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y l..mepa's Car Wash (Pacific Avenue) 1 Problem Assessment Report and Corrective Action Plan Page 6 Only one sample from either SB-3 or SB-4 contained a constituent of concern (MTBE at 0.28 mg/kg from 40 feet has in SB-4) . These borings were installed following the intermittent operation and shut down of the SVE system . MTBE (MW- 1 I , 0 . 005 mg/kg at 35 feet and 0 . 009 mg/kg at 40 feet) was the only constituent of concern found in soil samples collected from borings for monitor wells MW-9. MW- 10, and MW- 11 . The mass of petroleum hydrocarbon in the soil and the volume of contaminated soil were estimated using the exploratory data provided by WHF in 1994 . The estimated concentrations and distributions of hydrocarbons in soil are illustrated in plan view in Figure 7. The mass and volumes were estimated using the software "HyperVentilate 2. 0". The average soil TPH-G concentrations quantified in mg/kg were applied to a range of soil volumes to estimate the mass balance . The soil volumes were estimated based on the approximate thickness and radius of the contaminated soil column . The volume of petroleum hydrocarbons was estimated at approximately 13,000 pounds or 2,050 gallons (6.34 lbs/gal.). The volume of contaminated soil containing greater than 10 mg/kg TPII-G was estimated at 18,000 cubic yards. An array of nine soil vapor extraction wells was installed under the dispenser island in 1993 , and an SVE and combustion system was installed to remove VOCs from the extracted soil gas. The combustion unit was permitted in March 1994 and operated to mid-June 1995 . Average influent VOC concentrations measured with a photo ionization detector exceeded 2,200 ppm. The last influent measurement collected from the system was 2, 145 ppm in May 1995 . Operation of the system in 1995 configuration was halted in the fall of 1996 . A new thermal oxidation unit was installed, and the system was restarted in March 1999 and operated intermittently until February 2001 . From March 1999 to June 1999, an estimated 3 ,085 pounds of hydrocarbons were extracted from the soil and groundwater. Selected VOC concentrations in air influent to the thermal oxidation system on March 15, 1999 were 200 µg/l benzene and 190 µg/1 MTBE. In March 2002, the SVE was modified to use vapor phase activated carbon. Selected influent soil vapor VOC concentrations in March 2002 were 0 . 75 µg/1 benzene and < 0 . 15 µg/I MTBE. Depth to groundwater in the SVE wells was approximately 43 feet in March 1996 and 33 feet in April 2002 . Clearly, the soils contaminated with petroleum hydrocarbons are saturated and are now below the water table. The VOCs extracted by the system in 2002 are mostly vapors released from groundwater. 5.2 DISTRIBUTION OF GROUNDWATER CONTAMINATION Groundwater monitor wells were installed to investigate the vertical and lateral distribution of VOCs in groundwater beneath and adjacent to the site. The evolution and migration of the groundwater plume have been difficult to monitor because of rising water levels. Monitor wells MW-1 through MW-4, installed in 1993 , were fitted with screens extending from depths of 50 feet to 70 feet. Monitor well MW-5 was constructed in 1994 with the same screen position, and monitor wells MW-6, MW-7, and MW-8, also constructed in 1994, were constructed with screens extending from 45 feet to 65 feet. By March 1994, the rising water table approached the top of the screens in monitor wells MW- 1 through MW-5 . In 2001 , Condor installed monitor wells MW-9, MW- 10 and MW- 11 with screens positioned across the present water table elevation, a depth of approximately 35 feet. A deep monitor well, MW- 12d, screened from 100 to 110 feet, and nine hydropunch borings also were installed in 2001 to investigate the vertical extent of the plume and to sample a vertical sequence through the plume near the source area and in areas of potential lateral extent. 4M #L4 CONDOR