USING DISSOLVED AND VAPOR PHASE GAS ANALYSIS TO INVESTIGATE METHANOGENIC DEGRADATION OF PETROLEUM HYDROCARBONS IN VARIABLY SATURATED MEDIA

Petroleum hydrocarbon contamination of unconfined aquifers will induce microbially mediated degradation reactions involving a variety of electron acceptors. In the absence of more favourable electron acceptors, methanogenesis will dominate. Methane production will typically occur within and immediately downgradient of the source zone, and is not necessarily restricted to the saturated zone. The current study focuses on the investigation of methanogenesis and the fate of methane in an unconfined aquifer affected by a crude oil spill near Bemidji, MN. We have used dissolved and vapour phase gas analysis including Ar, CH₄, CO₂, N₂, and, O₂ to quantify the production and the fate of methane in the saturated and unsaturated zones.

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.