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GROUND ZERO ANWSIS,INC. • <br /> 1714 Main Street <br /> Escalon,California 95320-1927 <br /> Telephone: (209)838-9888 <br /> Facsimile:(209)838-9883 <br /> September 17, 1999AIVE® <br /> f, <br /> MAR 0 6 2000 <br /> Mr. Ron Rowe ENVIRONMENTAL HEALTH <br /> San Joaquin County Public Health Services PERMIT/SERVICES <br /> Environmental Health Division <br /> 304 E. Weber Ave., 3d Floor. <br /> Stockton, CA 95202 <br /> Subject: Workplan for Remedial Effectiveness/Case Closure Evaluation <br /> Knowles/Roek Property, 102 South Wilson Way, Stockton, CA (Figure 1) <br /> Dear Mr. Rowe: <br /> As per your August 23, 1999 comments in response to Ground Zero Analysis' Second <br /> Quarter 1999 Status Report (dated July 27, 1999) we are submitting this workplan for a <br /> remedial effectiveness/case closure evaluation. The proposed work is intended to evaluate <br /> the effectiveness of active and natural remedial processes to date, the cost effectiveness of <br /> continued active remediation, the fate of residual contamination under natural attenuation <br /> processes and the impact to human health and the environment from residual <br /> contamination. We are proposing this work based upon the following observations: <br /> Source Material Removal by Soil Vapor Extraction is No Longer Cost Effective <br /> Our data suggest that approximate 50% of the estimated mass of soil contaminants prior <br /> to instituting vapor extraction has since been removed. The remaining 50% is comprised <br /> ofor the most part of contaminated soil which is currently submerged due to high water <br /> levels. Water levels are not expected to decrease in the near future. Influent vapor <br /> o� concentrations to the VES have become asymptotic at relatively low levels (Figure 2). <br /> The cost per unit of contaminant mass removed from the subsurface has stabilized at a <br /> high value(Figure 3). <br /> Plume Stability <br /> Cumulative groundwater sampling data indicate that the plume of dissolved hydrocarbon <br /> (o e� contamination is stable. The exception is the increasing concentrations of dissolved <br /> gasoline and MTBE beneath the southern portion of the property. As discussed in the <br /> Second Quarter 1999 status report, the source of this encroaching contamination is offsite <br /> U9 <br /> ro k\c1mum%domp.dw <br />