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Samples for tritium anc carbon-14 will be analyzed by laboratories under contract with the U.S.Geological Survey, samples for noble gasses will be analyzed at a selected laboratory. Electromagnetic logs will be collected from existing observation wells and at the multiple-well sites drilled as part of this study.The logs will be used to determine if saline water is present at depths not sampled by well screens. Sequential logs done annually as part of this study will be used to determine if chloride concentrations are increasing at depths where screens are not located. Well-bore flow and depth-dependent water quality data(Izbicki and others, 1996)will be collected from selected production wells to determine at what depths high-chloride water enters the well under pumping conditions.Water movement through selected abandoned wells will be measured using low-flow current meters(such as an electromagnetic or heat-pulse current meter)to determine the direction and rate of water movement through the well casing under unpumped conditions. 3.Test dr lling_and well installation Three multiple-well sites,each containing three to four 2-inch diameter wells, will be drilled along one study flowpath.The wells will define movement of recharge water laterally and vertically through the flow system.Deeper wells at each site will define potential high-chloride source water from underlying bedrock.Similarly,shallower wells at each site will define potential high-chloride source water from irrigation return and,at the downgradient site,brackish water from delta sediments. 4.3-D Visualization Spatial data will be stored in a GIS will be the basis of a 3-D visualization of the ground water flow system using Earth Vision computer software.The visualization will incorporate hydrogeologic units and spatially connect data in the area of high-chloride water and along study flowpaths.The visualization will be a tool to evaluate data uncertainty and illustrate the effects of aquifer hydraulic properties and ground-water flow on the movement of high-chloride water toward wells. 5.Data Interpretation and Report Preparation Sources of high-chloride water to wells will be determined primarily from trace-element to chloride ratios using techniques developed by Piper Garrett and others(1953)and Izbicki (1991).These interpretations will be refined on the basis of major-ion data(Izbicki, 199 1)and changes in the 5180 and 8D composition of water from wells(Izbicki, 1996).Results will be compared to similar data collected in coastal aquifers elsewhere in California.The recharge temperature and tritium/helium-3 age of younger ground water will be estimated in accordance with Henry's Law using the computer program NOBLEGAS.Recharge