SITE INFORMATION AND CORRESPONDENCE

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page 2, 515 Charter Way However, the wells proposed for use in the test are dual cased nested wells. SJC/EHD is concerned that operation of the SVE system during the air sparge test could create skewed results by pulling the injected air through the artificial preferential pathway of the sand pack around the well screens, and with how the pressure effects of one test will be differentiated from those of the other. The addendum also proposes to begin intermittent operation of the SVE system. IT reported that influent concentrations of contaminants have significantly decreased since the system was started, indicating that the system may have reached an asymptotic level in its ability to recover contaminants. However, this has not been demonstrated graphically, and SJC/EHD notes that approximately 500 pounds of total petroleum hydrocarbons as gasoline (TPHg)were removed by the system during the 4'h Quarter 2001, which appears to indicate that the system is still performing efficiently. Beginning in the second quarter of 2002, IT will modify the operation of the system to be alternated on and off for two week intervals, to allow for rebound of contaminant vapors in the soil. SJC/EHD is concerned that such an operation will reduce the efficiency of the system prematurely. In discussion of these concerns with IT, it was agreed that careful monitoring of the system would be done during the second quarter of 2002, and that full time operation of the system will resume if efficiency drops more than expected. As this letter is being prepared during the second quarter 2002, the intermittent operation of the SVE system is tentatively approved with the qualifications noted. Please provide to SJC/EHD in the next quarterly report graphs showing mass removal rate verses time for the system. In summary, SJC/EHD approves the installation of the two shallow groundwater monitoring wells for lateral assessment of the plume that were proposed in the March 13, 2002 work plan, the installation of the monitoring well pair MW-4R for vertical assessment of the soil and groundwater contamination near the source area, and the change to intermittent operation of the soil vapor extraction system,with the provisions noted. More information is needed on the proposed method of the air sparge test. In the report of findings for the approved work, SJC/EHD requests the inclusion of a site conceptual model with maps and cross-sections depicting the geological and plume migration models; the newly acquired data should be incorporated into the model. If you have any questions or comments please call Lori Duncan at(209)468-0337. Donna Heran, REHS, Director Environmental Health Department (Iav , iia ,I/ � Lori Duncan, Senior REHS Nuel C. Henderson, Jr., R.G. LOP/Site Mitigation Unit IV LOP/Site Mitigation Unit IV cc: Marty Hartzell, CVRWQCB Kathleen Waldo, IT Corporation