Two ground water systems were identified in the Imperial Valley, California. The upper system consisted of water in the shallow subsurface. Stable isotopes and tritium showed that the chemical constituents were controlled by evaporative concentration of recent Colorado River water. Chemical analyses showed that concentrations were variable and influenced by the effects of irrigation and evaporation. Redox sensitive elements indicated that oxidizing conditions were present in this upper system. Trace elements important to the study included selenium and arsenic. Concentrations of these two elements appeared to be negatively correlated. Selenium and chloride covaried supporting isotopic data which showed that concentrations were controlled by evaporation and irrigation practices. A uniform regional ground water system was encountered beneath about 30 feet below land surface. Isotopes and tritium showed that evaporative concentration of old (pre 1952) Colorado River water had occurred in some areas. Chemical concentrations were generally lower and less variable which suggested that the deeper system had not evaporated to the extent of surface and irrigation waters. Selenium concentrations were generally lower as a result of sorption onto the soil whereas arsenic concentrations were generally high.
