Abstract

Arsenic is indigenous to aquifer sediments of the Kern River alluvial fan in California. Dissolved arsenic in shallow groundwater was found at a location on the Kern River Fan where residual petroleum hydrocarbons are undergoing remediation by natural attenuation. Property owners are concerned that geochemical reduction reactions associated with biodegradation may be increasing the amounts of arsenic that desorb from the aquifer matrix and enter groundwater. A study of the groundwater geochemistry, including direct speciation of dissolved arsenic, showed that oxidizing conditions are prevalent at this location and that As(V) is the predominant arsenic species. Only one well in the monitoring network is sited in conditions that are sufficiently reducing to produce dissolved arsenic as the As(III) species. The geographic distribution of dissolved arsenic concentrations at the property are random and unrelated to petroleum hydrocarbon biodegradation. Groundwater recharge via nearby percolation ponds, however, may influence dissolved arsenic concentrations. Dissolved arsenic also may be influenced by past applications of phosphate fertilizers on the property; groundwater containing the highest dissolved arsenic concentration also contains dissolved phosphate at a concentration higher than that generally found across the Kern River Fan.