Changes in Major and Trace Components of Landfill Gas During Subsurface Migration

Abstract

A gas plume emanating from the Foxhall Landfill in Suffolk (U.K.) 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, extends more than 100 m from the landfill boundary, and lies mainly between 2 m bgl (below ground level) 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. Oxygen appears beyond 80 m from the landfill boundary. Stable carbon and hydrogen isotope ratios in methane become heavier with distance, while carbon dioxide becomes isotopically lighter with respect to stable carbon. This provides strong evidence for microbially mediated methane oxidation. Zones of black reduced sediment near the landfill suggest that ferric iron [Fe(III)] may be acting as an electron acceptor for oxidation. No thermal anomaly was observed, thus suggesting that the rate of oxidation/flux of methane is low.

Volatile organic compounds in the plume were trapped using a combination of sorbants (Tenax GR, Haysep Q and Carbosieve S-III), and desorbed thermally into a GC/MS for semi-quantitative analysis. The 79 VOCs identified 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. Only 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, 1992) outside the landfill. Halogenated compounds (dichlorodifluoromethane, dichlorofluoromethane and trichlorofluoromethane) were found to be most mobile but their concentration profiles suggest that they may have been flushed out of the landfill during its early stages. It is suggested that the association of volatile halogenated compounds with methane is good evidence that they are derived from a landfill.