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WORK PLAN 0 10 FOR GROUND WATER WELL INSTALLATION AND MONITORING PORT OF STOCKTON-ROUGH AND READY ISLAND,CALIFORNIA Miocene, about 16 to 25 million years ago, and consists of consolidated rhyolitic tuffs, conglomerate, clay shale, and sandstone (Piper and Others, 1939). Similar to the Mehrten Formation, ground water in the Valley Springs Formation underlying the Stockton area is reportedly saline and contains natural gas (CDWR, 1955). Consolidated rocks of the Eocene age, older than 38 million years, underlie the Valley Springs Formation. These older rocks are considered to not yield ground water (CDWR, 1955). 2.2.3Beneficial Uses of Water Rough and Ready Island is located within the boundaries of the Sacramento-San Joaquin Delta, as defined in the Basin Plan (Figure 1; RWQCB, 1998). In general, the surface waters and Deltaic Deposits may not be significant sources of potable ground water because the water is typically brackish and the fine-grained geologic materials have a relatively low permeability. The Basin Plan acknowledges this general interpretation of surface water quality when it states "[b]eneficial uses vary throughout the Delta and will be evaluated on a case-by-case basis." In addition, the Basin Plan allows exceptions to the "beneficial use designation of municipal and domestic supply (MUN) [where] the total dissolved solids (TDS) exceed 3,000 mg/1 (5,000 µmhos/cm, electrical conductivity)... [or] there is contamination, either by natural processes or by human activity (unrelated to the specific pollution incident) that cannot reasonably be treated for domestic use... [or] the water source does not provide sufficient water to supply a single well capable of producing an average, sustained yield of 200 gallons per day." 2.3 LOCAL SETTING 2.3.1 Local Geologic Setting Borings advanced at RRI on Site 47, which is adjacent to MO-1, reveal a surface layer of native organic material ranging from peat to organic rich silts and clays to a depth of approximately 10 feet to 15 feet below ground surface (bgs). In some areas, especially those associated with development, silty and sandy fill materials may overlie this layer of organic matter. Beneath this layer of organic matter are mineral rich clays and/or silts with little or no sand content. Within the silts and clays are sandier layers that range from clean fine to coarse grained sands to mixtures of silt and sand. These sandy layers are ground water bearing zones that are generally confined and interconnected with distance and depth. September 2014 3 Environmental Risk Services r Corporation 1