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OBJECTIVES:The purpose of this study is to quantify 1)source,areal extent,and vertical distribution of high- chloride water to wells,and 2)the sources,distribution,and rates of recharge to aquifers along selected flowpaths in eastern San Joaquin County. RELEVANCE AND BENEFITS:Eastern San Joaquin County is located near the delta of the San Joaquin and Sacramento Rivers.Ground water in the area is pumped for agricultural and public supply and as part of conjunctive use agreements to maintain surface flows in the delta.Decreases in ground-water quality may decrease agricultural supplies and public supplies for Stockton and other cities—increasing reliance of surface sources,which may adversely impact the Delta and threaten water exports to cities in southern California during periods of low flow. This study builds on techniques and research expertise unique to the U.S.Geological Survey California District through application in a new hydrogeologic setting and provides the opportunity for technology transfer within the California District.This study is consistent with the Strategic Plan of the USGS Water Resources Division providing for the collection of high-quality hydrologic data using state-of-the-art technology(Item I-1),the application of innovative interpretive methods for the investigation of hydrologic issues(Item I-2),and publication of the final report and interim publications will contribute to national and international scientific publications(Item I-3).The study is consistent with the California District Science Plan providing information on Seawater Intrusion/ Continental-Ocean Interaction(Topic 2,Water Quality:Issue 2-9),Geohydrologic Characterization of Ground- Water Basins(Topic 1,Issue 1-1:Water Availability and Supply Reliability,and Sacramento-San Joaquin Delta Flows and In-Delta Water(Topic 1,Issue 1-7). APPROACH:The scope of this study includes: 1)assembly and review of existing geologic,hydrologic,and water-quality data,2)collection of geochemical and geophysical data(including well-bore flow and depth- dependent water-quality data),3)test drilling and well installation,4)construction of a spatial GIS database to be used for 3-D visualization of aquifer materials and ground-water chemistry,and 5)data interpretation and report preparation.The combination of chemical and isotopic data linked to depth-dependent measures of well hydraulics and sample collection is an especially valuable approach for the study of recharge processes and ground-water contamination resulting from brine invasion.