Water samples were collected from 44 wells in the northern part of the western San Joaquin Valley, California, between March and July 1985 to assess the geochemical relations and distribution of major ions and selected trace-element concentrations in ground water of the area. The ground-waterflow system consists of a semiconfined zone and a confined zone separated by a regionally extensive clay bed. The data show that the areal and vertical distribution of ground-water chemistry in the ground-water-flow system has been affected by different agricultural and natural sources of recharge and the source and redox status of the sediments. Tritium and stable-isotope data indicate extensive infiltration of the semiconfined zone by post-1952 irrigation water originating as runoff from the Sierra Nevada. Tritium concentrations greater than 2 tritium units in most samples from the confined zone indicate that the post-1952 water also has infiltrated to wells completed in this zone. Stable-isotope data indicate that ground water from the semiconfined zone, characterized by the enriched oxygen-18 that is indicative of a Coast Ranges source, occurs in many wells in the confined zone. Movement of water from the semiconfined zone to the confined zone likely is taking place by downward flow through the many wells that perforate the confining clay bed. Trace-element concentrations in the semiconfined and confined zones generally are similar. In contrast, concentrations were significantly different between ground water from Coast Ranges sediments and ground water from Sierra Nevada sediments in both zones. Ground water from Coast Ranges sediments contains significantly higher concentrations of nitrate, boron, and selenium than water from Sierra Nevada sediments. Ground water from Sierra Nevada sediments was significantly higher in arsenic, molybdenum, and manganese than ground water from Coast Ranges sediments. These differences result from a combination of variable availability of the constituents and redox controls on mobility. Selenium, the only priority pollutant to exceed the U.S. Environmental Protection Agency's drinking-water standard, equaled or exceeded the standard of 10 micrograms per liter in water from two wells completed in the semiconfined zone and one well in the confined zone. The conservative nature of selenium behavior in the oxidized Coast Ranges deposits is shown by the high correlation (r=0.88) between selenium concentrations and specific conductance in water from these deposits in the semiconfined zone. High selenium concentrations in both zones are spatially related to the location of Coast Ranges streams that have high selenium concentrations and that were historical sources of recharge to the zones. Principal-component analysis confirmed the association of boron, chromium, lithium, and selenium, and the association of arsenic, iron, manganese, and molybdenum to sediments in the study area. The analysis indicated that the first group is associated with Coast Ranges sediments and the second with Sierra Nevada sediments.