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Mr. Philip S. Isorena January 12, 2001 Page 2 including complete laboratory data packages was recently submitted in the 4th Quarterly Monitoring Report for 2000 dated January 4, 2001. Phase III of SVE treatment of the Source Area vadose zone soils was started on September 28th after a 7-month rest period. Sampling of soil vapor from all vapor monitoring points was performed immediately prior to the startup with analyses performed on-site by a mobile laboratory. Periodically, vapor samples were collected from the extraction wells using Summa canisters and the analyses were used to track mass removal. Based on decreasing soil vapor concentrations, it was decided to terminate Phase III SVE treatment on December 4 after 67 days of operation. Sampling for shutdown was conducted in the same manner as for startup. As shown on Figure IA, SVE was conducted in three phases during the past 1-1/2 years. Total VOC concentrations at the VEWs steadily declined during each phase of SVE operation and relatively small rebounds occurred after both rest periods. The overall reduction in concentration in the extracted vapor since the beginning of SVE treatment has been nearly three orders of magnitude. For example, the concentration at VEW-1 in the center of the Source Area decreased from approximately 900 mg/m3 during the April 1997 pilot test to ND (< 0.40 mg/m3 for each analyte) at the end of Phase III on December 14, 2000. At the end of Phase III, the VEW with the highest concentration was VEW-2 with 1.2 mg/m3 total VOCs. Table 1 shows that similar reductions of soil gas concentration have occurred at the VMPs. A standard of 1 ppmv (4.0 for 1,1-DCE and 5.5 mg/m3 for 1,1,1-TCA and TCE) is being utilized to initiate closure activities for SVE operations for remediation of chlorinated solvents at McClellan AFB. The recommendation at this threshold concentration is to consider termination of SVE, observe rebound and perform vadose zone modeling. By comparison, at the end of Phase III at the AASC site only a few samples from depths shallower than 35 feet had detections near this range (5 mg/m3, Table 2). The highest detection in the vadose zone shallower than 35 feet bgs on December 4, 2000, was 4.0 Mg/M3 total VOCs at VMP-3-27. Asymptotic Contaminant Mass Removal Rate Figure 1B summarizes mass removal data for both SVE soil treatment and groundwater extraction. The cumulative graph shows that mass removal by SVE approached an asymptotic curve at the end of Phase III. The figure also shows that the monthly mass removal for SVE during operational phases has been exceeded by groundwater extraction since January 2000. Phase III SVE removed approximately 0.7 lb of VOCs during 2 months of operation and only 0.2 lb during November. Confirmation Soil Vapor Sampling Confirmation soil gas borings are planned at 6 locations shown on Figure 2. The objective is to confirm that the low concentrations achieved at the VEWs and the VMPs have also been achieved at random intermediate points in the Source Area. Four (4) soil gas samples will be collected in each boring at various depths between 15 and 35 feet bgs for a total of approximately 24 samples. On November 6, 2000 the depth to the SVE CLOS.DOC 1/12/01