Cozzarelli, I.M., M.E. Schreiber, M.L. Erickson, and B.A. Ziegler.
Groundwater, Vol 54 No 1, 35-45, 2015

At hydrocarbon spill sites, natural attenuation (NA) relies on biodegradation of hydrocarbons coupled with reduction of electron acceptors, including solid-phase ferric iron (Fe(III)). Because arsenic (As) adsorbs to Fe-hydroxides, a potential secondary effect of hydrocarbon NA coupled with Fe(III) reduction is a release of naturally occurring As to groundwater. In a long-term study of anaerobic biodegradation of hydrocarbons coupled to Fe(III) reduction at a crude-oil-contaminated aquifer near Bemidji, Minnesota, groundwater samples collected at the site annually from 2009 to 2013 showed As concentrations in groundwater in the plume reached 230 µg/L, whereas groundwater outside the plume contained <5 µg/L As. Results suggest that (1) naturally occurring As is associated with Fe-hydroxides present in the glacially derived aquifer sediments; (2) introduction of hydrocarbons results in reduction of Fe-hydroxides, releasing As and Fe to groundwater; (3) at the leading edge of the plume, As and Fe are removed from groundwater and retained on sediments; and (4) downgradient from the plume, patterns of As and Fe in groundwater are similar to background. A conceptual model of secondary As release due to hydrocarbon NA can be applied to other sites where an influx of biodegradable organic carbon promotes Fe(III) reduction. This paper is Open Access at http://onlinelibrary.wiley.com/doi/10.1111/gwat.12316/pdf