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I L � beneath the subject site. The initial results were inconclusive. As a result, Chevron drilled and sampled deep soil boring VDSB-1 on July 14, 1997. Results obtained from that boring were successful in defining the vertical extent of contamination. The air sparge system was activated shortly after groundwater was sampled in April. It has been operating since. The introduction of air into the saturated zone causes dynamic changes to hydrocarbon distribution and concentration during the remediation process. As a result, Chevron and Geraghty&Miller feel that any results obtained now from these wells would be inconclusive and would not provide any benefit to our understanding of the remediation effectiveness or contaminant plume trend. Instead, it may be more appropriate to sample these wells at the conclusion of our remediation effort as a means to evaluate remediation effectiveness. Do you concur? As per instructions from your agency, Chevron has begun GC/MS analyses of groundwater beneath the subject site for oxygenates. All site-related monitoring wells and Municipal Well#10 were sampled on November 25, 1997 and analyzed for seven separate oxygenate compounds as described in the revised August 30, 1997 letter from the Central Valley Regional Water Quality Control Board. The results indicated that MTBE was present in seven of the ten wells sampled. The highest reported concentration was 2,100 ppb at well MW-9. In addition, this well contained 65 ppb TAME. There were no other oxygenate compounds detected at any of the remaining wells. This includes methanol. As previously discussed, it appears that the MTBE detected at well MW-9 originated from the ARCO facility across West Benjamin Holt Drive. This suspicion is supported by the unusually high concentration detected at this location relative to other MTBE detections at the subject site and by the occurrence of TAME where it is not associated with any other MTBE detection at the subject site. Chevron will continue to analyze groundwater samples by GC/MS methods for the purpose of understanding oxygenate plume trends and remediation effectiveness. Your letter also suggested appending existing geologic cross sections with those data acquired from ten new soil borings. The lithology logs from the new borings appear to agree quite well with the subsurface picture depicted by both existing cross sections. As such, Chevron and Geraghty&Miller see no incremental value to be gained by constructing more cross sections at this time. »•t.:=r. As requested in your letter, Chevron will begin submitting quarterly updates on the status of our remediation efforts. As mentioned above, these updates will include hydrocarbon mass calculations.For your information, I have included an informal copy of the most recent remediation operation update. A formal update will be submitted shortly after each calendar quarter. If you have any questions or comments, I can be reached at(510) 842-8695. I Sincerely, CAI Brett L. Hunter Environmental Engineer Site Assessment and Remediation Attachment cc: Elizabeth Thayer, Central Valley RWQCB, Sacramento, CA Ed Formosa, City of Stockton,Department of Municipal Utilities, 2500 Navy Dr., Stockton, CA 95206 Paul Supple, ARCO Products Co.,P. O. Box 6549,Moraga, CA 94570 Steve Misner,Brown and Caldwell, 9616 Micron Ave., Suite. 600, Sacramento, CA 95827 Aaron O'Brien, Geraghty&Miller, 1050 Marina Way South, Richmond, CA 94804 i