A MASS BALANCE APPROACH TO INVESTIGATE AS CYCLING IN A PETROLEUM HYDROCARBON PLUME

A crude oil pipeline rupture near Bemidji, MN in 1979 released 10,700 barrels of petroleum to a surficial sand-and-gravel aquifer. Since the oil spill, hydrocarbons have been biodegraded in the aquifer primarily coupled with reduction of ferric (Fe(III)) hydroxide minerals in the aquifer sediments. Biodegradation under Fe(III) reducing conditions has resulted in the mobilization of naturally occurring arsenic (As) from aquifer sediments into groundwater. In uncontaminated regions of the aquifer where geochemical conditions are oxic, As is associated with Fe(III) hydroxide minerals and has a mean sediment concentration of 2.6 mg/kg. Dissolved As concentrations in oxic groundwater are below detection (< 0.5 μg/L). However, in the Fe-reducing zone of the hydrocarbon plume, microbially mediated dissolution of the Fe(III) hydroxides has resulted in elevated As in groundwater above the 10 µg/L drinking water standard, with a maximum observed concentration of 230 µg/L near the oil source.

This study uses a mass balance approach to assess how biodegradation, coupled with Fe-reduction, redistributes naturally occurring As between groundwater and aquifer sediments under continuous, long-term anoxic conditions. From 2011-2015, groundwater and sediment data were collected and analyzed for As and Fe. Sediments were extracted to target As associated with bioavailable Fe(III) and near total concentrations of As. Preliminary results suggest that the distribution of As mass closely follows that of Fe. Most of the system’s As mass (>99%) exists in aquifer sediments; less than one percent of As in the system is in groundwater. Results also suggest that long-term biodegradation at the Bemidji site has created different redox zones that have different implications for As cycling. Depending on the redox, sediments can act as a source, sink, or both a source and a sink for dissolved As. Results from this study will be used to evaluate the capacity for the aquifer to naturally attenuate As and assess As mobilization under current and simulated future geochemical conditions.