Introduction of acetate into groundwater at the Rifle Integrated Field Research Challenge (Rifle, CO) has been used for biostimulation aimed at immobilizing uranium. While a promising approach for lowering groundwater-associated uranium, a concomitant increase in soluble arsenic was also observed at the site. An array of field data was analyzed to understand spatial and temporal trends in arsenic release and possible correlations to speciation, subsurface redox conditions, and biogeochemistry. Arsenic release (up to 9 μM) was strongest under sulfate reducing conditions in areas receiving the highest loadings of acetate. A mixture of thioarsenate species, primarily trithioarsenate and dithioarsenate, were found to dominate arsenic speciation (up to 80%) in wells with the highest arsenic releases; thioarsenates were absent or minor components in wells with low arsenic release. Laboratory batch incubations revealed a strong preference for the formation of multiple thioarsenic species in the presence of the reduced precursors arsenite and sulfide. Although total soluble arsenic increased during field biostimulation, the termination of sulfate reduction was accompanied by recovery of soluble arsenic to concentrations at or below prestimulation levels. Thioarsenic species can be responsible for the transient mobility of sediment-associated arsenic during sulfidogenesis and should be considered when remediation strategies are implemented in sulfate-bearing, contaminated aquifers.