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AVOCs have been detected in vadose zone soil at scattered locations <br /> throughout the site. Concentrations range from 3.6 mg/kg to <br /> 209.2 mg/kg. <br /> Appendix A contains location-specific of estimates of VOC mass in site <br /> soil. <br /> Ground Water Impacts <br /> Semiannual isoconcentration contour maps have been prepared since 1995 <br /> to define the extent of VOCs in groundwater at the site. Historical <br /> semiannual isoconcentration maps for these VOCs can be found in <br /> Appendix R of the 1996 Annual Monitoring Report and in Appendix K of <br /> the 1995 Annual Ground Water and Ground Water Treatment System <br /> Monitoring Report. The highest historical concentrations of total HVOCs <br /> in ground water were detected in TW-2 and TW-3. Concentrations at <br /> TW-2 have ranged from 857 micrograms per liter (pg/L) to 2124 pg/L. <br /> The total HVOC concentrating in ground water at TW-3 during the single <br /> sample event conducted at this well was 2124.3 pg/L. The highest <br /> historical concentrations of total AVOCs in ground water were detected in <br /> MW-6A and MW-11A at concentrations of 30.5 and 29.9 pg/L, <br /> respectively. <br /> Existing Ground Water Extraction System <br /> An existing interim ground water extraction and treatment system is <br /> currently in operation at the site and is regulated by Waste Discharge <br /> Requirements (WDRs) Order No. 92-226. This system was installed in <br /> 1992 and consists of 16 ground water extraction wells which are <br /> positioned along the property boundary (downgradient of the source <br /> area) to mitigate potential off-site migration of impacted ground water. <br /> The extracted ground water is treated in the Engine House using two <br /> 1,800-pound liquid phase carbon vessels placed in series. The effluent <br /> from the carbon vessels is discharged to the subsurface through two <br /> existing parallel infiltration trenches. Drawing C-1 shows the location of <br /> existing ground water extraction wells and infiltration trenches. <br /> ■ Ground water is extracted from each existing extraction well through <br /> suction tubes. The motive force for ground water extraction at extraction <br /> wells are pneumatic diaphragm pumps. These wells are currently <br /> extracting ground water at a rate of approximately 0.5 gallons per minute <br /> (gpm) per well. To increase the ground water extraction rate at the plume <br /> 2-3 <br />