ARSENIC RELEASE FROM SEDIMENT DURING NATURAL ATTENUATION OF HYDROCARBONS IN A CRUDE-OIL CONTAMINATED AQUIFER

Monitored natural attenuation of organic contaminants in hydrogeologic environments is widely applied as a remediation strategy at hydrocarbon spill sites. At a crude-oil contaminated aquifer near Bemidji, Minnesota, anaerobic biodegradation of benzene and alkylbenzenes that is coupled to microbial iron (Fe) reduction has been well documented. A consequence of this natural attenuation is an increase in dissolved arsenic concentrations. It is well known that arsenic (As) release from ferric (Fe (III)) oxyhydroxides is a common cause of As concentrations exceeding 10 μg/L in groundwater. We investigated the presence of arsenic-containing Fe(III) oxides in the Bemidji aquifer and therelease of As during Fe(III) reduction. Water samples collected from monitoring wells in 2009 and 2010 from the anaerobic plume showed elevated concentrations of As, with maximum concentrations > 200 μg/L. In contrast, groundwater outside the plume, in oxic sediment, had As concentrations < 1 μg/L. Analyses of digestions of sediment cores collected in the iron-reducing zone showed As was present in aquifer sediment at concentrations up to 5.6 mg/kg, with an average of 2.7 mg/kg. Arsenic was highly correlated (R²= 0.87) with total Fe present in the sediment. Closer to the oil source zone where the redox system is dominated by methanogenesis, the Fe(III) oxyhydroxides have been largely depleted and average As concentrations in the sediment are low (0.8 mg/kg). Closer to the oil source zone where the redox system is dominated by methanogenesis, the Fe(III) oxyhydroxides have been largely depleted and average As concentrations in the sediment are low (0.8 mg/kg). We hypothesize that the degradation of oil components during Fe (III) reduction results in As release to the groundwater. Over the 30+ years since the spill, a redistribution of As and Fe has occurred. We expect that when the anoxic plume mixes with oxygenated groundwater downgradient, As may precipitate with Fe (III) oxyhydroxides in an As-rich front at the leading edge of the plume.