University of California Water Resources Center

Arsenic, a toxic trace element, enters surface waters from abandoned mines and geothermal springs. Once arsenic is discharged to surface waters, photochemical reactions can alter the oxidation state of the metal or cause the dissolution of the mineral phases onto which it could adsorb. To assess the role of these photochemical reactions arsenic fate and transport, we conducted laboratory studies and collected samples from arseniccontaminated surface waters. Results of laboratory studies indicate that hydroxyl radical (OH)is the only photo-produced transient in sunlit surface waters that is capable of oxidizing As(III) at significant rates. A field study conducted at a geothermal spring (Hot Creek) indicated As(III) is oxidized by photo-produced hydroxyl radical with a half-life of approximately minutes. Photochemical oxidation of As(III) is likely to be faster in acid mine drainage streams, where hydroxyl radical concentrations are much higher. Results of laboratory studies also indicate that arsenic is released during the photoreductive dissolution of iron-containing sediments collected an acid-mine drainage site (West Squaw Creek).