by molecular hydrogen. Biogenic CH4 formed in near-surface, non-marine environments (e.g., marshes, swamps, and landfills) is primarily formed by acetic acid fermentation (Cole man et ai, 1995). Methanogens are capable of metabolizing a wide variety of substrates. From the standpoint of environmental liability, it is useful to distinguish between near-surface CH4 occurrences into those with natural and anthropogenic origins. Natural occurrences include (1) seeps originating from natural gas accumulations, (2) coal-bed gas, and (3) gas produced by biodegradation of indigenous sedimentary organic matter (e.g., organic-rich shales or marsh deposits). Anthropogenic CH4 occurrences are caused by human activities and therefore are associated with potential environmental liability. Anthropogenic CH4 can be derived fi-om: (1) gas pipelines, (2) oil and gas wells, (3) underground gas storage facilities, (4) sewer pipes and septic systems, (5) buried compost, (6) buried animal waste, (7) landfills, and (8) spilled petroleum. In addition, hazards associated with natural occurrences of CH4 can be increased by certain types of human activities. For example, the underground mining of coal seams creates opportunities for the degassing of coal into mine shafts, and thus increased risk of explosion. Construction of buildings, roads, and parking lots over natural gas seeps may cause up-ward moving gases to accumulate in places where they would otherwise escape to the atmosphere at low concentration or biodegrade to safe levels. While correlations between CH4 concentration in soil gas and petroleum-contaminated soil have led some environmental scientists to infer a direct causal relationship, the process by which carbon in petroleum would be converted to CH4 is not clear. Methanogenesis requires (1) strict anaerobic conditions (i.e., an absence of oxygen), and (2) either molecular hydrogen and CO2, or certain oxygenbearing precursor compounds (e.g., simple organic acids and alcohols). Most natural and refined petroleum liquids

TYPES OF FORENSIC DATA

do not contain significant amounts of the short-chain precursor compounds necessary for direct fermentation reactions. However, under certain conditions, petroleum degradation can generate the necessary precursor compounds for methanogenesis by fermentation (Eganhouse et al., 1993; Revesz et ai, 1995). Though less well understood, some groups of bacteria produce H2 under anaerobic conditions (Claypool and Kaplan, 1974). The molecular H2 is then available for methanogenesis by CO2 reduction (Whiticar et al., 1986). Elevated H2 concentrations have been found in contaminated ground water (Lovely et ai, 1994), and soil gas associated with petroleum contamination (Ririe and Sweeney, 1995; Lundegard et ai, 2000). While these studies indicate that generation of CH4 from petroleum by indirect processes can occur, the abundance of other anthropogenic and natural sources of CH4 requires that the possible causal relationship between petroleum and elevated CH4 concentrations be evaluated on a sitespecific basis. 6.3 COLLECTION OF SOIL GAS SAMPLES

Most forensic investigations of CH4 occurrences involve the collection and analysis of soil gas samplesfi*omthe vadose zone. The collection of valid, high quality soil gas samples is not unduly difficult but it does require care. There is generally nothing in the appearance of a gas sample to tell us at the time of collection whether it is a good or bad sample. Consequently, rigorous, proven sampling protocols should be followed, and procedures should be validated at appropriate intervals. Analysis of samples in the field at the time of collection is highly beneficial for assuring the quality of the final data. The gas sample that is analyzed must represent the chemical composition of the soil pore gas at a known location. The sample must be collected in such a manner that it is not diluted with air, and all the materials that contact a soil gas sample during collection and storage must not adversely alter its chemical composition. Probably the biggest cause of bad soil gas data is dilution of the sample with air as a result of leaky sampling devices and collection probes that are not adequately sealed to the native soil matrix. Several soil gas sampling guidance documents are available that address this and other sampling issues and should be consulted prior to sample collection (USEPA, 1996; CSDDEH, 2004; Wilson et al., 2004).

sources to be distinguished (Schoell, 1980). For example, the content of higher molecular weight hydrocarbon gases (e.g., ethane, propane, butane, and pentane) relative to CH4 may be used. Exploration geochemists have traditionally described the bulk composition of natural gases in terms of their "dryness" or "wetness". A dry gas is one with a high concentration of methane. Conversely, a wet gas is one with a lower concentration of methane and higher concentration of higher molecular weight compounds. A variety of gas dryness and gas wetness indices have been used, depending on the purpose of the investigation and the data that are available. An example of a gas dryness index might be the ratio of CH4 to the sum of C^ to C5 hydrocarbons (Ci/[Ci-C5]). Biogenic gases (e.g., landfill gas) are very dry, consisting predominantly of CH4 and CO2 and do not typically contain significant concentrations of C2 through C5 hydrocarbons (Cj/fCj-Cs] > 0.98) (Rice and Claypool, 1981). In contrast, most thermogenic gases (e.g.,fi-omnatural gas pipelines) do contain significant concentrations of C2 through C5 hydrocarbons (Ci/[Ci-C5] = 0.6 to 1.0). Using a gas dryness index, thermogenic and biogenic gases can often be distinguished (Figure 6.4.1). Natural gas at different depths or reservoir zones within a geologic basin typically varies in bulk composition or dryness. Consequently, produced natural gas, natural gas seepage, and gas leaking from damaged or improperly abandoned wells tends to vary in composition. From shallow to deep locations within a basin, natural gas tends to vary from dry biogenic gas, to wet thermogenic gas, to dry thermogenic gas (Figure 6.4.2). Landfill gas is a consideration in the forensic investigation of many CH4 occurrences. Landfills generate and emit large quantities of CH4 produced by the anaerobic decomposition of the wastes they contain. Emissions from landfills account for an estimated 5-15% of the global anthropogenic sources of atmospheric CH4 (Doom and Barlaz, 1995). Undiluted landfill gas typically contains 50-60% CH4, 40-50% CO2, and negligible oxygen. This composition does not vary greatly, apparently reflecting similarity in the composition of landfill waste and the processes of methanogenesis. The consistently high content of CH4 and CO2 (in undiluted gas) is a useful preliminary indication of the possible contribution by landfill gas. However, gas of other origins can have similar concentrations of CH4 and CO2. Landfill gas commonly contains trace concentrations of other constituents, including

