Coupled Vapour Phase and Dissolved Gas Analysis to Elucidate Physical and Geochemical Processes in Reactive Zones

Dissolved and vapour phase gas data, including Ar, N₂, O₂, CH₄ and CO₂, were collected at a crude oil spill site near Bemidji, MN. The dataset includes sampling points in the saturated and unsaturated zones from upgradient of the source zone, within the source zone, and down-gradient extending beyond the anoxic vapour and dissolved phase plumes. In the source zone dissolved Ar and N₂ concentration are depleted to concentrations below 50% of water equilibrated with the atmosphere. This is indicative of gas bubble formation caused by increased levels of CH₄ from methanogenic degradation of the oil, and partitioning of Ar and N₂ into the bubbles. In the unsaturated zone, above the most methanogenic areas, vapour phase Ar and N₂ concentrations also show a depletion, to concentrations less than 90% of atmospheric levels, indicating significant methane production in this zone and/or gas influx from the saturated zone. Directly surrounding these areas of depletion vapour phase Ar and N₂ concentrations show an enrichment whereas CH₄ and O₂ concentrations are low indicating advective gas flux from the atmosphere toward this zone. Furthermore, directly downgradient of the methanogenic zones dissolved Ar and N₂ concentrations return to levels that approach water equilibrated with the atmosphere where advective flow calculations would predict depleted concentrations to be transported much further. This observation suggests either limited flux through the source zone or increased mixing of water from the dissolved plume with stagnant water in low permeability zones, or possibly with trapped gas bubbles. 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, gas bubble formation and ebullition, methane oxidation in the unsaturated zone, and mixing within the groundwater plume. Additionally, this study highlights the importance of considering the saturated and unsaturated zone as a coupled system. In the methanogenic source zone, geochemical parameters such as CH₄ and CO₂ and pH will be affected by the gas flux from the saturated zone to the unsaturated zone. An accurate accounting of these parameters can be particularly important for models where rate estimates of geochemical processes depend on accurate mass balances. Future work will involve laboratory experiments and the use of reactive transport modelling to demonstrate these principles in a quantitative manner.