University of California Water Resources Center

Arsenic is a widely distributed constituent of geologic materials, with an average crustal abundance of 1.8 ppm. The natural processes of weathering of arsenic-containing minerals and volcanism contribute arsenic to groundwaters, surface freshwaters, and seawater. Recently, increased attention has focused on arsenic geochemistry in natural waters. This attention has been motivated by concern over the human health effects of arsenic exposure; consumption of drinking water can be a significant, if not the primary, route for ingestion of inorganic arsenic. Since the concentrations of inorganic arsenic in various foods and source waters can vary widely, the exposure of any individual or group depends strongly on both diet and the quality of locally supplied drinking water.

Two field studies were conducted to examine arsenic occurrence and redox speciation in natural waters in California with elevated arsenic concentrations. In Hot Creek, located in the Owens Valley, elevated arsenic concentrations are the result of geothermal inputs. Hot Creek flows into the Owens River, a source of drinking water to the City of Los Angeles. In Hanford, located in the San Joaquin Valley, elevated arsenic concentrations occur in groundwater wells emplaced in Quaternary alluvial sediments. These municipal supply wells serve the City of Hanford (population ca. 39,000).

At Hot Creek, concentrations of total arsenic, arsenic(III) and chloride (as a conservative tracer of geothermal inputs) were measured above, within, and downstream of the major geothermal inputs of arsenic in Hot Creek Gorge. Consistent with previous studies, extremely elevated arsenic concentrations were found in waters collected from geothermal pools. Lower concentrations were found in Hot Creek as a result of dilution by meteoric surface waters. Total arsenic behaved conservatively in Hot Creek. Rapid in situ arsenic(III) oxidation was observed. Comparison with batch oxidation studies conducted in the field demonstrated that the in situ arsenic(III) oxidation is microbially mediated.

At Hanford, total arsenic concentrations were found to vary considerably in the 16 municipal supply wells and 4 storage tanks tested. Higher arsenic concentrations (up to 75 ug/L) were found in the shallower wells; deeper wells contained lower arsenic concentrations. Arsenic was found to occur predominantly in the +III oxidation state. Occurrence of arsenic(V) in one of the storage tanks tested indicated that As(III) is oxidized within the distribution system.