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URS Mr Mike Infurna Page 2 San Joaquin County Environmental Health Department June 6, 2003 The DRW records search did not identify any domestic or public groundwater production within a 2,000-foot radius of the site The EDR report identified a total of seven groundwater production wells located between 1/4 and 1/2 mule from the site (Appendix B) The City of Stockton Public Works Department (CSPWD) was contacted for information regarding the construction details and precise locations of these wells The CSPWD confirmed that there are two municipal supply wells located within the 2,000-foot radius,well#4 and well# 12 The locations of these wells are shown on Figure 1,and well construction details are provided in Table I The nearest production well is city well#4,located approximately 1,300 feet southeast of the site This well produces approximately 1,000 gallons per minute and is screened at various intervals between 192 and 600 feet bgs The other municipal well located within the search area, city well# 12, is located approximately 1,600 feet south of the site and pumps approximately 700 gallons per minute Site Geology and Hydrogeology Subsurface sediments generally consist of silt,with manor lenses of sand from 0 to 50 feet bgs,and then sands interbedded with silts from approximately 50 to 75 feet bgs (the maximum depth explored) A three- dimensional geologic block diagram of the site is included as Figure 3, and boring logs are included in Appendix C Previously prepared geologic cross-sections are included in Appendix D Figure 3 illustrates the complex nature of the site lithology The sand lenses are former stream channel deposits and are not laterally continuous across the site Depth to groundwater in site monitoring wells has ranged from approximately 54 feet bgs when the wells were first installed in 1995, to approximately 35 feet bgs during the first quarter of 2001 (Table 2) During the December,2002 groundwater monitoring event,depth to groundwater was approximately 39 feet bgs in site wells,approximately 15 feet higher than in 1995 The increase in groundwater elevations has caused the screen intervals of the eight existing groundwater monitoring wells and six of the twelve soil vapor extraction (SVE) wells to become submerged Table 3 contains monitoring and remediation well construction information Groundwater monitoring and sampling is currently being conducted on a quarterly basis from all existing monitoring wells,air sparge wells,and SVE wells The inferred direction of groundwater flow across the site has ranged from northeast to southeast with a relatively low average hydraulic gradient of approximately 0 0015 feet/foot(Table 4) Site History Three on-site groundwater monitoring wells(MW-1,MW-2, and MW-3)were installed in December 1991 Two additional on-site wells(MW-4 and MW-5)were installed August 1992 Two off-site wells(MW-6 and MW-7) were installed in November 1992 In August 1993,four former gasoline USTs of various capacities were replaced with the four existing 10,000- gallon USTs The product piping was also replaced at this time Soil samples collected beneath the former USTs product piping indicated that soil was impacted with TPHg and BTEX in these areas K 1WpT xessV5 BP ARCO\021301Sne Conceptual Model Repnrt12134 Site CmLeptu.J Model dnc