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US t Mr Mike Infurna Page 2 San Joaquin County Environmental Health Department January 5, 2004 y 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 1 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 Geologyand H dro eolo Y 9 9Y ' Subsurface sediments generally consist of silt,with minor 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 and shows that 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 and Table 4 contains well screening and water level 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 5) Site History A subsurface investigation was performed in June 1991 during which one boring located,lust north of the UST complex was drilled and sampled between 15 and 68 feet bgs Soil samples identified the presence of petroleum hydrocarbons in all the samples collected at maximum concentrations of 2,400 mg/kg total ' petroleum hydrocarbons as gasoline(TPHg), 13 mg/kg benzene,75 mg/kg toluene, 31 mg/kg ethylbenzene, and 170 mg/kg total xylenes A follow-up investigation was conducted in December 1991 During this investigation,seven borings were drilled and sampled for TPHg and benzene,toluene,ethylbenzene,and total xylenes(BTEX) The analyses generally revealed increasing concentrations of petroleum hydrocarbons with depth The highest concentrations (TPHg, 3,000 mg/kg, benzene, 14 mg/kg, toluene, 37 mg/kg, ethyl- benzene,22 mg/kg,and total xylenes, 130 mg/kg)were detected north,south,and west of the UST complex Three of the borings(B-2,B-6,and B-S)were converted to monitoring wells(MW-1 through MW-3)and four of the bonngs(B-3 through B-5,and B-7)were converted to vapor extraction wells (VW-1 through VW-4) The four vapor extraction wells were used in a vapor extraction test performed in December 1991 At a wellhead flow rate of 110 cubic feet per minute(cfm)and a vacuum of 70 inches of water,the effective radius of influence was estimated to be approximately 70 feet Alternately, a wellhead flow rate of 30 cfm and ,1 K 1Wprocess115 BP ARCO\02130\Sjte Concepwal Model Report12130 SCM Rev 2 doc