6.4 TYPES OF FORENSIC DATA

Rarely is a single type of data sufficient to resolve an environmental forensics question. More often, it is the integration of several types of information that leads to an effective resolution. In the case studies presented here, it will be evident that it is the S5mthesis of geological, geochemical, and historical land use data that leads to a credible interpretation of the origin and source of the soil gas CH4. Spatial relationships and trends in the data are particularly important. Below, several types of geochemical data useful in investigations of soil gas CH4 are discussed. 6.4.1 Molecular Composition Data Soil gas compositional analysis is generally the starting point for most investigations, since one first needs to determine the abundance of CH4 and its spatial variation. Real-time, gas chromatographic analysis in the field is strongly preferred so that the quality of soil gas samples can be assured and useful sampling locations can be properly selected. Sometimes the composition of the trace constituents in the CH4 gas mbcture is very revealing. In some instances, gross gas compositional data alone allow different potential gas

99

Thermogenic Gas

Biogenic Landfill Gas

Dry

Wet 1.0

2.0

3.0

— I —

—1~

4.0

5.0

Log (C1/C2J Figure 6.4.1 Frequency histogram of a gas dryness index of thermogenic gases from the Appalachian and Turim Basins (Laughrey and Baldassare, 1998; Chen et al., 2000) and biogenic landfill gases from Pennsylvania (Baldassare and Laughrey, 1997).

100

METHANE

Thermogenic vs. Biogenic Gas dry biogenic -j o °

-100thermogenic

2000 H ^

-200-

I O Q ^

-300H

S 4000Q. 0)

Q

6000 H

dry thermogenic

800020

40 60 % CH.

80 100

^00-120

r—p-?—p-T—j-100 -80 -60

-40

-20

6 ^^C CH4 (°/oo) Figure 6.4.2 Plot of subsurface deptti versus percent CH4 in gas from thie Sverdrup Basin (after Snowdon and Roy, 1975).

halogenated hydrocarbons. The presence of halogenated volatile organic compounds in a gas sample is strong evidence of an anthropogenic component in the gas mixture. Such compounds can also be derived directly from spills of products such as chlorinated solvents into soil and groundwater. Gas transmission companies typically add low concentrations of odorants (e.g., mercaptans), or other tracers, to their gas to aid in the detection of leaks and to enhance consumer safety. The detection of these compounds is strong evidence that pipeline gas is present. Local gas companies should be contacted to determine what compounds are used in a particular market area. 6.4.2 Stable Isotope Data

Carbon and hydrogen isotopic data have been successfully used to discriminate natural gases (including CH4) of various origins (Figure 6.4.3; Schoell, 1988; Suchomel et ai, 1990; Coleman et al, 1995; Baldassare and Laughrey, 1997; Hackley et ai, 1999). Carbon isotopic data are typically reported using the delta (8) notation relative to the PDB international standard (Hoefs, 1980). Similarly, hydrogen isotopic data are reported using the delta (5) notation relative to the SMOW international standard. 6 X (per mil) = ( ^ e - R s t a n d a r d ) ^ ^^3 Standard

where X refers to either ^^C or D (D represents deuterium, ^H) and R refers to either the ^^C/^^C or D/H isotopic ratio. Analytical laboratories typically request a gas sample that contains a few milliliters of the compound of interest (e.g., ~ 100 ml of a gas that is 2% CH4). Analytical precision is typically about 0.2 per mil (%o) for 8 ^^C and about 1 per mil (%o) for 8 D on standard size samples. The carbon isotopic composition of CH4 depends on the composition of the organic source, fractionations that depend on the process by which it is formed, and postgeneration alteration. Microbial methanogenesis yields CH4 that is highly depleted in 8^^C of relative to the organic matter from which it is formed. Depending on environmental conditions and the microbial pathway, 8^-^C of biogenic CH4 can be 25 to 90 per mil more depleted (i.e., negative) than the source organic matter. Once formed, transport processes in shallow subsurface settings do not significantly

Figure 6.4.3 Plot of hiydrogen and carbon isotopic composition of CH4 in gas samples of different origins and from different sources. Crosses are tfiermogenic gas samples. Open circles are biogenic gas samples. Arrow indicates general direction of isotopic shift resulting from partial oxidation of CH4. Genetic fields after Coleman et al. (1995). Data from various sources (Schoell, 1980; Rigby and Smith, 1981; Smith et a!., 1985; Whiticar et al., 1986; Coleman et al., 1988; Jenden et al., 1988; Coleman et al., 1993; Jenden et al., 1993; Kaplan, 1994; Baldassare and Laughrey, 1997; Laughrey and Baldassare, 1998; Lundegarde\a\., 1998; Conrad e\a\., 1999a,b;Chene\a\., 2000; Pierce and LaFountain, 2000)

alter the stable isotopic composition of CH4, but partial oxidation by methanotrophs can impart large isotopic changes in the residual CH4 (see arrow in Figure 6.4.3; Coleman et at., 19%). Together, carbon and hydrogen isotopic data can often be used to distinguish gas samples of different origins (e.g. Schoell, 1980; Coleman etaL, 1995; Whiticar, 1996). Compositional ranges have been suggested for CH4 produced by acetate fermentation, CO2 reduction, and by thermogenic processes (Figure 6.4.3). These suggested ranges are useful guides but should not be used as absolute indications of gas origin because exceptions do occur. Methane in natural gas produced from oil and gas reservoirs and gas associated with coal beds is predominantly thermogenic in origin and generally has a carbon and hydrogen isotopic composition that distinguishes it from biogenic gas. Such CH4 generally is more enriched in ^^C than biogenic CH4 produced by CO2 reduction, and more enriched in deuterium than biogenic CH4 produced by acetate fermentation. Biogenic CH4 produced by acetate fermentation is generally more enriched in ^^C and more depleted in deuterium than biogenic CH4 produced by CO2 reduction (Whiticar et al, 1986). Isotope studies have shown that methanogenesis by acetate fermentation predominates in near-surface freshwater environments such as lakes and swamps (Whiticar et al, 1986), yet it has been shown that in some systems the fraction of CH4 produced by carbon dioxide reduction and acetate fermentation varies with time (Martens et al, 1986; Sugimoto and Wada, 1995). Biogenic CH4 in marine sediments and glacial drift is predominantly produced by the CO2 reduction pathway (Claypool and Kaplan, 1974; Coleman etal, 1988).

