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Integrated investigation of natural attenuation in a petroleum hydrocarbon contaminated aquifer Amos, Richard Theodule

Abstract

This research investigates natural attenuation processes at a crude oil spill site near Bemidji, MN, with a focus on the contribution of methanogenesis. An important aspect of this work is evaluating the use of naturally occurring gases in the saturated and unsaturated zones to aid in the assessment of methane production rates, the fate of methane, and more generally to quantitatively assess the effect of gas production in source zones on contaminant release and attenuation. Field data from the Bemidji site demonstrate that naturally occurring stable gases such as Ar and N₂ can be effectively used to better understand and quantify physical and chemical processes related to methanogenic activity. These include biochemically induced gas advection in the unsaturated zone, degassing and gas bubble formation due to methanogenic gas production in the saturated source zone, and attenuation of dissolved gases in the groundwater plume. To allow a more quantitative assessment, the reactive transport code MIN3P has been enhanced to simulate bubble production, bubble entrapment due to water table rise, and permeability changes due to trapped bubbles. Simulations for a hypothetical case study suggest that these processes work to attenuate the dissolved gas plume emanating from the source zone. A detailed simulation, constrained by Bemidji field data, is conducted to provide an integrated assessment of natural attenuation processes in the unsaturated and saturated zones. Although gas bubble ebullition is potentially an important process at the site, it could not be included in the site-specific simulations, because this process is poorly understood and not well constrained by field data. To investigate this process, column experiments were conducted that show clear evidence for ebullition from a methanogenic environment, using dissolved gas concentrations to monitor the extent and rate of ebullition. An empirically derived formulation describing ebullition, integrated into MIN3P, provides insights into the parameters controlling ebullition and allows the process to be quantified. Overall, this work contributes to a better understanding of natural attenuation processes in contaminated aquifers and offers novel field and modeling techniques to investigate and quantify these processes.

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