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IV. REGIONAL GEOLOGY AND HYDROLOGY The subject site is located at the north end of the San Joaquin Valley which constitutes the southern two-thirds of the Great Central Valley of California. The San Joaquin Valley is both a structural and topographic basin, and is bounded by the Sierra Nevadas to the east and the Coast Ranges to the west. The valley floor consists of unconsolidated alluvial sediments derived from the surrounding highlands. The Stockton area is located within a geomorphic province described as overflow land and lake bottom. Ground water in the Stockton area consists principally of surface water collected in drainage basins tributary to the northern San Joaquin Valley. Surface runoff is collected, transported, and stored in the alluvial sand and gravel deposits forming the valley floor. These water bearing sediments are divided into layers described as the upper and lower units of the Tulare Formation. These units are separated by the Corcoran Clay Member, a confining clay layer between 0 and 130 feet thick which has been correlated over much of the northern portion of the San Joaquin Valley. In the area south of Stockton, the upper unit is reported to be about 100 feet thick. In general, the depth to ground water is shallowest at the northern end of the San Joaquin Valley. Historic well drilling and other hydrologic information suggest that ground water in the vicinity of the subject site has been encountered at depths of between about 10 and 40 feet. No ground water was encountered during our exploratory drilling at the subject site to a depth of 16.5 feet. V. SUBSURFACE INVESTIGATION The subsurface investigation of the subject site was performed on March 13, 1995. The investigation consisted of advancing four soil probes; drilling three soil borings; and the recovery of soil samples. The soil samples obtained from the probes and exploratory borings were submitted for analytical testing. Detailed descriptions of the soil probes, exploratory borings, and the analytical testing are presented in respective sections below. The approximate locations of the soil probes and borings relative to the project site are indicated in Figure 3. Soil Probes Four soil probes were advanced at the suspect areas to obtain soil samples. Soil probes SP-1 through SP-4 were advanced to a maximum depth of seven feet with soil samples collected from selected depths. The approximate locations of the soil probes and depths of recovered soil samples are indicated in Figure 3. Soil samples were obtained by driving a 2-inch nominal diameter sampler with a brass liner insert using a 7-pound slide hammer in a stainless steel hand-augured hole. The sampler and brass liner were washed in a phosphate-free solution, rinsed with tap water, and rinsed again with distilled water prior to the collection of each sample. The liners were sealed using teflon and plastic end caps, and tape. Labels indicating the date, time and sampling locations were affixed to each sample. Samples were stored on ice and transported under chain- of-custody, to Western Environmental Science and Technology, a California State Certified Laboratory. 2 -