TYPES OF FORENSIC DATA

101

The vast majority of samples of CH4 produced in land- within or near the acetate fermentation field, most values fills plot within the acetate fermentation field (Figure 6.4.4). are shifted toward higher 8 ^^C and lower 8 D values than Those samples that plot within the thermogenic gas field landfill CH4. In fact, a significant number of the samples of likely represent the effects of partial oxidation of landfill CH4 produced through the process of petroleum biodegraCH4 or mixing of landfill and thermogenic CH4. The iso- dation have isotopic compositions outside the previously topic composition of CH4 produced from biodegradation of known range for biogenic CH4 of any origin. Careful exampetroleum contamination has not yet been studied in a wide ination of data sets from individual sites also shows that range of settings. Available data, however, suggest some within the vadose zone, partial or complete oxidation of CH4 interesting characteristics (Figure 6.4.5). While data plot producedfi^ompetroleum contamination seems to be common (e.g., Conrad et al. 1999b) and is sometimes reflected in its isotopic composition. Landfill Gas The hydrogen isotopic composition of biogenic CH4 is a -100function of the isotopic composition of the ambient water and pathway-dependent fractionation. In the case of CO2 thermogenic reduction, all four H atoms in CH4 come fi-om ambient water (Daniels et al., 1980). In the case of acetate fermentation, three of the H atoms originate from the methyl group of acetate, and one comes from ambient water (Pine and -200Barker, 1956). As a result, CH4 produced by CO2 reduction is more sensitive to the hydrogen isotopic composition of ambient pore water than CH4 produced by acetic acid CO2 ^ X fermentation (Figure 6.4.6). reduction o Carbon isotope ratios are used to estimate end member Q to abundances in two-component mbctures, provided the con-300 H centration and isotopic composition of the CH4 (or other gas species) in each end member is known. This is made possible by the degree of natural variation in isotope ratios Ac. ferment. and the high precision with which isotope ratios can be measured. The applicable mathematical expression is (Schoell et al., 1993) -400-

T

-120

-80 -80

-100

-60

^0

20

A=

6 ^^C CH4 (7oo)

Q2*(^M-^2) [Q2*(^»M - ^.2) - Ql*(S«M - ^/l)]

In the above expression, /i is the fraction of gas 1 in the mixture. C^ and Qg are the concentrations of species / Figure 6.4.4 Plot of hydrogen and carbon isotopic com- (e.g., CH4) in gases 1 and 2, respectively. 8,1 and 8,2 are the position of CH4 in gas samples collected from land- isotopic compositions of species i in each end member. 8,^ fills. Data from various sources (Coleman et a!., 1993; is the isotopic composition of species / in the gas mkture. Kaplan, 1994; Baldassare and Laughrey, 1997; Pierce and Partial oxidation of CH4 can have a substantial effect on LaFountain, 2000). the isotopic composition of residual CH4 and associated CO2. The potential for oxidation effects must be carefully considered before makingfinalinterpretations of stable isoPetroleum Contamination Derived topic data. Using a Rayleigh distillation model, the isotopic -100shifts caused by partial CH4 oxidation can be predicted for thermogenic

-^uu — ^

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CO2 ^ reduction

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o

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I ' -40

-20

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1

1

1

,

.

,

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.

100

Figure 6.4.5 Plot of hydrogen and carbon isotopic comFigure 6.4.6 Predicted relationship between the hydroposition of CH4 thought to be derived from biodegradation gen isotopic composition of CH4 and H2O for the acetate of spilled petroleum in near-surface environments. Data fermentation pathway (solid line) and the CO2 reduction from various sources (author's unpublished data; Conrad et al., 1999a,b). pathway (dashed line) (after Sugimoto and Wada, 1995).

102

METHANE

1

1 1

D)

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a =1.024

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\

\

0.8- •

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en

0 0 0 0

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0 0 0 0

y-

Age (years BP)

\ ' 1 ' 1 ' 1 1 1 1 1 1 -80 -60 -40 -20 0 20 40 6^3c(7oo)

Figure 6.4.7 Plot showing the isotopic shift in residual CH4 and produced CO2 during oxidation. An isotopic fractionation factor [a) of 1.024 was used in the calculations.

assumed isotopic fractionation factors and initial isotopic compositions (Hoefs, 1980). For example, -55 per mil CH4 might shift to -34.5 per mil after 60% oxidation and to -2.6 per mil after 90% oxidation, using a fractionation factor of 1.024 (Figure 6.4.7). Deuterium is also enriched in residual CH4 undergoing oxidation (Coleman et ai, 1981; Whiticar, 1996). Rates of CH4 oxidation by methanotrophs in natural environments can be appreciable. Using measured emission rates and isotopic fractionation of residual CH4, CH4 oxidation in landfill cover soil has been estimated. Schuetz et al. (2003) estimated average CH4 oxidation rates of 1.3-1.5g m~^ d~^ in soil cover over a landfill in France. Methane oxidation rates up to 166g m"^ d~^ have been measured in soil microcosms (Kightley et a/., 1995). Using a method based on measured diffusion rates for CH4 and oxygen, the author estimated an oxidation rate of 0.6g m"^ d"^ for CH4 diffusing upward from a zone of free product at a petroleum release site. Although rarely used in environmental forensic studies, there may be useful supplementary information in the carbon isotope ratio of individual C2+ gaseous hydrocarbons Games, 1983; Schoell et al, 1993). Such data are likely to be most applicable to the recognition and allocation of thermogenic gas sources where CH4 isotopic compositions are very similar or microbial alteration has affected isotope ratios.

