WORK PLANS - Laserfiche WebLink

Home Browse Search

GeosynteC consultants 2. HYDROGEOLOGIC CONCEPTUAL SITE MODEL RRI is located in Stockton, California along the eastern margin of the Sacramento-San Joaquin Delta(Delta; Figure 4). The Delta is formed by the confluence of several rivers, including the San Joaquin, Sacramento, Mokelumne, Calaveras, and Cosumnes Rivers. The Delta extends about 48 miles north,from Tom Paine Slough to Clarksburg,California and 31 miles west to Suisun Bay(Figure 4). Much of the Delta is located below mean sea level (msl). RRI consists of approximately 1,459 relatively flat acres that were originally marshy grassland(Tetra Tech,2002). The United States Geological Survey(USGS)has developed a CSM for the Delta islands, which is presented in Figure 5 (USGS, 2000). Prior to development, the Delta islands consisted of marshy grassland with organic clay and peat being formed. Once the islands were developed, levees were installed to prevent flooding from the surrounding surface water bodies, subsidence of the soils caused the ground surface in the center of the islands to subside below msl, and drainage channels in the interior of the islands were used to maintain the groundwater table below the ground surface (USGS, 2000). 2.1 RRI CSM Background In the 1990s, the Navy developed an initial CSM of the hydrogeology at RRI that subdivided the upper 70 to 80 feet of alluvial sediments into units of varying thickness and permeabilities including an uppermost confining unit (PRC, 1997a; Tetra Tech, 2002).However,after the Navy completed their investigation activities in 2002,the Navy revised their hydrogeologic CSM for RRI to a single heterogenous water-bearing unit in the uppermost 70 to 80 feet below ground surface (bgs), consisting of laterally and vertically discontinuous sedimentary packages in hydraulic communication with each other and the surface water bodies surrounding RRI, under semi-confined to unconfined conditions (Tetra Tech, 2002). In 2003, the Navy transferred RRI to the Port and the Port selected ERS as their environmental consultant. ERS used a CSM that subdivided the shallow alluvial sediments into multiple separate water-bearing units,similar to the initial CSM developed by the Navy in the 1990s(ERS, 2012). In 2017, the Port selected Geosyntec as their consultant for the RRI groundwater monitoring program and asked Geosyntec to re-evaluate the hydrogeologic CSM for RRI. Geosyntec reviewed historical reports and data, including groundwater elevations, RRI Groundwater Monitoring Work Plan 7 1 June 2018