Geohydrologic aspects of water-quality problems of the 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. High salinity and selenium concentrations are associated with ephemeral-stream deposits. Low salinity and selenium concentrations are associated with intermittent-stream deposits, which represent the major alluvial fans of the area. The highest salinity and selenium concentrations at the margins of the alluvial fans are the result of evaporation and evapotranspiration of shallow ground water, as indicated by oxygen- and hydrogen-isotopic data. The extent and direction of movement of saline ground water to drain laterals in agricultural fields are influenced by the time since installation of the drainage system, drain lateral spacing, and the regional ground-water-flow system.