Biogeochemical Changes in Landfill Gas During
Sub-Surface Migration

R. S. Ward Fluid Processes Group, British Geological Survey, Keyworth, Nottingham, NG12 5GG, UK

G. M. Williams Fluid Processes Group, British Geological Survey, Keyworth, Nottingham, NG12 5GG, UK

The presence of methane and carbon dioxide in the subsurface is a problem which must be addressed by the construction industry, landfill operators and developers of contaminated land. In recent years, a number of explosions, resulting in fatalities, have highlighted the seriousness of the methane problem and carbon dioxide has also been responsible for a number of deaths through asphyxiation. Although landfills constitute major sources of methane and carbon dioxide, there are many other natural and anthropogenic sources of these gases in the environment. Consequently, whenever they are detected, their source (or sources) must be established. Without knowledge of the source it is difficult to design remedial measures. Furthermore, inappropriate action could exacerbate the spread of the gas and worsen the problem.

Landfill gas is primarilly composed of methane and carbon dioxide but it also contains a wide range of volatile organic compounds. Invariably, it is the composition of the primary components in any methane containing gas which is used as the first indication of its provenance. As landfill gas migrates biogeochemical processes change the composition of the gas so that the use of methane/carbon dioxide ratios and light hydocarbon ratios for identification become less accurate with increasing distance from the source. Stable isotope ratios of methane and associated carbon dioxide may also be used, but these too can be affected by biogeochemical processes.

Detecting the presence of volatile organic compounds is potentially a better method of confirming a landfill source but very little is known about the fate of the volatile organic compounds during migration. A study was therefore made at a landfill where gas was known to have migrated beyond the landfill boundary.

A gas plume emanating from the landfill has been defined within unsaturated ferruginous sands on the basis of elevated concentrations of methane, carbon dioxide and volatile organic compounds (VOCs). The plume is relatively narrow but extends more than 100 m from the landfill boundary, and lies mainly between 2 m bgl and the water table at 9.5 m bgl. With increasing distance along the axis of the plume the ratio of methane to carbon dioxide gradually decreases, while nitrogen increases. Significant increases in oxygen concentration, however, only appear beyond 80 m from the landfill boundary. Stable carbon and hydrogen isotope ratios in methane and carbon dioxide within the plume show that methane becomes isotopically heavier and carbon dioxide isotopically lighter with distance. The resulting residual isotopic composition of the methane resembles a gas of thermogenic origin such as coal gas demonstrating the disadvantages of using stable isotope ratios for methane source identification. The compositional and isotopic observations are consistent with microbially mediated methane oxidation and zones of black reduced sediment near the landfill suggest that ferric iron (Fe(III)) in the sediment may also be acting as an electron acceptor for oxidation.

Volatile organic compounds (VOCs) in the plume were trapped using a combination of sorbants (Tenax GR, Haysep Q and Carbosieve S-III) and thermally desorbed into a GC/MS for semi-quantitative analysis. The 79 VOCs identified within the plume were similar to those found in other landfills and their concentrations both in the landfill and in the soil gas were broadly related to their volatility. Two compounds (vinyl chloride and dichlorofluoromethane) approached or exceeded the long term exposure limit (LTEL, as defined by the U.K. Health and Safety Executive) outside the landfill. Halogenated compounds (especially dichlorodifluoromethane and trichlorofluoromethane) were found to be most mobile and persistant and so it is suggested that the association of volatile halogenated compounds with methane provides good evidence that the gas is derived from landfill. The use of VOCs therefore appears to offer a very useful tool where methane source identification is a problem.