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r C v VATic #me A S S O C I A T E S I N C 6-1 STRATIGRAPHY Tracy, California is located in the San Joaquin Valley,the southern extension of the California Great Central 6w Valley. The Great Central Valley is a deep alluvial plain extending nearly 500 miles from the Siskiyou Range of the Cascade Mountains in the north to the Tehachapi Range to the south. At Tracy, the San Joaquin Valley is approximately 40 miles wide. The western Valley boundary consists of the low, rolling foothills of the California Coast Range Mountains (maximum elevation due west is less than 3,000 feet) and the eastern boundary consists of the more rugged foothills of the Sierra Nevada (maximum elevation due east is over 10,000 feet). The Great Central Valley contains a thick sequence of sediment, which, in places, reaches a depth of ten miles. These sediments range in geologic age from Jurassic (205 million years before present) to Recent (present time) and include both marine and continental deposits. The site's immediate subsurface geology �"+ consists of Pleistocene and Recent alluvial deposits. These alluvial deposits consist of heterogeneous sequences of sand and gravel originating from inactive stream channels, and silt and clay originating from overbank and marsh depositional environments and alluvium originating from the Coast Range. Boring logs prepared by Kleinfelder for MW-1 and MW-2 indicate clay and silty clay were encountered to 26.5 feet bgs. Silty clay, silty sand, sandy clay, clay, and silt were encountered in the borehole of MW-3. Silty clay, clay, and sandy silt were encountered in the borehole for MW-4. Boring logs prepared by Parker for borings BH-1 through BH-7 that were advanced at the site in 1993, indicate variable layers of clayey sand, silty and sandy clay, and sand were encountered. A generalized boring log prepared by Parker for borings B-8 to B-21 advanced on and adjacent to the site in 1995 indicated clay and silt were present from beneath the paved surface to approximately six feet bgs. The interval from six to seven and one-half feet bgs consisted of sand and silt underlain by clay and silt. Boring logs prepared by Parker for MW-5 through MW- ;_, 8 are generally consistent and indicate variable layers of silty clay, silty sand, clayey silt, silty clay sandy clay and clay to approximately 25 feet bgs. Boring logs prepared by Kleinfelder and Parker are included in Appendix A. Figure 4 shows line of cross section A-A'. Cross section A-A', based on boring logs prepared by Kleinfelder and Parker, is presented as Figure 5. Recent Deposits Recent alluvium consists of sediments deposited on top of the erosional surface that was formed near the end of the last glacial period. Recent deposits are generally composed of alluvial deposits consisting of �•• unconsolidated gravel, sand, silt, and clays. Subsurface soils encountered during drilling at the site reportedly consisted of silty and sandy clay with minor layers of fine-to medium-grained sands to a depth of approximately 25 feet. Groundwater is encountered in these sediments at a depth of approximately five to i., six feet bgs. HYDROCARBON-IMPACTED SOIL AND GROUNDWATER Information obtained from previous soil investigation and groundwater monitoring reports indicates that hydrocarbon impacted soil and groundwater are present beneath the site. v v Former Cheaper#37 January 29 2004 ATC Project Number 5425 847.003 7 Page 4 V