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The presence beneath the central and western San Joaquin Valley of a large body of confined ground water of lower mineral content than overlying unconfined ground water has long been an enigma to hydrogeologists. The confined waters beneath the diatomaceous Corcoran Clay Member of the Tulare Formation occupy a zone 300 to 730 m thick, are of sodium sulfate composition, and are notably uniform in chemical and stable isotope content as compared to the overlying unconfined water. The confined waters clearly were derived neither from the modern streams of the Coast Ranges to the west, which have higher mineral content, nor solely from streams draining the Sierra Nevada to the east, which contain relatively little sulfate.

A paleohydrologic model, consistent with the structural and stratigraphic history of the region, attributes the uniform chemical and isotopic composition of the confined water to mixing and recharge that occurred in temporary lakes during extreme flood events. The confined waters appear to be ancient, probably representing recharge in pre-Corcoran time (older than 615,000 to 725,000 yr B.P.). Low structural relief on the west side of the San Joaquin Valley during much of Pleistocene time, together with small differential head across the thick, poorly permeable, lacustrine Corcoran Clay Member, have combined to preserve the confined paleowaters from being flushed. A combined chemical-stable isotope model suggests that waters with a δ18O value of about −10 permil from the Sierra Nevada mixed in intermittent lakes with runoff waters from the then lower Coast Ranges of about −6 permil, resulting in blends of about −9 permil and 450 to 500 mg/L sulfate content in the principal confined aquifer system. Shallower ground waters above the Corcoran Clay Member are more variable in chemical and isotopic composition and appear to represent recharge from streams similar to those now crossing the area.

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