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1.Assembly and review of existing geologic,hydrologic,and water-quality data.Existing well logs,water-level,and water-quality data will be compiled and assembled into a Geographic Information System(GIS)using ARCINFO and ARCVIEW software.Although much of the development of the GIS will occur early in the study the GIS will be used,updated,and revised throughout the study and will be the basis for a 3-D visualization.Data within the GIS will be used to evaluate the areal extent of high-chloride water,and to draw geologic sections through the study area that define the areal and vertical extent of aquifer deposits along three selected flowpaths from sources of recharge to discharge areas near the delta.The areal extent of high-chloride water and the geologic sections will be used to define data gaps that guide test-drilling and installation of observation wells.Existing water-quality data in the area of high-chloride water and along the three study flowpaths will be used to define the quality of native ground water and its geochemical evolution prior to collection of new data. 2.Collection of geochemical and geophysical data.Water chemistry data will be collected from up to 60 existing production and the 12 observation wells installed as part of this study.The data will be used to define the source, movement,and age of water from wells and the areal and vertical extent and source of high-chloride water to wells along the three study flowpaths.Samples will be analyzed for major ions,nutrients,selected trace elements,and the stable isotopes of oxygen and hydrogen by the U.S.Geological Survey National Water Quality Laboratory(NWQL) in Denver,CO.Quality assurance will be provided through the use of blank and replicate samples collected as part of the study. Selected trace elements,including bromide,iodide,boron,and barium,will be used in conjunction with chloride data to determine the source of high-chloride water in wells using methods originally developed by Piper and other (1953)in the Long Beach area of southern California and extended to other parts of coastal California by Izbicki (199 1)and Izbicki and others(2002).The stable isotopic composition of water from wells also will be used to determine the hydrologic(oceanic versus meteoric)and evaporative history of the water using methods developed by Izbicki(1996).Selected samples from 20 wells will be analyzed for tritium and its decay product helium-3 to determine the age of younger(less than 50 years)ground water.Selected samples from 20 other wells will be analyzed for carbon-14 and carbon-13 to determine the age of older(greater than 50 years)ground water that does not contain tritium.Selected samples from 20 wells will be analyzed for noble gasses to determine the recharge mechanism—focused recharge from stream infiltration versus areal recharge from precipitation or irrigation return.