A previously published field-experimental investigation showed that injection of nitrate in anoxic groundwater that contained aqueous and sediment-bound Fe(II) diminished concentrations of As(V) and As(III) to below drinking-water limits. In the current study, reactive transport modeling confirmed that the observed attenuation was consistent with oxidation of Fe(II) by nitrate, leading to precipitation of hydrous ferric oxide, which, in turn, sorbed both As(V) and As(III). After calibration with site-specific observations, reactive transport modeling could aid in designing effective treatment to remove arsenic using injection of nitrate to oxidize Fe(II).