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Geosyntec consultants 2.3 RRI Geology and Hydrogeology 2.3.1 Site Setting and Topography RRI is bounded by the Burns Cutoff to the south and west, the Stockton Deep Water Channel to the north, and the San Joaquin River to the east (Figure 2). RRI is protected from the higher elevation water in the rivers and channels by levees that surround the perimeter of RRI (Figure 11). Surface water elevations in the surrounding rivers and channels are tidally influenced and range from approximately 1 to 10 feet above msl (Figure 12). The levees have average crest elevations ranging from between 12 and 18 feet above msl (Tetra Tech, 2002). The ground surface elevations in the interior of RRI range from approximately 5 feet below to 8 feet above msl, with an average elevation of approximately 1 foot below msl (Geosyntec, 2018a). 2.3.2 Drainage Channels RRI contains a system of drainage channels designed to collect rainfall runoff and intercept the shallow groundwater to keep the naturally marshy island dry(Figure 13). The RRI drainage channel system was excavated from native soils and is mostly unlined(Tetra Tech, 2002). The drainage channel system directs the collected water to the pump house located at the southwestern corner of RRI(north of Site 5G). The depths of the drainage channels range from 1 to 2 feet bgs in the central part of the island to 12 feet bgs in the Landfill Areas along the western side of the island near the pump house (Tetra Tech, 2002). The water from the surface water drainage channels is pumped into Burns Cutoff under MS4 General Order(Order No. R5-2016-0040,NPDES No CAS0085324). Sampling of the discharge water from the pump house is conducted and reported to the CVRWQCB under the MS4 General Order. Additionally, from 2018 to 2020 surface water samples were collected in the drainage channel prior to entering the pump house at sample location SW-7 to evaluate if there were ecologic risks associated with groundwater CDCs discharging to the surface water drainages being pumped into Burns Cutoff. The results were screened against the Los Alamos National Laboratory (LNAL) minimum no effect environmental screening level for aquatic community organisms and aquatic snails, the California Toxics Rule for dissolved metals (United States Environmental Protection Agency [USEPA], 2000), and the Basic Plan Criteria for dissolved metals (CVRWQCB, 2018b). The results from SW-7 were as follows (Geosyntec, 2020b; Geosyntec, 2020c; Geosyntec, 2020d): • VOCs, TPHg, and PCBs were not detected. • TPHd was reported at concentrations ranging from not detected to 120 µg/L. • TPHmo was reported at concentrations ranging from 240 to 950 µg/L. • Radionuclides and dioxins and furans concentrations were below the ecological screening levels. • The only metals exceedance of an ecological screening criteria was for arsenic, which has been demonstrated to be naturally occurring at elevated concentrations in groundwater at RRI (ERS, 2016; CVRWQCB, 2016). RRI Groundwater Monitoring Work Plan 10 June 2022