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Eastern San Joaquin Groundwater Basin Groundwater Management Plan Northeastern San Joaquin County Section 5 Groundwater Banking Authority 93 Groundwater Monitoring Program are ongoing; however, the cumulative effect of ongoing groundwater recharge projects on water levels and water quality in aquifers is unknown. The scope of Study is explained in detail below. 1. Assembly and review of existing geologic, hydrologic, and water-quality data Existing well logs, groundwater level, and groundwater quality data will be compiled and assembled into a GIS database. The GIS database will be used, updated, and revised throughout the study and will be the basis for a 3-D visualization. The GIS database will be used to evaluate the aerial extent of high-chloride water, and to draw geologic sections through the study area that define the aerial and vertical extent of aquifer deposits along three selected flow paths from sources of recharge to discharge areas near the delta. The aerial 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 flow paths 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 aerial and vertical extent and source of high-chloride water to wells along the three study flow paths. Samples will be analyzed for major ions, nutrients, selected trace elements, and stable oxygen and hydrogen isotopes. 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. The stable isotopic composition of water from wells also will be used to determine the hydrologic and evaporative history of the Basin. Selected samples will be analyzed for tritium, carbon-14, and carbon-13 to determine the age of groundwater. Selected samples will also be analyzed for noble gasses to determine the recharge mechanism as either focused recharge from stream infiltration or aerial recharge from precipitation or irrigation return. 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 non-pumping conditions. 3. Test drilling and well installation Three multiple-well sites, each containing three to four 2-inch diameter wells, will be drilled along one study flow path. 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 down gradient site, brackish water from delta sediments.