Relation of salinity and selenium in shallow ground water to hydrologic and geochemical processes, western San Joaquin Valley, California

Salinity and selenium concentrations in shallow ground water of the western San Joaquin Valley, California, are related to the geomorphology and hydrology of the alluvial fans. The highest salinity and selenium concentrations in shallow ground water occur in alluvium deposited by ephemeral streams and at the margins of the major alluvial fans, where there were naturally saline, fine-grained soils. Low-to-moderate salinity and selenium concentrations in shallow ground water are associated with upper and middle areas of the major alluvial fans deposited by intermittent streams. Areas with the most naturally saline soils have been irrigated in the last 40 years. These are now the areas of highest salinity and selenium concentrations in the shallow ground water. Present-day (1986) ground-water salinity is significantly correlated with natural soil salinity in these areas. Isotopic data indicate that the highest salinity and selenium concentrations in ground water present at low altitudes resulted from evaporative concentration when the water table was shallow.