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Geosyntec
<br /> consultants
<br /> • The only organochlorine pesticide exceedances of ecological screening criteria were
<br /> for 4,4-dichlorodiphenyldichloroethane (4,4-DDD) and
<br /> 4,4-dichlorodiphenyltrichloroethane (4,4-DDT), which are rarely detected in
<br /> groundwater at RRI and are most likely associated with historical surface applications
<br /> (Geosyntec, 2020d).
<br /> The results from the SW-7 sampling indicated there were no ecologic risks resulting from
<br /> groundwater discharging into the surface water drainages (Geosyntec, 2020b; Geosyntec, 2020c;
<br /> Geosyntec, 2020d).
<br /> 2.3.3 Lithology
<br /> Unconsolidated deltaic geologic deposits underlie RRI to a depth of approximately 1,300 feet
<br /> bgs, which are composed of interbedded layers and mixtures of clay, silt, sand, gravel, and
<br /> organic soils (Tetra Tech, 2002; CDWR, 2006; CDWR, 1967). The sedimentary deposits at RRI
<br /> are related to meandering river facies in a back-delta environment of wetlands,bogs, and oxbows
<br /> (Tetra Tech, 2002). Several sub-facies including back swamp, flood plain, shallow channel, and
<br /> abandoned channel sequences have been identified at RRI (Tetra Tech, 2002). Peat layers at
<br /> shallow depths in the western portion of RRI represent a back swamp or bog environment(Tetra
<br /> Tech, 2002). At most of RRI, the soils to a depth of about 80 feet bgs represent flood plain
<br /> deposits consisting of clays and silts of various thickness with interbedded thin lenses of sand
<br /> (Tetra Tech, 2002). A representation of the variation in lithology encountered at RRI is
<br /> illustrated on cross-sections in Figures 14 to 19.
<br /> 2.3.4 Water-Bearing Zone
<br /> Evaluations of groundwater elevations,pumping tests, geochemistry data, and geologic logs
<br /> provided multiple lines of evidence that the uppermost approximately 80 feet bgs of soils at RRI
<br /> are part of a single, heterogenous, hydraulically connected, water-bearing zone (Tetra Tech,
<br /> 2002; Geosyntec, 2018a). The monitoring wells at RRI are all screened at depths within 85 feet
<br /> bgs, and a thick clay unit, starting at approximately 80 feet bgs, has been reported to extend
<br /> under most of RRI below the uppermost water-bearing zone (Tetra Tech, 2002).
<br /> Hydrographs and head difference graphs for monitoring well clusters at RRI are included in
<br /> Appendix D. The hydrographs and head difference graphs show that wells screened at different
<br /> depth intervals within the same well cluster generally have similar groundwater elevations. This
<br /> indicates that there is generally little to no vertical gradient above the aquitard at approximately
<br /> 80 feet bgs and supports the 2018 hydrogeologic CSM that the monitoring wells at RRI are all
<br /> screened within a single connected water-bearing zone.
<br /> Pumping tests conducted by the Navy at Sites 3, 4, and IAS-11 had drawdown responses to
<br /> pumping observed in monitoring wells screened at different depth intervals than the extraction
<br /> well, indicating a high degree of hydraulic interconnection and supporting the interpretation of a
<br /> single water-bearing zone (PRC, 1997b; Tetra Tech, 2002). For example, at Site IAS-11, well
<br /> W-082 screened from 38 to 48 feet bgs was pumped at 8.5 gallons per minute (gpm) and an
<br /> immediate drawdown response to pumping was observed at W-058 screened from 5 to 9 feet bgs
<br /> and located approximately 15 feet from W-082 (PRC, 1997b). Navy pumping test data from the
<br /> 1997 PRC report was included in the 2018 GMWP.
<br /> RRI Groundwater Monitoring Work Plan 1 1 June 2022
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