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Geosynteco consultants surface water elevations, boring logs, pumping tests, soil physical parameter testing, and analytical results. The updated hydrogeologic CSM for RRI is illustrated in Figure 5 and described in the following sections. The updated CSM consists of a single heterogenous water-bearing unit in the top 70 to 80 feet bgs under unconfined conditions in hydraulic communication with the surface water bodies. The updated CSM presented herein is similar to the CSM developed by the Navy in 2002. 2.2 Regional Geology and Hydroaeology 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 (13 million years ago) in age and are encountered to a depth of about 1,300 feet bgs in the vicinity of RRI (CDWR, 1967; 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 5). 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 5). RRI Groundwater Monitoring Work Plan 8 1 June 2018