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CLOSURE REQUEST REPORT Section No 3 0 Former American Forest Products Company 07/31/01 2801 West Lane Page 3-2 Stockton,California 3.2 SITE GEOLOGY AND HYDROGEOLOGY The site is situated at the northern end of the San Joaquin Valley of the Great Valley geomorphic province of California The site rests on over 1,000 feet of sediments consisting of recent alluvium and unconsolidated Pleistocene sediments of the Victor Formation and Arroyo Seco gravels These units are underlain by the semi-consolidated to consolidated Tertiary to early Pleistocene sediments of the Laguna and Mehrten Formations The near-surface sediments directly underlying the site are Holocene-age flood-basin deposits characterized by fine silt and clay These sediments were deposited by flood waters from the Calaveras and San Joaquin rivers Recent alluvial deposits overlie the Victor Formation and generally consist of a heterogeneous assemblage of sand, silt, gravel, and clay deposited by shifting streams on broad alluvial fans As is typical in alluvial depositional environments, sediments are heterogeneous laterally and vertically Information on subsurface conditions at the site collected during the remedial site investigations show that the upper 80 feet of soils underlying the site are composed of interbedded clays, silts, and fine sands In general, the upper 15 to 25 feet of sediments consist predominantly of dense clayey silts and silty clays with locally discontinuous fine sand layers grading into fine sandy silts and silty sands Generalized geologic cross-sections in the vicinity of the maintenance shop are presented in Figures 5 and 6 The cross-sections have been interpreted from well installation boring logs and soil boring information Well construction data is presented in Table 1 The depth to groundwater at the site in March 2001 was approximately 38 feet below ground surface (bgs) Water levels have risen approximately 34 feet since July 1991, when a maximum depth to groundwater of approximately 69 feet bgs was reported The highest groundwater levels were reported in May 2000 (approximately 35 feet bgs) Historical water level measurements since 1990 are compiled in Table 2 Seasonally, water levels vary from three to 4 feet, with the shallowest water levels typically measured in the spring/summer and the deepest water levels in the fall/winter At least two groundwater- producing zones have been reported (Dames & Moore, 1994a), and the impacted aquifer at the site is reported to be located within a shallow unconfined aquifer extending to 150 feet bgs The direction of groundwater flow is typically to the north-northeast, based on interpretation of groundwater elevation contour maps included in quarterly reports Groundwater elevation contour maps for November 2000 and March 2001 are presented in Figures 7 and 8, respectively Historically, the direction of flow has shifted from the west-northwest in 1987 to the east in 1995 and is now generally in the direction of north-northeast to northeast The magnitude of the gradient has varied from 0 0005 to 0 0018 foot/foot since 1991 Groundwater use in the vicinity of the site was assessed by perforinmg a water well survey within a 1-mile radius of the site in I988 and in 2001 The surveys were conducted using sources including the California Department of Water Resources, the State of California Department of Health Services — Public Water Supply Branch, and the San Joaquin County Flood Control and Water Conservation District A total of seventeen water wells, excluding monitoring wells, were identified within a 1-mile radius of the site Well information and locations are presented in Appendix A Six water wells operated by the California Water Service Company were reported within a 1-mile radius of the site All six wells were reported to have screens deeper than 200 feet bgs, which correlates to a deeper, semi-confined aquifer, as opposed to the shallow unconfined aquifer impacted by TPH at the site One domestic water supply well appears to be Iocated within a 2,000-foot radius of the site The well was completed to a depth of 125 feet bgs It is uncertain if the well is actively used The well is located south K 1840191afp1afp_c1osure_rpt doc