RAPID OXIDATION OF GEOTHERMAL ARSENIC(III) IN STREAMWATERS OF THE EASTERN SIERRA-NEVADA

Arsenic redox cycling was examined in source waters of the Los Angeles Aqueduct specifically at Hot Creek, a tributary of the Owens River. E levated arsenic concentrations in Hot Creek result from geothermal inp uts. Total arsenic and As(III) concentrations were determined in the c reek and in hot spring pools along its banks. Samples were processed i n the field using anion-exchange columns to separate inorganic As(III) and As(V) species. Downstream of the geothermal inputs, decreasing co ntributions of As(III) to total arsenic concentrations indicated rapid in-stream oxidation of As(III) to As(V) with almost complete oxidatio n occurring within 1200 m. Based on assumed plug flow transport and a flow velocity of about 0.4 m/s, the pseudo-first-order half-life calcu lated for this reaction was approximately 0.3 h. Conservative transpor t of total dissolved arsenic was observed over the reach. Pseudo-first -order reaction rates determined for As(III) oxidation in batch studie s conducted in the field with aquatic macrophytes and/or macrophyte su rface matter were comparable to the in-stream oxidation rate observed along Hot Creek. In batch kinetic studies, oxidation was not observed after sterile filtration or after the addition of antibiotics, which i ndicates that bacteria attached to submerged macrophytes are mediating the rapid As(III) oxidation reaction.