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i • • ithe installation of the groundwater monitoring well network for the Phase 1 development; the monitoring well network for Phase 2 shall be documented in a ireport submitted at a later date. Local Hydrogeologic Conditions iThe Lathrop area is located in the basin-trough geologic zone of the San Joaquin Valley, which consists of continental Pleistocene and Holocene stream-laid and i still-water deposits3. These deposits are derived from a mixture of Sierra Nevada and Coast Range rocks'. The near surface deposits are mapped primarily as Quaternary alluvial flood plain deposits5. The local depositional environment is a i sedimentary river basin, and the shallow sediments are primarily flood plain and over bank deposits of sand, silt and clay. The near surface deposits are characterized by discontinuous inter-bedded silty sand, sandy silt, silty clay, clay, i and sand deposits6. Soils are derived from mixed Sierran and Coast Range source materials, and are characterized by poor drainage. i In the Mossdale subarea, borehole samples collected as part of monitoring well installation show variable mixtures of sediment color and texture. (See Section 2.1 for a discussion of drilling details; the boring and construction logs are i provided in Appendix A). The shallow (within 20-feet of land surface) water- bearing sediments generally exhibit a fining upward sequence; the upper sediments are predominantly fine-grained deposits, whereas the lower deposits i are predominantly sands. First water was encountered during drilling at depths that ranged from 7 to 19.5 feet below land surface; depth to water may have i been influenced at some locations by construction dewatering. In the northwestern and southeastern portions of Mossdale, the fine-grained i deposits consist of variable mixtures of brown fine sand, clay, and silts; lenses of olive gray silt and clay are frequently encountered. A fine to medium grained olive-gray sand underlies the fine-grained deposits at depths ranging from 3 to 16 i feet below land surface. These general lithologic trends are characterized by the borings for monitoring wells MWM-1 through MWM-11, and MWM-16, MWM-20, and MWM-21 (see Figure 2 for monitoring well locations). In the central portion iof the Mossdale subarea, there is a general reduction in the frequency and i 3 Jennings, C.W. and Strand, R.G., 1958, "Geologic map of California, Santa Cruz Sheet', California Division of Mines and Geology, Sacramento, California. " Miller, R.E., Green, J.H., Davis, G.H..,1971, US Geological Survey Professional Paper 497-E. 5 Atwater, B. F., 1982, Geologic maps of the Sacramento-San Joaquin Delta, California, US i Geological Survey Miscellaneous Field Studies Maps, MF-1401. 6 Kleinfelder, "Hydrogeology of the Mossdale and Reiter Property, Lathrop, California", November 24, 2004. i 7 Soil Conservation Service, 1992, Soil Survey of San Joaquin County 8 Appendix A("Construction and Boring Logs for Monitoring Well") includes borehole sample descriptions based on the Unified Soil Classification System, and Appendix C ("Pore Water Extraction Results") includes similar lithologic descriptions based on feel as described in Shaw i (1928). i