The behavior of As in the subsurface environment was examined along a transect of groundwater monitoring wells at a Superfund site, where enhanced reductive dechlorination (ERD) is being used for the remediation of groundwater contaminated with chlorinated solvents. The transect was installed parallel to the groundwater flow direction through the treatment area. The ERD technology involves the injection of organic C (OC) to stimulate in situ microbial dechlorination processes. A secondary effect of the ERD treatment at this site, however, is the mobilization of As, as well as Fe and Mn. The concentrations of these elements are low in groundwater collected upgradient of the ERD treatment area, indicating that, in the absence of the injected OC, the As that occurs naturally in the sediment is relatively immobile. Batch experiments conducted using sediments from the site inoculated with an Fe(III)- and As(V)-reducing bacterium and amended with lactate resulted in mobilization of As, Fe and Mn, suggesting that As mobilization in the field is due to microbial processes. In the areas of the transect downgradient of the ERD treatment area, however, the concentrations of OC, As, Fe and Mn in the groundwater are not elevated relative to background levels. The decrease in the dissolved concentration of OC can be attributed to mineralization by microorganisms. The losses of As, Fe and Mn from the dissolved phase must presumably be accompanied by their uptake onto aquifer solids, but chemical extractions provided evidence only for the enrichment of Fe(II). Nor could sorption of As(III) onto sediments be detected by X-ray absorption spectroscopy (XAS) against the background of native As in the sediments, which was present as As(V). © 2009 Elsevier Ltd.