BIODEGRADATION OF OIL AT THE OIL-WATER INTERFACE AT THE OIL SPILL SITE NEAR BEMIDJI, MN

A pipeline transporting crude oil, near Bemidji, MN, broke in 1979 resulting in about 146 m³ of product remaining at the water table. Previous work at five locations in the north Bemidji oil pool indicated that the oil was undergoing degradation at different rates within the oil pool. The current work is a more comprehensive analysis of the oil and associated groundwater at the oil/water interface. Samples of oil and water were collected with a Telfon bailer in wells screened at the interface. The concentration of organic C in the oil increases with increasing extent of degradation of the oil, which is consistent with trends observed in petroleum source rocks. Normal alkanes (C₆-C₁₂) are absent or are present in low concentrations, indicating more degraded oil, near the location where it accumulated and moved through the unsaturated zone immediately after the spill. Farther downgradient from this area, the oil is not degraded or only slightly degraded in these alkanes. Summed concentrations of benzene, toluene, ethylbenzene and xylenes are 1-8 mg/g in the oil and 1-7 mg/L in water associated with the oil. The concentrations of benzene and naphthalene in water are positively correlated with their concentrations in oil. Benzene is subject to loss by dissolution, volatilization, and biodegradation whereas naphthalene is more recalcitrant. As shown in previous work, the dominant anion in groundwater is bicarbonate, which is generated by biodegradation and carbonate dissolution. The dominant cations are calcium, magnesium and iron. The highest concentrations of bicarbonate in groundwater coincide with the highest methane concentrations, which are observed at sampling sites downgradient of the spill area. In contrast, dissolved ammonium and iron concentrations are higher in groundwater associated with the most degraded oils. The δ¹³C of dissolved organic carbon in water near the oil spill is a few per mil more negative than in the downgradient area and is close to the δ¹³C of the crude oil. The δ¹³C of the dissolved inorganic carbon is more positive where dissolved methane concentrations are highest. Examination of processes near the oil-water interface indicate that once the most labile hydrocarbons, including normal alkanes and some aromatic hydrocarbons are degraded, the rate of hydrocarbon degradation decreases.