0 IT) CD

0 CO CJ)

0 f^ G)

0 CX) O)

0 Oi CD

Year Figure 6.4.8 Temporal changes in ^"^C concentrations. (A) Change related to radioactive decay of ^"^C. (B) Change in ^"^C concentration in atmospheric CO2 related to nuclear bomb testing in the 1950s and 1960s. Note that CH4 derived from very old (>50, 000 years) carbon sources like petroleum will have a^'^C concentration of 0 pMC.

the ^^C concentration in its tissue gradually decreases as the ^"^C undergoes radioactive decay with a half-life of 5730 years. After approximately 50,000 years, the remaining ^"^C is too dilute to detect by standard methods. Organic matter greater than 50,000 years old, and CH4 derived from such organic matter, will have a ^"^C concentration of OpMC. This would include CH4 formed from coal, oil, kerogen, and petroleum products made from these materials (e.g., gasoline, kerosene, and diesel fuel). Methane formed from organic matter less than 50,000 years old will have detectable ^'^C (greater than OpMC) and can be "dated". A ^"^C "date" obtained on CH4 reflects not the time of methanogenesis, but the age of the carbon source from which CH4 formed. The second factor giving time significance to ^'^C concentrations is the enormous increase in ^^C resulting from 6.4.3 Radiogenic Isotopic Data atmospheric testing of nuclear bombs during the 1950s Carbon-14 analysis has forensic value because of the infor- and 1960s (Figure 6.4.8b; Levin et al., 1980). Consequently, mation it provides about the age of the organic carbon organic material formed since this time has an elevated ^^C source. Carbon-14 is present in all living things. It is a nat- concentration. Methane produced by bacterial degradation urally occurring isotope of carbon that is formed in the of such young organic matter will also contain the so-called upper atmosphere by the reaction of cosmic-ray neutrons "bomb carbon", and have a ^'^C concentration greater than with nitrogen (Faure, 1977). Plants extract CO2 from the 100 pMC. This is typically the case for landfill and sewage atmosphere, thereby incorporating a ^'^C concentration in gases since they are largely produced from organic material their cells that reflects the ^"^C concentration in atmospheric less than a few decades in age (Coleman et ai, 1995). CO2. This ^'^C concentration is passed up the food chain. By considering mathematical mixing relationships, ^"^C In most environmental investigations, the ^^C concentration results can be used to investigate source contributions to in a sample is reported in terms of percent modem car- a methane sample of mbced origin. In a two component bon (pMC), where 100 pMC is defined as the "normal" ^^C mixture, theft-actionof CH4 derived from each source can concentration in atmospheric CO2 prior to anthropogenic be calculated ft-om the measured ^"^C concentration of the disturbances such as nuclear bomb testing and extensive CH4, and the anticipated ^^C concentration of CH4 from the burning of fossil fuels (Stuiver and Polach, 1977). two suspected sources. The time significance of ^"^C concentrations comes from (^"^CgPL ~ ^2) two factors. The first is the natural decay of this radioactive fi = isotope (Figure 6.4.8a). When a plant or animal dies,

CASE STUDIES

where fj is the fraction of CH4 from source 1 in the sample, and ^^CspL, ^^c^, and ^^€2 are the ^^c concentrations in CH4 in the sample, source 1, and source 2, respectively. Much like ^"^C, tritium has forensic value in identifying CH4 produced from modem carbon sources. Tritium is a naturally occurring radioactive isotope of hydrogen (^H) and has a half-life of 12.3 years. Its content in the atmosphere and hydrosphere was also markedly increased by atomic bomb testing in the 1950s and 1960s. Consequently, elevated tritium content in soil gas CH4 could indicate the CH4 was produced from organic matter no more than a few decades in age. Methane in some landfill gases is very high in tritium {ColemmetaL, 1995). 6.4.4 Microbial Data

103

active methanogenesis (and CH4 oxidation) is taking place anagaki et ai, 2004; Kleikemper et al, 2005). By combining information on the distribution of active methanogens, methanotrophs, CH4, oxygen, and organic carbon types, a fully integrated picture of biogenic CH4 generation and oxidation might emerge. In the mean time, established methods for soil microcosms can be used for demonstrating the CH4 generation potential of different soil/organic matter samples, and perhaps the relative generation rates from different samples (Figure 6.4.9). In a set of anaerobic microcosm experiments, several soil/organic matter samples were shown to naturally generate CH4. Methane generation was most rapid for samples containing modem plant debris in landscaped soil. Soil containing tarry, weathered cmde oil also generated CH4, but at a lower rate.

While still developing, methods for the analysis of microbial DNA have promise as a way to identify soil zones where 6.5 CASE STUDIES 25 (148 days)

E 20H O 0}

o

CO Q. (0

•o CO

o I

w 15 H

Tarry soil

Several case studies, based on the author's personal investigations and others in the literature, are presented to illustrate application of the forensic tools discussed above. Note that in each case the spatial relationships of data and land use information are key to the interpretations. 6.5.1 Case Study #1

j^

Surface Ag soil Ag soil soil #1 #2

Figure 6.4.9 Methane concentration in the headspace of canned soil samples after 148 days of incubation. The tarry soil contains weathered crude oil from a former oilfield sump. The surface soil is a sandy soil that contains 4 weight percent organic carbon from modern plant debris. Ag Soil #r and Ag Soil #2 are clay-rich agricultural soils that contain plant debris.

A service station, located in an urban area near a lake, was the site of a large accidental release of gasoline in 1980. An estimated 303 m^ (80,000 gallons) of leaded, premium gasoline was released. Light non-aqueous phase liquid (LNAPL) gasoline eventually covered an area roughly the size of a city block (~2 acres; Figure 6.5.1). During subsequent monitoring of a soil vapor extraction system, it was noticed that CH4 comprised a substantial percentage of the influent vapor. The concentration of CH4 in the SVE influent was greater on average than the concentration of total volatile hydrocarbons, and reached values greater than 10% (by volume). The CH4 was initially attributed to bacterial degradation of the spilled gasoline. However, the persistence of high CH4 concentrations over time raised questions about its origin and led to further investigation (Lundegard et at., 1998, 2000).

Figure 6.5.1 Site map for case study #7 showing original gasoline LNAPL plume and CH4 concentrations measured in monitoring wells.

