SITE INFORMATION AND CORRESPONDENCE

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VMr. Werner Sicvol • - 2 - • 13 October 199V'))�`—', BP Oil should operate the SVE and P&T systems until it makes the demonstration that Items 1 and 2 or 3 above have been met. BP Oil should make the demonstration that the vadose zone cleanup has been achieved for Item 1 using contaminant fate and transport modeling, trend analysis, mass balance, and/or other means. This demonstration should include examination of the effects of remaining vadose zone contamination on the groundwater, using an appropriate vadose zone model, if necessary. Groundwater cleanup has been achieved when contaminant concentrations are at background concentrations. To demonstrate compliance with Item 3, the facility should operate the SVE and P&T systems until VOC removal is no longer technologically or economically feasible. The feasibility analysis will include but not be limited to consideration of the following factors: 1. Whether the predicted concentration of the leachate from the vadose zone (using an appropriate vadose zone model that interprets soil gas data) will exceed the background groundwater concentration; 2. Whether the mass removal rate is approaching asymptotic levels after temporary shutdown periods and appropriate optimization of the SVE and P&T systems; 3. The additional cost of continuing to operate the SVE and P&T systems at concentrations approaching asymptotic mass levels; 4. The predicted effectiveness and cost of further enhancements to the SVE and P&T systems (e.g., additional vapor extraction wells, air injection, air sparging) beyond system optimization, which should occur throughout operation of the remedial action, to remove additional VOCs; 5. Whether the cost of groundwater remediation will be significantly more if the residual vadose zone contamination is not addressed; 6. Whether residual mass in the vadose zone will significantly prolong the time to attain the groundwater cleanup standard; and 7. The incremental cost over time of vadose zone remediation compared to the incremental cost over time for groundwater remediation on the basis of a common unit (e.g., cost of pound of contaminant removed). BP Oil should cycle the SVE and P&T systems on and off in order to optimize system operations and/or to evaluate the factors listed above. Additional Comments The work plan proposes to conduct a 6 -hour pumping test to determine hydraulic properties of the aquifer beneath the facility. During a telephone conversation on 5 October with your consultant, Mr. Fred Kintzer of Parsons Engineering Science, Inc., he stated that the pumping test had been completed. Typically, pumping tests are run for 72 hours for unconfined aquifers and 24 hours or less for confined aquifers. I informed Mr. Kintzer that the 6 -hour test seemed insufficient and may have to be repeated if it did not provide the needed hydraulic information. He said that the test was shortened because of concerns regarding treatment and disposal of extracted groundwater, they got the needed information on aquifer properties, and trenching for the SVE and P&T systems piping will start during the first week of November.