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URS <br /> Mr Mike Infurna Page 5 <br /> San Joaquin County Environmental Health Department <br /> January 5, 2004 <br /> MW-6 currently delineates the downgradient extent of the dissolved MtBE above laboratory reporting lmuts <br /> A block diagram of the dissolved MtBE plume is presented as Figure 12 This figure illustrates that the <br /> highest MtBE concentrations(20 pg/L)are located beneath the former UST complex <br /> As demonstrated in the groundwater monitoring program.,the groundwater contaminant plume has remained <br /> fairly stable overtime This is likely the result of the relatively fine-grained lithology of the site(Figure 3)and <br /> the relatively low historical hydraulic gradients across the site(Table 5) <br /> Contaminant mass calculations for TPHg and benzene in groundwater identified 29 pounds and 3 pounds, <br /> respectively, that were located in the groundwater prior to remedial action (Appendix D) Remediation of <br /> groundwater to date has included use of fresh air injection into site wells However, measurements of the <br /> concentration of effluent are not part of this remediation system and, hence, mass removal rates are not <br /> possible <br /> CONCLUSIONS <br /> Based on the subsurface investigations conducted to date, the following conclusions can be made <br /> • Two active groundwater production wells are present within a 2,000-foot radius from the site These <br /> wells are screened at various depths ranging from 168 to 600 feet bgs and are located upgradient or <br /> crossgradient of the site Off-site migration of petroleum hydrocarbons from the site is estimated to <br /> extend only 50 feet to the southeast, crossgradient of the nearest drinking water well <br /> • Soil impacted with petroleum hydrocarbons is generally restricted to beneath and adjacent to the UST <br /> complex Off-site borings did not detect petroleum hydrocarbons in soil Maximum concentrations of <br /> contaminants identified prior to remedial actions are as follows TPHg,4,000 mg/kg,benzene, 13 <br /> mg/kg, toluene,75 mg/kg,ethylbenzene, 31 mg/kg, and total xylenes, 170 mg/kg <br /> • Remediation of soil and groundwater at the site included the operation of an SVE system utilizing 12 <br /> SVE wells from August 1995 to November 1997 The system removed approximately 9,109 pounds <br /> of TPHg from the site Subsequent fresh air infection into groundwater has further contributed to site <br /> remediation efforts <br /> • Based on the relatively low permeability soils(predominantly silts)and the relatively low hydraulic <br /> gradient (approximately 0 0015 ft/ft), the plume has remained stable as demonstrated through the <br /> quarterly groundwater monitoring program at the site <br /> • The remaining petroleum hydrocarbon contamination of groundwater is identified in on-site wells in <br /> the vicinity of the UST complex and dispenser islands and approximately 50 feet downgradient and <br /> off site The highest concentrations is currently being measured in well VW-10, located <br /> approximately 10 feet south of the western dispenser island TPHg and benzene were reported at <br /> 14,000µg/L and 2,800 gg/L in VW-10 during the December 2002 sampling event As these levels <br /> currently represent a threat to the beneficial use of groundwater,the site is being evaluated for further <br /> remedial action <br /> K\wprocess\J5 BP ARCO\02130\Site Conceptual Model Report\2130 SCM Rev 2 doe <br />