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The investigations and nature and extent of contamination in soil, sediment, and groundwater are described in Site 47 RI/FFS. Section 2.2 summarizes the nature and extent of soil and sediment contamination. Section 2.3 summarizes the nature and extent of groundwater contamination. 1.6 Previous Removal Actions Taken No previous remedial or removal actions have been taken at Site 47. 1.7 Site Geology and Hydrogeology 1.7.1 Regional Geologic and Hydrogeologic Setting RRI is located within the Delta, which is formed by the confluence of the generally south-flowing Sacramento River and the generally north-flowing San Joaquin River to create the largest estuary on the western coast of the United States. The Delta deposits consist of organic-rich clay and silt, with less abundant peat, sand, and gravel (California Department of Water Resources [CDWR], 2006). The Delta deposits are encountered to a depth of about 1,300 feet below ground surface (bgs) in the vicinity of RRI(CDWR, 1967; 2006). The Delta was originally a marsh that received annual floods from the Sacramento and San Joaquin Rivers (United States Geological Survey [USGS], 2000). As development occurred in the Central Valley, the marshes in the Delta were drained and levees were constructed to use the Delta for agricultural and other purposes (USGS, 2000). The groundwater gradients in the Delta are generally low, and rainfall, irrigation, and seepage from surface water are the main sources of recharge to Delta groundwater(USGS, 2000). On Delta islands, shallow groundwater typically discharges into drainage channels and then is pumped into one of the surface water channels in the Delta surrounding the island(USGS, 2000). 1.7.2 Local Geology and Hydrology At Site 47, the first 10 feet bgs consist of a low permeability layer comprised of organic materials ranging from peat to mineral rich silts and clays. Some developed sections of RRI have sandy fill material that lies over the organic-rich layer. The uppermost 70 to 80 feet bgs are part of a single, heterogeneous, hydraulically connected, water-bearing zone. A thick clay unit appears to extend under most of RRI below the water-bearing zone (beginning at 70 to 80 feet bgs; Geosyntec, 2018). Similar to many islands in the Delta, the ground surfaces of many parts of RRI are below msl while the San Joaquin River is several feet above msl. This hydraulic condition requires levees to control flooding and also causes the river to exert hydrostatic pressure on groundwater system at RRL As a result, groundwater at RRI generally flows radially inward from the surrounding surface water towards the island's interior. To keep the ground surfaces at RRI dry, a network of drainage channels funnel groundwater that comes to the surface back outside the levees. The drainage channels conduct water to the pump house located north of Site 5G in the southwestern corner of RRI (Figure 2), where it is pumped into Burns Cutoff under the General Permit for Discharges from Municipal Separate Storm Sewer Systems, CVRWQCB Order No. R5-2016- 0040-11,National Pollutant Discharge Elimination System permit CAS0085324. Site 47 RAP for Soil 4 October 2023