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GROUND WATER INVESTIGATION WORK PLAN BULK TERMINALS SITE, PORT OF STOCKTON, STOCKTON, CALIFORNIA The PAH chrysene (EPA Method 8310) was reported at 17 !AWL in ground water from MW-3. 2.3 GEOLOGIC AND HYDROGEOLOGIC SETTING 2.3.1 Regional Setting Unconsolidated Deltaic Deposits underlie the site to a depth of approximately 1,500 feet bgs. These mainly Pliocene, Pleistocene, and Recent deltaic sediments are composed of interbedded layers and mixtures of clays, silts, sands, gravels, and organic soils. The Deltaic Deposits are laterally contiguous with the relatively coarser grained Valley Fill Deposits to the east. 2.3.2 Local Geologic Setting Geologic and hydrogeologic conditions in the upper 25 feet bgs were documented by Dames and Moore (1991b). Based on the geologic logs of borings and monitor wells at the bulk terminals site and nearby sites within the Port of Stockton, three distinct types of geologic materials are generally present in the shallow subsurface. The uppermost 10 feet consist of interbedded layers of brown silt and sand mixtures. Underlying these upper soils is a dark gray, soft, moist, medium- to high-plasticity clay, characteristic of native geologic materials in the delta. The clay is high in organic content, and ranges from approximately three to six feet in thickness. The clay layer grades into an underlying sequence of silty clay to clayey silt with lenses of silts and sands. In 1991, the capillary fringe was observed in this transitional zone beneath the clay layer. Ground water monitor wells MW-1 through MW-4 are screened in these fine-grained materials between 5.5 feet and 25.5 feet bgs (approximately 0 feet to —20 feet nisi). 2.3.3 Local Hydrogeologic Setting Although borings advanced at and near the bulk terminal site did not encounter a shallow sandy aquifer in the upper 25 feet bgs, one of the objectives of this Work Plan is to characterize the shallow hydrogeologic system, including the quality of ground water in the shallow aquifer. Typical delta geology and experience at nearby sites suggests a shallow sandy, or silty sand, aquifer at least two feet thick is likely present beneath the maximum depth of the existing monitor wells. Pending hydrogeologic data resulting from implementation of this Work Plan, silt and sand units underlying the clay are interpreted as the upper portion of a more regional shallow aquifer system. Depth-to-water measured in 2003 indicate ground water in wells MW-1 through MW-4 varies from 1.8 feet to —4.2 feet msl. These ground water elevations are approximately three to eight feet higher than those measured in 1991. Ground water elevation contours prepared by Dames July 2005 Page 6 Environmental Risk Services Corp.