| Summary: | Naturally occurring contaminant attenuation processes are investigated in a petroleum-hydrocarbon contaminated shallow aquifer, near Bemidji, MN. At this site, the biodegradation of hydrocarbons operates mostly under methanogenic conditions and generates CO₂ and CH₄. The main objectives of this study are to determine whether the full suite of noble gases, including He, Ne, Ar, Kr, and Xe, can be used to further delineate the fate of contaminants in the saturated and vadose zones and to identify mass transfer processes between these two compartments. Noble gases are sampled in the field and analyzed by way of an extraction line and mass spectrometry. In the vadose zone, gas consumption and production will induce pressure gradients, causing advective gas transport, which can be identified through concentration gradients of the inert gases. Noble gas data collected at the Bemidji site confirms the occurrence of advective gas transport, providing verification for previous field investigations and modeling that focused on Ar and N₂ as gas tracers. In addition, the present study reveals that heavier noble gases provide the strongest signal for identifying reaction-induced gas advection in the vadose zone, as a result of their lower diffusion coefficients. The biogenic addition of gas to the saturated zone promotes gas exsolution and bubble formation, which can be marked in the source zone by the depletion of dissolved noble gas concentrations in relation to atmospheric values. Modeling results support the hypothesis that ebullition, the buoyancy-driven upward migration of gas bubbles, is taking place locally in the source zone. The flux of gas across the water table, as a result of ebullition, is estimated at 0.177Lm-²day-¹. Ebullition is further investigated under laboratory closed system conditions. Results indicate that both atmospherically derived Ar and injections of SF₆ can be used as tracers for ebullition. However, the partitioning of gas tracers into free-phase hydrocarbons limits the applicability of gas tracer injections. An oil-gas partitioning experiment is carried out to assess the feasibility of SF₆ as a tracer in hydrocarbon contaminated settings. The results suggest that partitioning of SF₆ into oil is extensive, with a dimensionless oil-gas partitioning coefficient of 0.73.
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