ASSOCIATION OF ARSENIC WITH IRON IN A CRUDE OIL CONTAMINANT PLUME

A 1979 pipeline rupture released 100,000 gallons of crude oil into the subsurface near Bemidji, MN, creating a dissolved hydrocarbon plume. Groundwater data collected by USGS from 2009-2013 show elevated concentrations of arsenic (As) (up to 72 µg/L) with elevated Fe(II) in the iron (Fe)-reducing zone of the plume. Upgradient from the plume, As concentrations in groundwater are below detection (< 1 μg/L). These data suggest that As is released to groundwater via microbially-mediated reductive dissolution of Fe(III) oxyhydroxides, coupled to the oxidation of hydrocarbons. At the leading edge where the anoxic plume interacts with more oxygenated groundwater (transition zone), Fe and As are below detection in groundwater, suggesting that As is removed from groundwater by adsorption to or co-precipitation with Fe oxyhydroxides.

This study evaluates the association of As with Fe in sediment from different regions of the plume. In summer 2013, we collected four sediment cores along the plume transect: upgradient from the plume, in the Fe-reducing zone, the oxic/anoxic transition zone, and downgradient from the plume. Arsenic and Fe were extracted from selected core intervals; the extracts were analyzed for As and Fe species to determine if redox-specific associations exist between As(III) and As(V) with Fe(II) and Fe(III). Preliminary results reveal elevated concentrations of Fe(II) in sediment in the Fe-reducing zone, but not in three other locations. The Fe(II) extracted from sediment could be adsorbed to mineral surfaces, or could be in mineral phases such as siderite or magnetite. The transformation from Fe(III) to Fe(II) in the solid phase may impact the association between As and Fe by changing Fe mineralogy, and consequently, the adsorption affinity of As to Fe oxyhydroxides. We hypothesize that dissolution and re-precipitation of Fe oxyhydroxides controls As mobility in the plume, and that identifying specific interactions of the As with these minerals is critical to understand As fate in this system.