NOBLE GASES AS TRACERS OF BIOGENIC GAS DYNAMICS AT A HYDROCARBON-CONTAMINATED SITE

Naturally occurring contaminant attenuation processes were investigated in a shallow aquifer contaminated by a crude oil spill near Bemidji, MN, USA. The biodegradation of petroleum hydrocarbons at this site results from methanogenesis in both the vadose zone and saturated zone. The primary objective of this study was to assess whether the full suite of noble gases concentrations (i.e., ^3,4He, ^20,22Ne, ^36,40Ar, Kr, and Xe) can offer unique insight into mass-transfer and mass-transport processes linked to subsurface hydrocarbon degradation. Noble gas abundances were measured for samples collected from multilevel vadose zone gas wells and groundwater wells. The samples were stored in crimped Cu tubing, and noble gas concentrations were determined using an extraction line and mass spectrometry. Concentrations of major component gases (i.e., CH₄, CO₂, O₂ and N₂) in the vadose zone were determined by gas chromatography. In the vadose zone, the production and consumption of biogenic gases (i.e., CH₄ and CO₂) can induce pressure gradients coupled with advective gas transport. Spatial trends in noble gas concentrations were indicative of advective gas transport from zones of methanogenesis to methanotropy. This result provided verification for previous field investigations and modeling that focused on Ar and N₂ as gas tracers at this site. Moreover, the heavier noble gases (i.e., Kr and Xe) provided the strongest signal for gas advection due to their lower diffusion coefficients. In the saturated zone, biogenic addition of gases can promote gas exsolution, bubble formation and ebullition. This buoyancy-driven transport of biogenic gases across the phreatic surface has been hypothesized as an important mass transport processes at hydrocarbon-contaminated sites. Mass-dependent trends in noble gas concentrations reflected depletion in groundwater corresponding to equilibrium partitioning into gas bubbles. These results agree with the hypothesis that ebullition occurs at the study site, and provide the first direct evidence of this process in a field setting. Noble gas concentrations provided new insight into biogenic gas dynamics and hydrocarbon degradation, and results confirm their potential as tracers of biogeochemical and physical processes in contaminated and pristine groundwater systems.