Arsenic contamination of groundwater and surface water is a worldwide problem, which poses health risks to millions of people in the world. The arsenic in the groundwater is of natural origin, and is released from the weathering of arsenic bearing minerals into the groundwater, owing to the anaerobic conditions of the subsurface. In this study, the oxidation of arsenite (As(III)) to arsenate (As(V)) linked to oxidizing compounds under anoxic conditions was shown to be a widespread microbial activity in anaerobic sludge and sediment samples that were not previously exposed to arsenic contamination. The results indicate that microbial oxidation of As(III) and Fe(II) linked to denitrification resulted in the enhanced immobilization of aqueous arsenic in anaerobic environments by forming Fe(III) (hydr)oxide coated sands with adsorbed As(V). Thus, the elevated oxidizing compounds could play critical roles in influencing the biogeochemistry of arsenic in subsurface environments.