104

METHANE

Exploratory borings and review of historical land use records revealed that the site vicinity was underlain by other carbon sources, including fill material with abundant zones of sawdust and wood, as well as historic lake sediments. Forensic investigations focused on whether gasoline or these other carbon sources could be the source of the CH4. Soil gas was collected from 23 monitoring wells within the limits of the original gasoline spill and beyond. Gas compositional analysis was conducted in the field with a portable gas chromatograph. Samples were also collected for subsequent isotopic analysis in the laboratory. Methane content of soil gas samples varied by several orders of magnitude, fi-om 0.002 to 26% (by volume) (Figure 6.5.1). Carbon dioxide concentrations ranged from 0.265 to 27% (by volume). High CH4 and CO2 concentrations were not restricted to the original area of contaminated soil and LNAPL, as is the case at other contaminated sites (Marrin, 1987, 1989, 1991; Suchomel et al, 1990). Elevated CH4 concentrations were encountered upgradient, downgradient, and cross-gradient of the original LNAPL plume (Figure 6.5.1). The occurrence of high CH4 concentrations well beyond the limits of the original gasoline spill suggested that, perhaps, gasoline was not the sole source of the CH4. Within the limits of the gasoline spill some C2-C6 gasoline constituents were observed using gas chromatographic techniques. However, in high-CH4 samples well beyond the limits of the original gasoline spill, less than 0,01% €9^ hydrocarbons (ethane and heavier) were observed. Low concentrations of Cg-Cg alkanes are characteristic of biogenic gas accumulations. Stable isotope data on the CH4 showed that it was produced bacterially by the acetate fermentation pathway (Figure 6.5.2). Together with the gas composition the isotope data rule out a thermogenic source of the CH4. Sources of thermogenic gas would include natural seepage from subsurface oil and gas deposits, and leaking gas pipelines. However, stable isotope data alone did not resolve whether the source of the CH4 was the spilled gasoline or the other carbon sources in the environment (e.g..

-100" thermogenic

^

-200 H

X

o '^ -300 H

CO2 reduction Ac. ferment o Case study #1 + Case study #2

-400

^ 120

'

I ^ \ ' r -100

-80 -60 6^3CCH4(7oo)

\ -40

-20

Figure 6.5.2 Hydrogen and carbon isotopic data for CH4 samples in case study #7 (open circles) and case study #2 (crosses).

woody fill debris or lake sediments). Carbon isotopic analysis of wood fi-om the fill beneath the site and of gasoline from a monitoring well indicated that the wood and gasoline were essentially indistinguishable (8 ^^C wood = -25.95%o; h^^Q gasoline = -25.83%o). Therefore, stable isotope analyses, while identifying the predominant mechanism for CH4 formation (acetic acid fermentation), were unable to uniquely identify the carbon source for the CH4. When potential carbon sources have very similar stable isotopic compositions, or when bacterial oxidation has occurred, the source and origin of near-surface CH4 is more difficult to determine. Under these conditions, ^"^C (or tritium) data can be a powerful supplement to stable isotope data. Five high-CH4 samples were selected for ^'*C analysis. Included were tsvo samples more than 49 m beyond the limits of the original spill, on both the upgradient and downgradient sides (Figure 6.5.3). The ^^C concentrations in the CH4 in the five samples were 60, 87, 92, 96, and 97pMC, respectively. The two samples farthest from the original gasoline spill had the highest ^^C concentrations. If CH4 were derived wholly from the spilled gasoline its ^"^C concentration would be 0.0 pMC. It is clear, therefore, that the CH4 in these samples is not derived predominantly from gasoline. Using the ^^C data, the possible proportions of CH4 from different sources can be explored through a simple mixing model. The sample with a CH4 ^^C concentration of 60pMC came fi-om a well within the original extent of the gasoline plume. The ^^C concentration of this sample could be explained by formation solelyfi-omorganic matter approximately 4000 years old. Alternatively, it could represent a mixture of two or more carbon sources. For example, if a two-component mbrture involved contributions from the woodyfillmaterial (estimated ^^C concentration of 97pMC) and the gasoline Q^C concentration of OpMC), 38% of the CH4 could theoretically have been produced from gasoline. Any contribution of CH4 fi-om organic matter older than the woodfill,such as the underlying lake sediments, would reduce the calculated percentage of gasoline-derived CH4. The ^^C concentration in the other four samples suggests that the actual contribution of CH4fi-omgasoline is probably much lower than 38%. The CH4 in the other four samples, with ^"^C concentrations of 87, 92, 96, and 97pMC, could be produced entirely from organic matter with an average age of between 200 and 1100 years. While the precise age of lake sediments underlying the site is not known, their age could plausibly lie within this range. Derivation of CH4 exclusively from the wood fill beneath the site (estimated age of 150-250 years) would yield a CH4 ^^C concentration of approximately 97pMC. One of these four CH4 samples, therefore, could be completely derivedfi-omthe woodfill.A two-component mixture calculation for these four samples, assuming the two sources are the gasoline (OpMC) and the wood fill debris (97pMC), indicates that the maximum contribution fi-om gasoline rangesfi-om0 to 10%. Contributions of CH4 from organic matter in the lake sediments would reduce the calculated contribution from gasoline. With the knowledge that two of these four samples came from well beyond the limits of the original gasoline spill, it is very unlikely that more than a few percent of the CH4 in these samples was derived fi-om the gasoline spilled at the site. The ^^C data indicate that the primary sources of the CH4 are the woody debris within thefillmaterial and the underlying lake sediments. 6.5.2 Case Study #2

At a large industrial facility where crude oil was stored, a number of accidental releases of the crude oil were known

CASE STUDIES

105

87.4

96.8

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59.5 30 m

LNAPL A

(flow 1

96.8

92.8

96.2 to Figure 6.5.3 Site map for case study #7 showing CH4 ^^C results. Note that CH4 derived from gasoline would have a ^"^C concentration of OpMC. High values indicate little input from gasoline.

