Crude oil at a spill site near Bemidji, Minnesota has been undergoing aerobic and anaerobic biodegradation for over 30 years, creating a 150-200 meter (m) dissolved contaminant plume. Continuous temperature measurements at multiple locations in the vadose and saturated zones revealed a thermal plume of 2 to 6 degrees above background emanating from near the residual oil body at the water table. The thermal plume is associated with heat produced by aerobic methane oxidation in the unsaturated zone overlying the oil body. Temperatures and concentrations of methane, oxygen, and carbon dioxide in the unsaturated zone were measured throughout the year. Oxygen concentrations decreased from 21% at the land surface to approximately 0% in the methane oxidation zone (about 3 m depth), while methane concentrations peaked at about 8% immediately above the oil body and decreased to 0% in the methane oxidation zone. Carbon dioxide and methane concentrations above the oil body varied seasonally (~7 to 15% and 2.6 to 7.4%, respectively), suggesting seasonal variations in microbial activity. In the methane oxidation zone, temperatures increased to 4-6 degrees above background and also showed seasonal variations. This presentation will discuss the sources of observed seasonal variations at the site.

Seasonal changes in temperature and vadose zone gases due to microbial activity at a crude oil-contaminated site

249th National Meeting (2015)

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