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• GROUND ZERO ANALYSIS, INC. 1714 Main Street Escalon,California 95320 Telephone: (209)838-9888 Facsimile: (209)838-9883 w June 27, 2000 N '— N Mr. Ron Rowe San Joaquin County Public Health Services Environmental Health Division 304 E. Weber Ave., P Floor Stockton, CA 95202 Subject: Addendum to Workplan for Confirmation Borings and Remedial Effectiveness Evaluation-Knowles/Roek Property, 102 South Wilson Way, Stockton, CA(Figure 1) Dear Mr. Rowe: • As requested in your April 10, 2000 letter commenting on Ground Zero's Workplan for Remedial Effectiveness/Case Closure Evaluation (dated September 17, 1999) we are submitting this addendum to the workplan. The proposed work is intended evaluate the effectiveness of remediation to date, the cost effectiveness of continued active remediation, the fate of residual contamination under natural processes and the potential impact to human health and the environment from residual contamination. In addition, in accordance with your directive, the work is intended to further define the extent of contamination and identify preferential flowpaths, if any. We are proposing this work based upon the following observations: Source Material Removal by Soil Vapor Extraction is No Longer Cost Effective Our data suggest that approximate 50% of the estimated mass of soil contaminants prior to instituting vapor extraction has since been removed. The remaining 50% is comprised for the most part of contaminated soil which is currently submerged due to high water levels. Water levels are not expected to decrease in the near future. As of December, 1999, influent vapor concentrations to the VES had become asymptotic at relatively low levels while he cost per unit of contaminant mass removed from the subsurface had stabilized at a high value. • rmk\cbsum\addendum