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The mean isotopic composition of CH4 emitted from six New England (United States) landfills was 13C and D enriched (−48.1 to −50.4‰ and −273 to −281‰) relative to anoxic zone landfill CH4 (mean values of −55.9 to −56.2‰ and −296 to −300‰) owing to the oxidation of methane as it was transported from the landfill to the atmosphere through the soil cap. The fraction of methane oxidized f0 during its passage through the soil cap was calculated from the degree of 13C enrichment in emitted CH4 relative to anoxic zone CH4 in conjunction with values determined for the preference of soil methane oxidizing bacteria for 12CH4 over 13CH4 (α = 1.022 ± 0.008). Mean values for methane oxidation in six landfills were from 24 to 35% of the total flux through the soil during the warm season, depending upon how the data were grouped. Our results bracket recent estimates of methane oxidation of about 30% in the warm summer period produced using a model with the input terms of soil temperature, moisture, depth, and oxygen concentration. Because of variations in the response of methane oxidation to temperature at these New England sites, our study is consistent with the modeling results of Czepiel et al. [1996b] that the best estimate for the annual value for methane oxidation in the landfills considered is about 10%.

Field measurement of landfill methane emissions indicates natural variability spanning more than 2 seven orders of magnitude, from approximately 0.0004 to more than 4000 g m{sub -2} day{sup -1}. This wide range reflects net emissions resulting from production (methanogenesis), consumption (methanotrophic oxidation), and gaseous transport processes. The determination of an {open_quotes}average{close_quotes} emission rate for a given field site requires sampling designs and statistical techniques which consider spatial and temporal variability. Moreover, particularly at sites with pumped gas recovery systems, it is possible for methanotrophic microorganisms in aerated cover soils to oxidize all of the methane from landfill sources below and, additionally, to oxidize methane diffusing into cover soils from atmospheric sources above. In such cases, a reversed soil gas concentration gradient is observed in shallow cover soils, indicating bidirectional diffusional transport to the depth of optimum methane oxidation. Rates of landfill methane oxidation from field and laboratory incubation studies range up to 166 g m{sup -2} day{sup -1} among the highest for any natural setting, providing an effective natural control on net emissions. Estimates of worldwide landfill methane emissions to the atmosphere have ranged from 9 to 70 Tg yr{sup -1}, differing mainly in assumed methane yields from estimated quantities of landfilled refuse. At highly controlled landfill sites in developed countries, landfill methane is often collected via vertical wells or horizontal collectors. Recovery of landfill methane through engineered systems can provide both environmental and energy benefits by mitigating subsurface migration, reducing surface emissions, and providing an alternative energy resource for industrial boiler use, on-site electrical generation, or upgrading to a substitute natural gas.

Field, laboratory, and computer modeling methods were utilized to quantitatively assess the capability of aerobic microorganisms to oxidize landfill-derived methane (CH4) in cover soils. The investigated municipal landfill, located in Nashua, New Hampshire, was operating without gas controls of any type at the time of sample collection. Soil samples from locations of CH4 flux to the atmosphere were returned to the laboratory and subjected to incubation experiments to quantify the response of oxidation in these soils to temperature, soil moisture, in situ CH4 mixing ratio, soil depth, and oxygen. The mathematical representations of the observed oxidation reponses were combined with measured and predicted soil characteristics in a computer model to predict the rate of CH4 oxidation in the soils at the locations of the measured fluxes described by Czepiel et al. [this issue]. The estimated whole landfill oxidation rate at the time of the flux measurements in October 1994 was 20%. Local air temperature and precipitation data were then used in conjunction with an existing soil climate model to estimate an annual whole landfill oxidation rate in 1994 of 10%.

Methane (CH4) emissions were measured from the Nashua, New Hampshire municipal landfill using static enclosure and atmospheric tracer methods. The spatial variability of emissions was also examined using geostatistical methods. One hundred and thirty nine enclosure measurements were performed on a regular grid pattern over the emitting surface of the landfill resulting in an estimate of whole landfill emissions of 15,800 L CH4 min−1. Omnidirectional variograms displayed spatial correlation among CH4 fluxes below a separation distance of 7 m. Eleven tracer tests, using sulfur hexafluoride (SF6) as a tracer gas, resulted in a mean emissions estimate of 17,750 L CH4 min−1. The favorable agreement between the emission estimates was further refined using the observed relationship between atmospheric pressure and CH4 flux. This resulted in a pressure-corrected tracer flux estimate of whole landfill emissions of 16,740 L CH4 min−1.

The Harvard Forest research site located in central New England is influenced by numerous anthropogenic methane sources on a year-round basis. Methane is strongly correlated to other chemical species that have an anthropogenic component, including acetylene, propane, ethane, hexane, and additional short-lived nonmethane hydrocarbons. The correlation between methane and acetylene is due to the colocation of landfills and cities. The correlation between methane and other short-lived species implies that emissions from local and regional rather than distant sources are the primary cause of elevated events. Wind roses of chemical species are examined for annual and seasonal time periods with enhancements in anthropogenic species corresponding to the location of large cities and landfills. The southwest quadrant is subjected to the most severe pollution events and is impacted by outflow from nearby cities in that sector, including Northampton and Springfield, Massachusetts. Emissions from cities in other quadrants, including Boston and Worcester, Massachusetts, Providence, Rhode Island, and the close-by town of Petersham, Massachusetts, also affect the site, but to a lesser degree. Case studies are used to identify atmospheric conditions that lead to high concentrations of methane and other species. The co-occurrence of a persistent wind direction, light wind speed, and stable atmospheric conditions is the ideal scenario in which emissions from nearby cities and landfills are advected to the site. Emissions from local and regional, rather than distant sources, are the primary cause of elevated events.