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WATER RESOURCES RESEARCH, VOL. 41, W02001, doi:10.1029/2004WR003433, 2005

Use of dissolved and vapor-phase gases to investigate methanogenic degradation of petroleum hydrocarbon contamination in the subsurface

Richard T. Amos

Department of Earth and Ocean Sciences, University of British Columbia, Vancouver, BC, Canada


K. Ulrich Mayer

Department of Earth and Ocean Sciences, University of British Columbia, Vancouver, BC, Canada


Barbara A. Bekins

U.S. Geological Survey, Menlo Park, California, USA


Geoffrey N. Delin

U.S. Geological Survey, Mounds View, Minnesota, USA


Randi L. Williams

Department of Earth and Ocean Sciences, University of British Columbia, Vancouver, BC, Canada


Abstract

At many sites contaminated with petroleum hydrocarbons, methanogenesis is a significant degradation pathway. Techniques to estimate CH4 production, consumption, and transport processes are needed to understand the geochemical system, provide a complete carbon mass balance, and quantify the hydrocarbon degradation rate. Dissolved and vapor-phase gas data collected at a petroleum hydrocarbon contaminated site near Bemidji, Minnesota, demonstrate that naturally occurring nonreactive or relatively inert gases such as Ar and N2 can be effectively used to better understand and quantify physical and chemical processes related to methanogenic activity in the subsurface. In the vadose zone, regions of Ar and N2 depletion and enrichment are indicative of methanogenic and methanotrophic zones, and concentration gradients between the regions suggest that reaction-induced advection can be an important gas transport process. In the saturated zone, dissolved Ar and N2 concentrations are used to quantify degassing driven by methanogenesis and also suggest that attenuation of methane along the flow path, into the downgradient aquifer, is largely controlled by physical processes. Slight but discernable preferential depletion of N2 over Ar, in both the saturated and unsaturated zones near the free-phase oil, suggests reactivity of N2 and is consistent with other evidence indicating that nitrogen fixation by microbial activity is taking place at this site.

Received 20 June 2004; accepted 3 November 2004; published 2 February 2005.

Keywords: gas advection; methanogenesis; nonreactive gases; petroleum hydrocarbons.

Index Terms: 0432 Biogeosciences: Contaminant and organic biogeochemistry (0792); 1829 Hydrology: Groundwater hydrology; 1831 Hydrology: Groundwater quality; 1832 Hydrology: Groundwater transport; 1875 Hydrology: Vadose zone.

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Citation: Amos, R. T., K. U. Mayer, B. A. Bekins, G. N. Delin, and R. L. Williams (2005), Use of dissolved and vapor-phase gases to investigate methanogenic degradation of petroleum hydrocarbon contamination in the subsurface, Water Resour. Res., 41, W02001, doi:10.1029/2004WR003433.