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Geosyntec consultants hydrogeologic CSM.No new hydrogeologic data were collected between 2018 and 2021 that indicated the 2018 hydrogeologic CSM needed to be modified. The hydrogeologic CSM is illustrated in Figure 11 and described in the following sections. The hydrogeologic CSM consists of a single heterogenous water-bearing unit in the top approximately 80 feet bgs under unconfined conditions in hydraulic communication with the surface water bodies. 2.2 Regional Geology and Hydrogeology 2.2.1 Geologic Units The Delta deposits consist of unconsolidated, organic-rich clay and silt, with less abundant peat, sand, and gravel (California Department of Water Resources [CDWR], 2006). The Delta deposits are recent to Pliocene in age and are encountered to a depth of about 1,300 feet below ground surface (bgs) in the vicinity of RRI(CDWR, 2006). Groundwater is generally not extracted from the Delta deposits because they are mostly fine-grained and contain brackish water(CDWR, 1967; CDWR, 2006). The Mehrten Formation underlies the Delta deposits and is comprised of andesitic conglomerate, silt, clay, andesitic agglomerate, and interbedded black sands (CDWR, 1967; CDWR, 2006). The Mehrten Formation is encountered at a depth of about 1,300 feet bgs, is reported to be about 600 feet thick, and is an important regional aquifer in the Sacramento and San Joaquin Valleys (CDWR, 1967; CDWR, 2006). 2.2.2 Groundwater The Delta was originally a marsh that received annual floods from the Sacramento and San Joaquin Rivers (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; Figure 10). 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; Figure 10). Islands in the Delta may have surface water contamination resulting from historical site activities, such as the use of pesticides for agricultural activities, and the discharge to the surrounding surface water channels may be regulated under a CVRWQCB permit. 2.2.3 Climate The climate in the Stockton area is moderated by the flow of marine air from the coast. The mean daily minimum temperature is 48 degrees Fahrenheit(°F), and the mean daily maximum temperature is 76 °F (National Centers for Environmental Information,National Oceanic and Atmospheric Administration [NOAA], 2018). In an average year, the Stockton area receives about 18 inches of rain, with 90% of rainfall occurring between November and April (NOAA, 2018). RRI Groundwater Monitoring Work Plan 9 June 2022