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Geosyntec
<br /> consultants
<br /> 2. HYDROGEOLOGIC CONCEPTUAL SITE MODEL
<br /> This section describes the hydrogeologic conceptual site model (CSM) for RRI, including the
<br /> regional geology and hydrogeology and the RRI site setting, topography, surface water drainage
<br /> channels, soils, water bearing zone, groundwater depth, and groundwater now.
<br /> RRI is located in Stockton, California, along the eastern margin of the Delta(Figure 9). The
<br /> Delta is formed by the confluence of several rivers, including the San Joaquin, Sacramento,
<br /> Mokelumne, Calaveras, and Cosumnes Rivers. The Delta extends about 48 miles north, from
<br /> Tom Paine Slough to Clarksburg, California and 31 miles west to Suisun Bay(Figure 9). Much
<br /> of the Delta is located below mean sea level (msl). RRI consists of approximately 1,459
<br /> relatively flat acres that were originally marshy grassland(Tetra Tech, 2002).
<br /> The United States Geological Survey(USGS) has developed a CSM for the Delta islands, which
<br /> is presented in Figure 10 (USGS, 2000). Prior to development,the Delta islands consisted of
<br /> marshy grassland with organic clay and peat being formed. Once the islands were developed,
<br /> levees were installed to prevent flooding from the surrounding surface water bodies. Subsidence
<br /> of the soils caused the ground surface in the center of the islands to subside below msl, and
<br /> drainage channels in the interior of the islands were used to maintain the groundwater table
<br /> below the ground surface (USGS, 2000).
<br /> 2.1 RRI CSM Background
<br /> In the 1990s, the Navy developed an initial CSM of the hydrogeology at RRI that subdivided the
<br /> upper 70 to 80 feet of alluvial soils at RRI into units of varying thickness and permeabilities,
<br /> including an uppermost confining unit(PRC, 1997b; Tetra Tech, 2002). However, after the Navy
<br /> completed their investigation activities in 2002, the Navy revised their hydrogeologic CSM for
<br /> RRI to a single heterogenous water-bearing unit in the uppermost 70 to 80 feet below ground
<br /> surface (bgs), consisting of laterally and vertically discontinuous soil type packages in hydraulic
<br /> communication with each other and the surface water bodies surrounding RRI, under semi-
<br /> confined to unconfined conditions (Tetra Tech, 2002).
<br /> In 2003, the Navy transferred RRI to the Port, and the Port selected ERS as their environmental
<br /> consultant. ERS used a hydrogeologic CSM that subdivided the shallow alluvial soils into
<br /> multiple separate water-bearing units, similar to the initial hydrogeologic CSM developed by the
<br /> Navy in the 1990s (ERS, 2012a).
<br /> In 2017, the Port selected Geosyntec as their consultant for the RRI groundwater monitoring
<br /> program and asked Geosyntec to re-evaluate the hydrogeologic CSM for RRI. Geosyntec
<br /> reviewed historical reports and data, including groundwater elevations, surface water elevations,
<br /> boring logs, pumping tests, soil physical parameter testing, and analytical results. The updated
<br /> hydrogeologic CSM for RRI was presented in the 2018 GMWP that was approved by DTSC and
<br /> CVRWQCB (Geosyntec, 2018a; DTSC, 2018; CVRWQCB, 2018a). The updated hydrogeologic
<br /> CSM was similar to the hydrogeologic CSM developed by the Navy in 2002 and consisted of a
<br /> single heterogenous water-bearing zone to approximately 80 feet bgs.
<br /> The data collected from 2018 to 2021 including the geologic logging of the new borings drilled,
<br /> and the groundwater elevations measured in the monitoring wells supported the 2018
<br /> RRI Groundwater Monitoring Work Plan 8 June 2022
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