USING DISSOLVED AND VAPOR PHASE GAS ANALYSIS TO INVESTIGATE METHANOGENIC DEGRADATION OF PETROLEUM HYDROCARBONS IN VARIABLY SATURATED MEDIA
Microbially mediated degradation processes lead to characteristic changes of the dissolved gas composition in the source zone, including increases of CH4 and CO2 and decreases of Ar and N2 due to degassing, which is driven by CH4-production. The degree of Ar and N2 depletion can be used to estimate rates of historical methane production. If degassing is neglected, and historical rates of methanogenesis are determined only based on measured CH4 concentrations, both methanogenesis and contaminant degradation are under-predicted. In the unsaturated zone above the oil body, the non-reactive gases Ar and N2 indicate that gas transport includes an advective component and can be used to determine the direction and magnitude of advective gas fluxes. Our data suggests that reaction-induced advective gas transport of CH4 and O2 may enhance CH4 oxidation in the unsaturated zone. In the groundwater plume down-gradient of the source zone, the slow advance of the methane plume is coincidental with that of the depleted Ar/N2 plume. Considering the non-reactive nature of Ar and N2 in this system, it can be concluded that methane attenuation is caused in large part by physical processes.
The results of this study demonstrate the use of naturally occurring non-reactive gases as effective tracers of both physical and geochemical processes in contaminated systems. This includes quantifying rates of methanogenesis in the source zone, methane oxidation in the unsaturated zone, and mixing within the groundwater plume.