to have occurred. Natural seeps of liquid and gaseous hydrocarbons from underlying petroleum reservoirs are also well known in the region surrounding the facility. Numerous pipelines and utility corridors exist in the area. In an area of the site where the soils were substantially impacted by spilled crude oil, high concentrations of combustible hydrocarbons were detected in soil gas. As a result of a property transaction, the source of the vapors was a liability issue and an important factor in determining the suitability of the area for construction. There was also interest in the fate of the CH4 and whether there was significant upward transport toward the ground surface. Shallow soil gas samples were collected in the area of the spilled crude oil. Driven soil probes were used to obtain samples at 0.6 m depth below grade. Gas chromatographic analysis revealed that the soil gas contained up to 24% CH4 (by volume) as the predominant hydrocarbon. C2 to C5 hydrocarbons were present in low concentrations, from a few ppmv to over lOOOppmv (Ci/iCi-Cs] > 0.98). Having identified areas of very high CH4 concentrations, vertical soil gas profiles to a depth of 6.1 m were conducted in order to understand better the occurrence and source of the CH4. The soils throughout the profiled depth range contained substantial amounts of crude oil. Carbon and hydrogen isotopic data on CH4 in selected samples confirmed that the CH4 is biogenic rather than thermogenic in origin (Figure 6.5.2). This origin is also consistent with the low concentrations of C2 to C5 hydrocarbons. Carbon-14 analysis indicated that the CH4 and associated carbon dioxide contained no detectable ^^C, indicating that these components were derived from a carbon source greater than 50,000 years in age. Together, this information rules out several possible sources for the CH4. The ^"^C data rules out "modem" carbon sources such as buried refuse, compost, plant debris, and sewer gas. The stable isotope data excludes sources of thermogenic gas such as leaking gas pipelines and natural seepage from thermogenic gas in the underlying sedimentary basin. Based on these geochemical data, the CH4 could be produced in the near-surface soil environment from biogenic degradation of the spilled crude oil, or produced biogenically at greater depth from

naturally occurring, old (>50,000 years) organic matter and transported to the near-surface soil environment. A detailed vertical profile of soil gas composition was developed by careful sampling at different depths within the zone of contaminated soil from 0.15 to 6.1 m below ground surface. Soil gas profiles shed light on both the source and fate of the CH4. In the upper 1.5 m the CH4 concentration increased dramatically with increasing depth, leveling off at between 70 and 75% (by volume) at 1.5 m and greater (Figure 6.5.4). Oxygen concentration showed the opposite depth trend, decreasing sharply in the upper 0.6 m and reaching zero at 0.9 m and greater, indicating the occurrence of anaerobic conditions. Carbon dioxide concentrations varied in a manner similar to CH4, increasing rapidly with increasing depth to about 23% at 2.1 m and greater. However, in the aerobic zone of the soil column (upper 0.9 m) the CO2/CH4 ratio is significantly greater than it is below. This observation, together with the fact that CH4 in the upper 1.5 m has a heavier (more positive) carbon isotopic composition, and CO2 has a lighter (more negative) carbon isotopic composition than deeper samples, is evidence that CH4 oxidation is occurring near the ground surface (Figure 6.5.5). In general, the presence of oxygen in soil gas will indicate that CH4 oxidation is likely occurring, provided that the oxygen was not derived from an air leak in the sampling equipment. The vertical profile of H2 concentration was found to be important to understanding the source and origin of the CH4 (Lundegard et at., 2000). From a depth of 0.15 to 0.9 m, H2 concentration increased more than 2 orders of magnitude from 41ppm to 15,378 ppm (1.5%). Below 0.9 m, the H2 concentration decreases sharply to less than 100 ppm by 2.1m depth, indicating that H2 utilization is occurring within the anaerobic zone of the soil column. In addition, the heavy carbon isotopic composition of CO2 in the anaerobic zone (greater than 10%o) is evidence of methanogenesis by CO2 reduction (Figure 6.5.4). These geochemical depth trends and isotopic data strongly suggest that CH4 is being generated from petroleum contamination in the interval from 0.9 to 2.1 m by the reduction of CO2 with molecular Hg. Thus, while the compositional and isotopic data indicate a biogenic origin for the CH4 at this industrial facility, it is the detailed

106

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Figure 6.5.4 Detailed vertical profile of soil gas composition at industrial facility (case study ^2). Carbon isotope results for CH4 and CO2 are posted next to data points. Entire soil column is contaminated with crude oil. Upper 1.5 m is a zone of inferred CH4 oxidation.

increase associated with the methanogenic degradation of solid, ligno-cellulose into the gases, CH4 and CO2 (approximated by equation below). 2CH20(s)->CH4(g) + C02(g)

O O O

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Second, the waste within the landfill compacts under the weight of overburden, which reduces the porosity of the waste. Near a municipal waste landfill, elevated concentrations of CH4 in soil gas samples from site periphery and offsite locations raised questions about the effectiveness of the facility's gas recovery system (Pierce and LaFountain, 2000). Landfill gas recovered by the vapor extraction system is burned at a flare station. Effectiveness of the recovery system and regulatory compliance is monitored by analyzing samples from numerous gas probes along the periphery of the facility (Figure 6.5.6). Unexpectedly high gas pressures (over 10 inches of water) and CH4 concentrations (over 60%) in some locations raised suspicions about the source oftheCH4.

6i3ccH4(7oo) Figure 6.5.5 Carbon isotopic composition of CH4 and associated CO2 for soil gas samples collected from depthis greater than (open circles) and less than 1.5 m (solid squares) at the site described in case study #2. The isotopic enrichment of CH4 and depletion of CO2 in the shallow samples is evidence of CH4 oxidation.

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1

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depth profile that suggests the crude oil contamination as the source of the CH4. 6.5.3 Case Study #3

Federal and state regulations in the US require that landfill gas containing greater than 5% CH4 be prevented from migrating beyond the facility boundary. Generally, if CH4 is found near a landfill, landfill gas is presumed to be the source. Such occurrences lead to compliance and sometimes litigation problems for the owner of the landfill. Advective escape of landfill gas can occur because positive pressure develops within the pores of the landfill waste. Pressures develop for two reasons. First there is a volume

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Figure 6.5.6 Map of landfill site described in case study #3. Surrounding features include residential neighborhoods, commercial property, an oil field, and an underground gas storage facility. Gray circles indicate those peripheral gas probes that showed anomalous methane concentrations or gas pressures.

