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GROUND WATER MONITORING WORK PLaN is DREDGE MATERIAL DISPOSAL SITE 1,ROBERTS ISLAND,PORT OF STOCKTON sediments are comprised of interbedded layers and mixtures of clay, silt, sand, gravel, and organic soils. The Deltaic Deposits are laterally contiguous with the relatively coarser grained Valley Fill Deposits to the east. The Mehrten Formation underlies the Deltaic Deposits from approximately 1,500 feet to 2,300 feet bgs. The Mehrten Formation was formed in the late Miocene, about 7 to 9 million years ago, and consists of andesitic conglomerate and agglomerate, and interbedded silt, clay, and black sands (CDWR 1956; Piper and Others, 1939). Ground water in the Mehrten Formation underlying the Stockton area is saline and contains natural gas (CDWR, 1955). The Valley Springs Formation underlies the Mehrten Formation from approximately 2,300 feet to 2,800 feet bgs. The Valley Springs Formation was formed in the middle 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). 2.3.2 Local Geologic Setting Based on the geologic logs for borings and monitor wells, three distinct types of geologic materials are present at DMD1. The dredge materials consist of interbedded layers of brown and gray-brown lean clay, silt, silty sand, and sand. A black lean clay, characteristic of native geologic materials on the delta islands, appears to underlie the dredge materials in most locations. A layer of peat was encountered beneath the clay in most borings. However, a few borings revealed peat directly beneath possible dredge materials and directly above black clay. The borings generally did not penetrate the full thickness of the peat, which appears to vary in thickness to approximately 8 feet. Department of Water Resources has mapped a peat layer with a thickness generally less than 10 feet beneath Roberts Island (DWR, 1995). Existing borings advanced at and near DMD1 did not encounter a shallow sandy aquifer because the borings were terminated in or above the shallow peat layer and did not extend below the relatively shallow depth of 13.5 feet below original ground surface. However, based on the typical delta geology and experience at other delta islands, a shallow sandy, or silty sand aquifer, is anticipated below the peat layer. Pending geologic data resulting from this Work Plan, sand or silty sand units underlying the peat are interpreted as the upper portion of a more regional, or at least island-wide, shallow aquifer system. Figure 2 depicts the existing boring locations and Figure 3 depicts a schematic geologic cross-section at DMD1. January 2005 Page 4 Environmental Risk Services,Corp.