ARSENIC MOBILIZATION IN A CRUDE OIL CONTAMINANT PLUME: RESULTS FROM PORE WATER AND SEDIMENT SAMPLING

A 1979 pipeline rupture released 100,000 gallons of crude oil into the subsurface near Bemidji, MN, USA. Data collected from 2009-2012 have revealed elevated concentrations of arsenic (up to ~200 µg/L) in groundwater of the iron-reducing zone downgradient from the hydrocarbon contaminant plume. Arsenic concentrations are below detection (< 1 μg/L) upgradient of the plume, suggesting that As in groundwater may be a secondary effect of the natural attenuation of the contaminant plume. This work evaluates As mobilization from sediment in the plume. We hypothesize that some microbial populations in the Fe-reducing zone of the plume may reduce As(V) to As(III), which could result in elevated As(III) concentrations in groundwater. In summer 2013, we collected sediment cores in the saturated zone at four locations along the plume transect: upgradient from the plume, within the Fe-reducing zone, within the oxic/anoxic transition zone, and downgradient from the plume. After core collection, pore water was extracted under nitrogen gas. Cores were then processed in an anaerobic glove bag using an oxalic acid solution to extract As from sediments by desorption. Both pore water and sediment extracts were speciated in the field for As. We expect that As concentrations in aquifer sediment upgradient and downgradient will be similar to background concentrations in glacial sediments in other parts of the state (~ 6 mg/kg). In the transition zone, where precipitation of Fe oxyhydroxides occurs due to mixing of groundwater from the anoxic and oxic zones, we expect that As concentrations in sediment will be elevated above background due to adsorption. Results of the As speciation will help identify if oxidation-reduction reactions involving As are occurring in the plume.