IMPORTANCE OF SECONDARY WATER QUALITY IMPACTS AT TWO CRUDE OIL SPILL SITES

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.