Arsenic is indigenous to aquifer sediments of the Kern River alluvial fan. Dissolved arsenic in shallow ground water was found at a location on the Kern River Fan where residual petroleum hydrocarbons are undergoing remediation by natural attenuation (RNA). The initial concern was that geochemical reduction reactions associated with biodegradation may be causing increased amounts of arsenic to desorb from the aquifer matrix and enter ground water. A study of the ground-water geochemistry including direct speciation of dissolved arsenic, shows that oxidizing conditions are prevalent at this location and As(V) is the predominant arsenic species. Only one site in the monitoring network has sufficiently reducing conditions to cause the occurrence of dissolved arsenic as the As(III) species. Dissolved arsenic concentrations appear to be random and unrelated to petroleum hydrocarbon biodegradation. Groundwater recharge via nearby percolation ponds may influence the dissolved arsenic concentrations. Arsenic may also be influenced by past applications of phosphate fertilizers at this location; ground water containing the highest dissolved arsenic concentration also contains dissolved phosphate at a concentration higher than generally found across the Kern River Fan.

AAPG Search and Discovery Article #90928©1999 AAPG Annual Convention, San Antonio, Texas