UNDERSTANDING THE LONG-TERM DYNAMICS OF NATURAL ATTENUATION OF CRUDE OIL IN THE SUBSURFACE

Long-term multi-disciplinary investigations into the processes that limit the extent of subsurface hydrocarbon contamination have been conducted at a site near Bemidji, Minnesota, USA, where crude oil was spilled in 1979. The present investigation was conducted to assess the long-term patterns of (1) benzene, toluene, ethylbenzene, and xylene (BTEX) in the source oil body and downgradient groundwater and (2) sediment Fe (III) concentrations, a source of electron acceptors critical for hydrocarbon biodegradation. Groundwater and oil samples were collected periodically from 1985 through 2010 and analyzed for BTEX. Sediment samples were collected and analyzed for Fe (III) concentrations from 1992 through 1995 and again from 2006 through 2008. Benzene and ethylbenzene concentrations in the groundwater near the source oil are explained well by concentrations of these constituents in the oil. Corresponding decreases in benzene concentrations in the oil phase and near-source groundwater were observed from 1995 through 2010. Ethylbenzene concentrations in both the oil body and the near-source groundwater have remained consistent throughout the study period. Ethylbenzene is less water-soluble than benzene, and hence a larger fraction remains in the oil despite similar exposure as benzene to recharge and groundwater. The dissolved benzene and ethylbenzene plumes have expanded at a rate of approximately 3 meters per year since 1995 and now extend to 105 meters downgradient from the center of the oil body. Previous research at the site indicated that iron-reducing bacteria utilize iron oxy-hydroxide coatings on the aquifer’s sand and gravel sediments to oxidize the hydrocarbons. Because BTEX is still being supplied by the source oil, the plume continues to expand as this Fe (III) source is consumed. From 1994 through 2006, Fe (III) decreased by an average of 20 micromoles per gram of sediment in the zone 60 to 120 meters downgradient from the oil body. In general, when BTEX compounds remain in the source zone at sites where natural attenuation is used as a remediation strategy, contaminant plumes are prone to expansion because of depletion of electron acceptors inside the plume and limited mixing with dissolved electron acceptors outside the plume.