Samples of shallow ground water that underlies much of the irrigated area in the western San Joaquin Valley of California were analyzed for various major ions and trace elements, including selenium. Concentrations of the major ions generally were similar for ground water collected in the two primary geologic zones--the alluvial fan and basin trough. Soils in the alluvial-fan zone are derived from Coast Range rocks, and soils in the basin-trough zone are from a mixture of Sierra Nevada and Coast Range sources. Most of the variance in concentrations of major ions, as determined by principal components analysis, was associated with ground-water salinity and the dominant ions--magnesium, sodium, sulfate, and chloride. Most of the variance in trace elements was associated with concentrations of boron, molybdenum, selenium, and vanadium, which are present as mobile oxyanions. The concentrations of oxyanions trace elements were significantly correlated (a=0.05) with groundwater salinity, but the correlations between selenium and salinity and molybdenum and salinity were significantly different (a=0.05) in the alluvial-fan geologic zone compared with the basin-trough geologic zone. In addition, selenium concentrations are significantly (a=0.05) higher in the ground water of the alluvial-fan zone than in the basin-trough zone.
The evidence suggests that the main factors influencing selenium concentrations in the shallow ground water are the degree of ground-water salinity and the geologic source of the alluvial soil material.
