Paper No. 7
Presentation Time: 10:15 AM


BEKINS, Barbara A., U.S. Geological Survey, Menlo Park, CA 94025, COZZARELLI, Isabelle M., U.S. Geological Survey, National Research Program, Eastern Branch, Reston, VA 20192 and NG, G.-H. Crystal, Department of Earth Sciences, University of Minnesota, 310 Pillsbury Dr SE, Minneapolis, MN 55455-0231,

Risk analysis at hydrocarbon-contaminated sites typically focuses on BTEX and especially benzene because it has the lowest maximum contaminant level (5µg/L). When sources remain in place for decades, high concentrations of biogenic methane (CH4) and poorly characterized partial metabolites also migrate from the source. These other reduced carbon species compete with benzene for natural or added electron acceptors with two important consequences: (1) increased mobilization of secondary inorganic contaminants and (2) delayed benzene degradation. These effects are illustrated with data from two crude oil spill sites. The first site, near Bemidji, MN, was contaminated in 1979 by a pipeline rupture. Only ~1% of the source oil at this site is comprised of BTEX, ~60% is composed of alkanes, and the remaining 39% includes PAHs, resins, asphaltenes, and longer chain alkylbenzenes. The observed peak BTEX concentration in the groundwater plume was 6.5 mg/L in 2010. Methanogenic degradation of the crude oil contaminants forms a secondary plume of dissolved CH4 with a peak concentration of 16 mg/L. In addition, non-volatile dissolved organic carbon (NVDOC) with a peak concentration of 68 mg/L is migrating from the oil source. The CH4 and NVDOC plumes are transformed downgradient indicating these reduced carbon sources are using substantial electron accepting capacity. The second site, near Cass Lake, MN, was contaminated between 1971 and 2002 by a leaking pipe connection. Degradation within the oil body occurs via methanogenesis and nitrate reduction, creating secondary plumes of ammonium, CH4, and NVDOC. Aqueous concentrations of BTEX, CH4 and NVDOC migrating from the oil at this site are 5.9, 19 and 21mg/L respectively. Benzene concentrations migrating from the oil have decreased with time at both sites, but CH4 has remained the same and NVDOC has increased at the Bemidji site. Relative to BTEX, the electron demands of CH4 and NVDOC are 13 times greater at Bemidji and 7 times greater at Cass Lake. The NVDOC, CH4 and BTEX plumes attenuate over Fe-reducing intervals in both aquifers generating elevated levels of Fe2+ and Mn2+. These results indicate the importance of fully accounting for all carbon sources when estimating required electron accepting capacity and predicting secondary impacts.