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Former Beacon Station No 12419 March 18,2002 Stockton California Problem Assessment Repo rt/Correcti ve Action Plan . The Great Valley geomorphic province is an elongate, northwest-trending, asymmetric structural trough filled with a thick sequence of marine and non-marine sediments ranging in age from Jurassic to Holocene Great Valley sediments generally thin to zero towards the adjacent Sierra Nevada and Coast Ranges provinces and are predominantly underlain by the westward continuation of Sierran granitic and metamorphic rocks The Coast Ranges are underlain, in part, by folded and faulted sedimentary rocks equivalent to intervals found at depth in the Great Valley stratigraphic sequence Generally unconsolidated to poorly consolidated alluvial, fluvial and lacustrine deposits of Miocene to Holocene age comprise the upper portion of the stratigraphic sequence of the Great Valley province and are generally underlain by consolidated marine rocks Surface sediments in the site area are generally Pleistocene to Holocene flood-basin deposits and Pliocene to Holocene alluvial and fluvial deposits (USGS, 1986) Lithologic data from soil borings indicate the site surface is underlain by interbedded sand, silt and clay as shown on the generalized cross sections A-A' and B-B' (Figures 3 and 4, respectively), located as shown on Figure 2 The Central Valley is divided into three hydrologic basins the northern Sacramento River Basin, the central San Joaquin River Basin, and the southern Tulare Lake Basin The city of Stockton is located within the San Joaquin River hydrologic basin The general movement of groundwater within the San Joaquin River hydrologic basin is from the adjacent flanks of the Sierra Nevada and Coast Ranges provinces toward the axis of the Great Valley structural trough, which is generally located in the western portion of the Central Valley, nearer to the Coast Ranges and west of the site The natural, regional groundwater flow then generally runs north-northwest parallel to the axis of the structural trough Depth to groundwater at the site generally rose from approximately 70 feet below surface grade (bsg) in 1994 to approximately 40 feet bsg in May 2000 Thereafter, groundwater levels generally dropped to approximately 54 feet bsg in November 2001 During the most recent four quarters of monitoring, groundwater flow directions were variable and generally 419PARCAP3-02 doc 3 Project No 1419 23 HORIZON ENVIRONMENTAL INC.