CASE STUDIES

The landfill is adjacent to residential and commercial properties, as well as an oil and gas field, and an underground gas storage facility (Figure 6.5.6). In the investigation of the CH4 anomalies, gas samples were collected from siteperiphery gas probes, offsite gas probes, the landfill flare station, the oil field, and the gas storage reservoir. Stable isotope data showed that most of the samples from the siteperiphery gas probes plot within thefieldgenerally accepted for biogenic landfill gas (Figure 6.5.7). Gas samples from the oil field and the gas storage reservoir plot within the thermogenic field. And, several of the gas probe samples also plot within the thermogenicfield,suggesting that some of the anomalous CH4 localities are affected by non-landfill thermogenic gas. A smaller number of samples were also analyzed for Carbon-14 activity (Figure 6.5.8). As would be expected for thermogenic gas, CH4 from the gas storage reservoir and the nearby oil field had ^'^C activities of less then 1 pMC. A sample of landfill flare gas had a ^^C activity of 119pMC, a value consistent with an origin from post1950 organic refuse. Methane from some of the peripheral gas probes had ^"^C activities consistent with a landfill gas origin (>119pMC), while at other peripheral and offsite locations lower ^"^C activities were evidence of gas of mixed origin containing variable amounts of non-landfill thermogenic gases. Methane from one of the peripheral gas probes had a ^"^C activity of 2.5 pMC, indicating that this gas was about 98% thermogenic. These results confirmed that several of the site-periphery and offsite locations are affected by non-landfill thermogenic gas. 6.5.4 Case Study #4

Owners of a house built on remediated petroleum-impacted soil had trees that were not growing well. Methane derived from residual soil contamination was alleged to be the cause of the stressed vegetation. The sandy soil originally contained tarry crude oil contamination. The soil had been remediated to less than lOOmg/kg total petroleum hydrocarbons, then re-placed and re-compacted before housing construction. An assessment of indoor air failed to detect elevated concentrations of CH4. A soil gas survey found

107

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150

Figure 6.5.8 Plot of hydrogen isotopic composition and ^"^C activity of methane samples collected in case study #3. Note that several gas probe samples do not plot in the vicinity of the landfill flare gas, and two gas probe samples plot very close to samples from the oil field and gas storage resen/oir. Data from Pierce and LaFountain (2000).

highly variable concentrations of CH4 in soil, from 0.15 to 3 m depth. Concentrations varied fi-om less than lOppmv to 12%. Carbon-14 analysis of the soil gas sample with the highest CH4 concentration yielded a result of 109 pMC, a value which indicates the CH4 was predominantly derived from post-1950 organic matter. The low concentration of petroleum hydrocarbons in the remediated soil, therefore, did not significantly contribute to the CH4. Microcosm experiments also demonstrated that local soils with modem organic debris readily generated methane when they became sufficiently wet (Figure 6.4.9). Agricultural experts attributed the stressed vegetation at the site to the unfavorable physical character of the densely compacted sand.

-100+ O D A

gas probes LF flare gas Oil field Gas reservoir

6.5.5 Case Study #5

-200I O Q -300 H

-400-100

1 80

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-20

Figure 6.5.7 Hydrogen and carbon isotopic data for CH4 samples in case study #3. Note that several gas probe samples plot in the thermogenic part of the diagram, near samples from the oil field and the gas storage reservoir. Data from Pierce and LaFountain (2000).

Methane was encountered during a soil gas assessment of a small petroleum spill at a 2900-acre residential development site. Follow-up investigations revealed that shallow CH4 anomalies were more widely distributed than the petroleum contaminated soil and multiple theories for the source of the CH4 emerged. Sources that were initially considered included leaking pipelines, spilled petroleum, organic matter in lowland or wetland soils, animal manure, sedimentary organic matter in underlying geologic strata, and landfill waste. Site-wide reconnaissance, review of historic air photos, and the molecular composition of soil gas helped to narrow the list of likely sources. Stable and radiogenic isotopic data confirmed that the CH4 was biogenic and derived from recent organic matter (Golightly, personal communication 2004). After soil gas sampling at every graded lot, it was noted that the occurrence of elevated CH4 correlated strongly with the location of re-compacted fill material (Figure 6.5.9; AMEC Earth & Environmental, 2001a). The re-compacted fill material consisted of on-site native soil that was removed during mass grading of the site and replaced in areas where soil was needed to reach final design grade. Mass grading engineering maps showed where soil was removed (i.e., cut areas) and where re-compacted fill was placed (i.e.,fillareas). Of the 128 lots

108

METHANE

50-j

The most important discovery of this case study was that CH4 generation and accumulation could be initiated in native soils simply by changing their physical character through routine grading operations. Based on the findings of this and other cases in the area, the County planning and land use department passed an ordinance specifying soil gas assessment and mitigation requirements in housing developments where mass grading occurs. An action level was set at 12,500 ppmv CH4 in soil gas. Mitigation is required on all lots with 3 m (10 feet) or more offill,unless soil gas testing demonstrates that CH4 is not present above the action level.

25-

6.6 SUMMARY

150-1 cut lots

125-J 100 H 0)

75-^

0

fill lots

r 0.5

1.5 2.5 3.5 4.5 >5 Methane Concentration (%) Figure 6.5.9 Histogram of soil gas CH4 results from a residential development site (case study # 5 | White bar shows data from cut lots. Stippled bars show data from fill lots. Data from AMEC Earth & Environmental (2001a).

built on cut areas, none had soil gas CH4 concentrations over 0.1%. In marked contrast, 85% of the 59 lots built on fill had soil gas CH4 concentrations over 5%. Tliis strong geographic relationship demonstrated that the CH4 was produced from organic matter within the fill material. The fill differed from the in-place native soil only in having a higher moisture content (water was added during fill operations) and greater degree of compaction. These two factors evidently allowed anaerobic conditions to develop and methanogens to produce CH4fi^omnative organic matter. These processes were further demonstrated in laboratory column studies (AMEC Earth & Environmental, 2001b). Native soil with 4% organic matter was incubated in sealed columns for 24 weeks. Shortly after water was added in the eleventh week, the headspace CH4 concentration rose sharply and reached values as high as 26% (Figure 6.5.10). 30-1

Discovery of near-surface occurrences of CH4 in soil gas can generate alarm as well as financial and legal consequences for responsible parties, property owners, developers, and other parties. Determining the origin and source of near-surface CH4 is very important to the determination of environmental liability and to the selection of appropriate mitigation measures. Near-surface occurrences of CH4 can have a variety of natural and anthropogenic sources. In many instances, certain sources of CH4 can be ruled out by molecular compositional and/or isotopic analysis of a few samples. However, to confidently identify the source, or sources, of CH4 it is generally necessary to integrate sitespecific geological, land use, and forensic geochemical data on a number of samples. Spatial trends in geochemical data (vertically and laterally) are especially important. As demonstrated by the case studies presented, CH4 associated with spilled petroleum is derivedfi*omthe petroleum in some cases, but not in others. 6.7 ACKNOWLEDGMENTS

The generous support of the Unocal Corporation is gratefully acknowledged. I would also like to thank the following Unocal colleagues, past and present, for much of what I have learned about soil gas investigations: Todd Ririe, Bob Sweeney, Bob Haddad, and Greg Ouellette. Bill Golightly and Clay Westling helped me find information pertaining to case study #5. Chris Kitts provided references to microbiological investigations of methanogenesis.

