title: Relating subsurface temperature changes to microbial activity at a crude oil-contaminated site.
abstract: Crude oil at a spill site near Bemidji, Minnesota has been undergoing aerobic and anaerobic biodegradation for over 30 years, creating a 150-200 m plume of primary and secondary contaminants. Microbial degradation generates heat that should be measurable under the right conditions. To measure this heat, thermistors were installed in wells in the saturated zone and in water-filled monitoring tubes in the unsaturated zone. In the saturated zone, a thermal groundwater plume originates near the residual oil body with temperatures ranging from 2.9°C above background near the oil to 1.2°C down gradient. Temperatures in the unsaturated zone above the oil body were up to 2.7°C more than background temperatures. Previous work at this site has shown that methane produced from biodegradation of the oil migrates upward and is oxidized in a methanotrophic zone midway between the water table and the surface. Enthalpy calculations and observations demonstrate that the temperature increases primarily result from aerobic methane oxidation in the unsaturated zone above the oil. Methane oxidation rates at the site independently estimated from surface CO2 efflux data are comparable to rates estimated from the observed temperature increases. The results indicate that temperature may be useful as a low-cost measure of activity but care is required to account for the correct heat-generating reactions, other heat sources and the effects of focused recharge.
authors.affiliation: U.S. Geological Survey, 345 Middlefield Road, Menlo Park, CA 94025, United States. Electronic address: ewarren@usgs.gov.
authors.forename: Ean
authors.initials: E
authors.lastname: Warren
authors.affiliation: U.S. Geological Survey, 345 Middlefield Road, Menlo Park, CA 94025, United States.
authors.forename: Barbara A
authors.initials: BA
authors.lastname: Bekins
mesh_terms: D001673:Biodegradation, Environmental; D002245:Carbon Dioxide / Q000032:analysis / Q000378:metabolism; D004784:Environmental Monitoring / Q000379:methods*; D060587:Groundwater / Q000737:chemistry / Q000382:microbiology*; D006358:Hot Temperature; D008697:Methane / Q000378:metabolism*; D008910:Minnesota; D010578:Petroleum / Q000378:metabolism; D059392:Petroleum Pollution; D012989:Soil Pollutants / Q000378:metabolism*; D013696:Temperature; D014874:Water Pollutants, Chemical / Q000378:metabolism
chemical_list: D010578:Petroleum; D012989:Soil Pollutants; D014874:Water Pollutants, Chemical; D002245:Carbon Dioxide; D008697:Methane
keywords: Crude oil; Groundwater; Heat; Microbial biodegradation; Temperature; Unsaturated zone