THE LEGACY OF SPILLED OIL AND FUEL IN GROUNDWATER: SOURCE ZONE PERSISTENCE AND PLUME GROWTH (Invited Presentation)

Nationwide there have been over half a million underground storage tank (UST) releases reported to the US EPA since 1984. Although 86% have completed cleanups, Wisconsin and Minnesota state data show that residual fuel is still present in the subsurface at 27-45% of sites. If these percentages apply in all states then upwards of 135,000 legacy UST release sources may exist nationwide. The potential longevity and ongoing impact on groundwater quality at these legacy petroleum hydrocarbon release sites is the subject of an ongoing study of a terrestrial crude oil release. The study site, located near Bemidji, Minnesota, was contaminated in 1979 when a pipeline rupture spilled ~1.7 million liters of light crude oil. The initial clean up removed ~75% of the oil and the remaining ~25% infiltrated the subsurface. A subsurface oil body covering an area of ~25 m by 75 m is present at the water table with oil saturations up to 65%. Oil composition data together with mass balance modeling indicate that the oil mass has decreased ~17% in the 37 years since the spill. Partial transformation of the oil compounds by a consortium of fermenters and methanogens in the source zone produces polar compounds that partition into the groundwater. These oil metabolites together with degradation products of dissolved hydrocarbons form the largest source of reduced carbon in the plume with total concentration of over 50 mg/L nonvolatile dissolved organic carbon (NVDOC). Over 14,000 individual partial transformation products are present. As these compounds migrate through the aquifer, they are further transformed under anaerobic conditions at a first-order rate of ~0.13% d^-1. The ongoing migration of metabolites from the source zone and biodegradation under iron-reducing conditions has resulted in reduction of the poorly crystalline iron oxy-hydroxides coatings on the aquifer sediments. As the electron accepting capacity of the aquifer has been attenuated, the plume of metabolites has expanded ~325 m from the center of the source zone to the shore of a small lake. Ongoing studies of the partial metabolites are examining their toxicity and fate as groundwater discharges into the lake and how they compare to metabolite plumes from refined fuel releases.