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Mr. Steve Sasson Project 22794-100.001 August 10, 1998 Page 4 Site Geology and Hydrogeology Geologic cross sections prepared by Weiss Associates depict the subsurface conditions m the vicinity of the former UST complex. The former UST location is underlain by low-permeability silts and clays to a depth of approximately 26 feet bgs. Interbedded sands and silty sands are locally present at depths of approximately 17 to 20 feet bgs in the vicinity of monitoring well MW-3 A laterally continuous silty sand to clean sand is present between approximately 26 and 35 feet bgs. In 1989, groundwater was approximately 26.8 feet bgs Groundwater rose approximately 7 feet between 1992 and 1995 monitoring events and was approximately 19 feet bgs in March 1995. Groundwater flow is generally eastward and the observed gradients during the 1992 through 1995 monitoring period vaned from 0.005 to 0 022 foot per foot. Safeway's 1996 Closure Report included a review of the San Joaqum County Flood Control and Water Conservation District (WCD) Map dated 1993 and other WCD maps from unspecified years. The WCD maps reportedly showed an eastward regional gradient. Well Survey Western Geologic Resources, Inc., conducted the well survey in 1990. Thirteen wells were reportedly identified within 112 mile of the site. Eleven wells are used for industrial purposes and two wells are used for domestic purposes. Safeway's March 1996 report included an updated well survey Four other wells were identified that produce groundwater for municipal and domestic purposes. The municipal well is California Water Service Pumping Station No 72 and has been out of service since 1978 Information presented in the report attributed to California Water Service cites high total dissolved solids (TDS) and methane in groundwater in the western part of Stockton as factors for taking the well out of service. Results of the well survey are presented in Appendix B. Background Water Quality According to the California Department of Water Resources (San Joaquin County Groundwater Investigation, Bulletin 146, July 1967), the shallow groundwater in the Stockton area is of generally poor quality and has Lusted beneficial use as a source of drinking water The recommended secondary MCL for chloride is 250 parts per million (ppm) and the 300 ppm chloride isoconcentration line was approximately 1 mule east of 60 the site in 1963 (DWR, 1967) Additional background water quality data collected in the SACW T=794\27941000 ICS-991tMomer8 Mon Rev 0.8110198