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U Ms.Lori Duncan,REHS Page 3 of 8 San Joaquin Environmental Health Department June 9,2005 Groundwater Monitoring The site has been on a quarterly groundwater monitoring schedule since February 1995. Groundwater concentrations during the first quarter 2005 included gasoline range organics (GRO) up to 29,000 micrograms per liter(µg/L)(MW-4), benzene up to 12,000 gg/L(MW4), and methyl tert-butyl ether up to 23 µg/L(MW-1). Groundwater is currently approximately 20 feet below the ground surface (bgs)and flowing in an easterly direction. Soil Remediation Approximately 300 cubic yards of hydrocarbon-impacted soil were removed from the site during the UST-and dispenser-removal activities. SVE was conducted from January 1997 through February 1998 to remediate vadose zone soil impacts. SVE was terminated after an estimated mass of approximately 10,000 pounds (approximately 1,600 gallons) of total petroleum hydrocarbons as gasoline (TPHg) was removed and influent benzene concentrations were less than laboratory reporting limits of 0.5 milligram per cubic meter(mg/m3). t--�ite Geology and Hydrogeology Based on observations during drilling, the subsurface beneath the site is dominated by silt. The upper 40 feet of soil beneath the site consists of clayey silt and silty clay. Below 40 feet bgs, sand and silty sand dominate to approximately 55 feet bgs. From 55-feet bgs to approximately 85 feet bgs, the primary material is sand. Below 85 feet bgs to the maximum depth explored (120 feet bgs), the lithology is silt with variable sand and clay fractions. A generalized geologic cross-section is provided as Figure 3. Investigations at the site have concluded that soil impacts are confined to the soils above 41.5 feet bgs in the shallow silt zone. The deepest soil impact was located in boring B16/MW-5 at 41 feet bgs in a stiff silt with TPHg and benzene concentrations reported at 1,700 milligrams per kilogram (mg/kg) and 6.9 mg/kg, respectively. Typically, the elevated TPHg concentrations are reported in soils located in the area of 25 feet bgs as seen with samples collected from boring SB-24 (440 mg/kg) and VW-9 (130 mg/kg) at 25 feet and 26.5 feet,respectively. Depth-to-water measurements at the site have ranged from approximately 20 to 47 bgs. Monitoring data from March 30, 2005, indicate that the dominant inferred groundwater flow direction is northeast to southeast, with average hydraulic gradients from 0.003 to 0.006 feet per foot. Historical data show groundwater direction and hydraulic gradient to be variable. The direction of flow and gradient will change between sampling events. K\WpmcmV5 BP ARCO\02168\2005 System Installation\SVE-AS Workplan Am