ARSENIC IN SEDIMENTS AND GROUNDWATER FROM A WELL IN THE SOUTHERN SAN JOAQUIN VALLEY, CALIFORNIA

Sediments and groundwater from a well in the Kern Water Bank approximately 25 km SW of Bakersfield, CA (30S/25E-23H01) with anomalously high dissolved As were evaluated to determine the source of the dissolved As. Sediment grab samples were analyzed for total organic carbon, grain size, magnetic susceptibility, and mineralogy, as well as subjected to sequential extraction of arsenic. In addition to water pumped from the well, depth-specific groundwater samples were collected under non-pumping conditions.

The sediments are arkosic arenites and wackes. Magnetic susceptibility decreases with depth, suggesting that Fe-oxides also decrease with depth. Organic matter is correlated with decreasing grain size. Plagioclase dissolution (1% of total volume) is ubiquitous. The finest grain size samples contain authigenic pyrite, occurring as spherical framboids and some large cubic-shaped crystals that contain up to 3% As and show dissolution features. Although detrital Fe-oxides were not found, authigenic Fe-oxides are present.

Total sediment As ranges from 1.08 to 15.56 ppm and is correlated with decreasing grain size and increasing organic matter. Significant amounts of As are contained in the exchangeable, Fe-Mn oxide, and the residual fractions of the sediments. Exchangeable As ranges from 0.31 to 7.55 ppm and is also correlated with decreasing grain size and organics.

Depth-specific groundwater samples collected under non-pumping conditions contain 5.78 to 20.88 ug/L As. Groundwater As is correlated with both exchangeable and total sediment As. Water pumped from the well contains 35.46 ug/L As (30% As(V)) and 4.7 ppm dissolved oxygen.

Plagioclase dissolution suggests significant movement of water through the aquifer. The inferred decrease of Fe-oxides with depth and the occurrence of authigenic pyrite in organic-rich sediments suggests that pyrite formation resulted from reduction of Fe-oxides and sulfates. Authigenic pyrite served as a sink for As released from Fe-oxides. Subsequent dissolution of pyrite by oxygenated water results in intermediate redox conditions and in elevated groundwater As. The presence of both As(III) and (V) and measurable oxygen in pumped water are consistent with intermediate redox conditions resulting from oxygenated water in contact with reduced, pyrite-containing sediments.