Arsenic behavior was examined in a contaminated stream by sampling the dissolved (<0.45 &mgr;m) arsenic and metals in surface water, shallow hyporheic zone water, and adjacent ground water. Surface water was oxic and slightly basic, and ground water was anoxic and acidic. Hyporheic zone water had pH values of 6--7, dissolved oxygen concentrations mostly between 0 and 3 mg L-1, and mean concentrations of most metals inbetween surface and ground water sample concentrations. However, arsenic and iron were enriched in the hyporheic zone. Most of the hyporheic zone dissolved arsenic was in the form of As(III), which is considered to be more toxic to some organisms than As(V). In the oxic surface water, 20% of the total dissolved As was found to occur in its reduced form. We hypothesize that upon burial and reduction of Fe-oxyhydroxides in the streambed, sediment-bound arsenic is transferred into the dissolved phase as As(III), and it is subsequently released into the surface water, where it does not immediately reoxidize. A continual flux of reduced As to the surface water maintains As(III) concentrations above that expected in oxygenated surface waters. ¿ 1999 American Geophysical Union