Wellhead and downhole water samples were collected and analyzed from a 114.3-m~well at Cos0 Hot Springs (Coso No.1) and a 1477-111 well (CGEH No. 1) 3.2 km to the west. The same chloride concentration is present in hot waters entering both wells (about 2350 mg/kg), indicating that a hot-water-dominated geothermal system is present: The maximum measured temperatures are 142 C in the Coso No. 1 well and 195'C in the CGEH No. 1 well. Cation and sulfate isotope geothermometers indicate that the reservoir feeding water to the Coso Hot Spring well has a temperature of about 240-250'C, and the reservoir feeding the. CGEH well has a temperature of about 205 C. The variation in the chemical composition of water from the two wells suggests a model in which water-rock chemical equilibrium is maintained as a convecting solution cools from about 245' to 205'C by conductive heat loss.

A total of 39 water samples collected from the Coso geothermal area and vicinity and were analyzed for major chemical constituents, 6D and 6180. Nonthermal ground waters from the Coso Range were found to be isotopically heavier than those from the Sierra Nevada to the west.,The similarity of the 6D value for the deep thermal water at Coso to that of the Sierra water suggests that the major recharge for the hydrothermal system comes from the Sierra Nevada rather than from local precipitation on the Coso Range. The 6)80 values of the thermal water are about 7 loo heavier than the of the Sierra water. This shift in is the result of water-rock reaction at high temperatures, and the magnitude of the shift indicates that the ratio of rock to total water has been large for the system up to its present stage of development. The isotopic data are compatible with the chemical model.

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