OIL IN THE SHALLOW SUBSURFACE: WHICH HYDROCARBONS ARE MOST RESISTANT TO DEGRADATION IN THE OIL AND IN THE GROUNDWATER PLUME

To understand the fate of hydrocarbons in the subsurface and how best to remediate petroleum-contaminated sites, long-term studies of spills are critical. Hydrocarbons in the oil phase and groundwater have been studied for over 30 years near Bemidji, MN where a pipeline break occurred in 1979. Oil resides about 6-10 m bls and is about 1 m thick or less floating at the water table. The residual oil and the groundwater plume extend downgradient from the spill site about 50 m and 200 m respectively. The oil was previously analyzed as approximately 60% alkanes and 35% aromatics. In the oil phase, the concentration of normal hydrocarbons (C₆-₃₀) originally was 140 mg/g oil; over time, concentrations have dropped below the detection limit at some locations. However, cyclohexanes (C_10-25) and a suite of seven acyclic isoprenoid hydrocarbons have undergone little degradation. The more soluble components, benzene, toluene, ethylbenzene, xylenes, C_3-4 alkylbenzenes, naphthalenes, and C_6-8 cyclohexanes were about 46 mg/g in the original oil. Although they have decreased in concentration, only a few compounds are below detection limit in the oil. The soluble components of the oil have served as a constant source of ground-water contamination over a 30-year period. In the groundwater associated with the oil, concentrations of the soluble components are high (8 mg/L) near the oil, and concentrations decrease due to degradation, volatilization and dilution as the groundwater moves downgradient. One hydrocarbon, 1,3-dimethyl-2-ethylbenzene appears to be the most recalcitrant hydrocarbon identified in the groundwater. It was the only identifiable hydrocarbon in seven wells of 29 wells sampled downgradient from the oil. Perhaps this hydrocarbon, is more difficult to biodegrade. Other recalcitrant volatile and semi-volatile hydrocarbons that were found in groundwater 140 m downgradient from the spill include benzene, cyclohexane, ethylbenzene, 1,2,4-trimethylbenzene, 1,2,3,4-tetramethylbenzene, 1,3-dimethyl-2-ethylbenzene, biphenyl, and dimethylnaphthalenes. Multiple factors control the fate of hydrocarbons in the oil and groundwater including the aquifer matrix, recharge, volatilization, flooding in low topographic areas, availability of electron acceptors, and microbial degradation.