REFERENCES Allison, M. L, 2001, Hutchinson, Kansas: A Geologic Detective Story; Geotimes Web Feature, American Geologic Institute, http://www.agiweb.org/geotimes/Oct01/ feature kansas.html AMEC Earth & Environmental, Inc., 2001a, Methane assessment summary report, 4S Ranch, Tract 5067, San Diego County, California. AMEC Earth & Environmental, Inc., 2001b, Methane gas column study, 4S Ranch, Ranch Bernardo, San Diego, California. Baldassare, F. J., and Laughrey, C. D., 1997, Identifying the sources of stray methane by using geochemical and isotopicfingerprinting.Environmental Geoscience, Vol. 4, No. 2, pp. 85-94. 5 10 15 20 25 Chanton, J., and Liptay, KL, 2000, Seasonal variations in Incubation Time (weeks) methane oxidation in a landfill cover soil as determined by an in situ isotope technique. Global Biochem. Cycles, Vol. 14, pp. 51-60. Figure 6.5.10 Concentration of headspace CH4 versus Chen, J., Yonchang, X., and Difan, H., 2000, Geochemiincubation time in a microcosm using native soil with 4 cal characteristics and origin of natural gas in Tarim weight percent organic carbon from a residential develBasin, China. Amer. Assoc. Petrol. Geol. Bull, Vol. 84, opment site (case study #5/ Data from AhAEC Earth & pp. 591-606. Environmental (2001b).

REFERENCES

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habitats at the Kuroshima Knoll, southern Ryukyu Arc, by analyzing pmoA, mmoX, mxaP, mcrA, and 16S rRNA genes. Applied and Envir. Microbiol., Vol. 70, pp. 7445-7455. James, A. T., 1983, Correlation of natural gas by use of carbon isotopic distribution between hydrocarbon components. Amer Assoc. Petrol. Geol. Bull., Vol. 67, pp. 1176-1191. Jenden, P. D., Drazan, D. J., and Kaplan, I. R, 1993, Mixing of thermogenic natural gases in northern Appalachian basin. Amer. Assoc. Petrol. Geol. Bull., Vol. 77, pp. 980-998. Jenden, P. D., Newell, K D., Kaplan, I. R., and Watney, W. L, 1988, Composition and stable isotope geochemistry of natural gases from Kansas, Midcontinent, USA Chemical Geology, Vol. 71, pp. 117-147. Kaplan, I. R, 1994, Identification of formation process and source of biogenic gas seeps. Israel. Jour. Earth Sci., Vol. 43, pp. 297-308. Kerfoot, H. B., Mayer, C. L, Durgin, P. B., and D'Lugosz, J. J., 1988, Measurement of carbon dioxide in soil gases for indication of subsurface hydrocarbon contamination. Ground Water Monitoring Review, Spring 1988, pp. 67-71. Khalil, M. A. K, and Rasmussen, R. A., 1983, Sources, sinks, and seasonal cycles of atmospheric methane. Jour. Geophysical. Res., Vol. 88, pp. 5131-5144. Khalil, M. A. K, and Rasmussen, R A., 1990, Atmospheric methane: Recent global trends. Environmental Science and Technology, Vol. 24, pp. 549-553. Kightley, D., Nedwell, D. B., and Cooper, M., 1995, Capacity for methane oxidation in landfill cover soils measured in laboratory-scale soil microcosms. Applied and Environ. Microbiol., Vol. 61, pp. 592-601. Kleikemper, J., Pombo, S. A., Schroth, M. H., Sigler, W. v., Pesaro, M., and Zeyer, J., 2005, Activity and diversity of methanogens in a petroleum hydrocarboncontaminated aquifer. Applied and Environ. Microbiol., Vol. 71, pp. 149-158. Laughrey, C. D., and Baldassare, F. J., 1998, Geochemistry and origin of some natural gases in the Plateau province, central Appalachian Basin, Pennsylvania and Ohio, Amer. Assoc. Petrol. Geol. Bull., Vol. 82, pp. 317-335. Levin I., Munnich, K. 0., and Weiss, W., 1980, The effect of anthropogenic CO2 and ^'^C sources on the distribution of ^^C in the atmosphere. Radiocarbon, Vol. 22, pp. 379-391. Lovely, D. R, Chapelle, F. H., and Woodward, J. C , 1994, Use of dissolved H2 concentrations to determine distribution of microbially catalyzed redox reactions in anoxic groundwater. Environmental Science and Technology, Vol. 28, No. 7, pp. 1205-1210. Lundegard, P. D., Haddad, R, and Brearley, M., 1998, Methane associated with a large gasoline spill: Forensic determination of origin and source. Environmental Geoscience, Vol. 5, pp. 69-78. Lundegard, P. D., Sweeney, R E., and Ririe, G. T., 2000, Soil gas methane at petroleum contaminated sites: forensic determination of origin and source. Environmental Forensics, Vol. 1, pp. 3-10. Marrin, D. L, 1987, Detection of non-volatile hydrocarbons using a modified approach to soil-gas surveying: Proceedings ofNWWA Conference on Petroleum Hydrocarbons and Organic Chemicals in Ground Water, Houston, pp. 87-95. Marrin, D. L., 1989, Soil gas analysis of methane and carbon dioxide: delineating and monitoring petroleum hydrocarbons. Proceedings ofNWWA Conference on Petroleum Hydrocarbons and Organic Chemicals in Ground Water, Houston, pp. 357-367.

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