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Geosyntec consultants Landfill Areas along the western side of the island near the pump house (Tetra Tech, 2002). 2.3.3 Sediments Unconsolidated deltaic geologic deposits underlie RRI to a depth of approximately 1,300 feet bgs, which are composed of interbedded layers and mixtures of clay, silt, sand, gravel, and organic soils (Tetra Tech, 2002; CDWR, 2006; CDWR, 1967). The sedimentary deposits at RRI are related to meandering river facies in a back-delta environment of wetlands, bogs, and oxbows (Tetra Tech, 2002). Several sub-facies including back swamp, flood plain, shallow channel, and abandoned channel sequences have been identified at RRI (Tetra Tech, 2002). Peat layers at shallow depths in the western portion of RRI represent a back swamp or bog environment(Tetra Tech, 2002). At most of RRI, the sediments to a depth of about 80 feet bgs represent flood plain deposits consisting of clays and silts of various thickness with interbedded thin lenses of sand (Tetra Tech, 2002). 2.3.4 Water-Bearing Zone Evaluations of groundwater elevations, pumping tests, geochemistry data, and geologic logs provide multiple lines of evidence that the uppermost 70 to 80 feet bgs of sediments at RRI are part of a single, heterogenous, hydraulically connected, water-bearing zone. The monitoring wells at RRI are all screened at depths within 60 feet bgs. A thick clay unit, starting at 70 to 80 feet bgs, appears to extend under most of RRI below the water- bearing zone(Tetra Tech, 2002). Hydrographs for 134 monitoring wells in 54 monitoring well clusters at RRI are presented in Appendix B.5 The hydrographs show that wells screened at different depth intervals within the same well cluster generally have similar groundwater elevations.This indicates that there is generally little to no vertical gradient within the top 60 feet bgs and supports the updated CSM that the monitoring wells at RRI are all screened within a single connected water-bearing zone. 5 The groundwater elevations shown in the hydrographs are from the project database. The groundwater elevations in the project database use the 2007 ERS surveyed top of casing elevations or 1990s Navy surveyed top of casing elevations for wells not surveyed in 2007 to calculate the groundwater elevations. RRI Groundwater Monitoring Work Plan 10 1 June 2018