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01/19/98 14:34 0510 842 8370 CHEVRON U.S.A. Z003/004 <br /> beneath the subject site.The initial results were inconclusive. As a result,Chevron drilled and sampled <br /> deep soil boring VDSB-1 on July 14, 1997.Results obtained from that boring were successful in defining <br /> the vertical extent of contamination.The air sparge system was activated shortly after groundwater was <br /> sampled in April.It has been operating since.The introduction of air into the saturated zone causes <br /> dynamic changes to hydrocarbon distribution and concentration during the remediation process. As a. <br /> result,Chevron and Geraghty&Miller feel that any results obtained now from these wells would be <br /> inconclusive and would not provide any benefit to our understanding of the remediation effectiveness or <br /> contarninant plume trend.Instead,it may be more appropriate to sample these wells at the conclusion of <br /> our remediation effort as a means to evaluate remediation effectiveness.Do you concur? <br /> As per instructions from your agency, Chevron has begun GCIMS analyses of groundwater beneath the <br /> subject site for oxygenates. All site-related monitoring wells and Municipal Well#10 were sampled on <br /> November 25, 1997 and analyzed for seven separate oxygenate compounds as described in the revised <br /> August 30. 1997 letter from the Central Valley Regional Water Quality Control Board.The results <br /> indicated that MTBE was present in seven of the ten wells sampled.The highest reported concentration <br /> was 2,100 ppb at well MW-9.In addition.this well contained 65 ppb TAME.There were no other <br /> oxygenate compounds detected at any of the remaining wells.This includes methanol. As previously <br /> discussed,it appears that the MTBE detected at well MW-9 originated from the ARCO facility across <br /> West Benjamin Holt Drive.This suspicion is supported by the unusually high concentration detected at <br /> this location relative to other MTBE detections at the subject site and by the occurrence of TAME where it <br /> is not associated with any other MTBE detection at the subject site.Chevron will continue to analyze <br /> groundwater samples by GGMS methods for the purpose of understanding oxygenate plume trends and <br /> remediation effectiveness. <br /> Your letter also suggested appending existing geologic cross sections with those data acquired from ten <br /> new soil borings.The lithology logs from the new borings appear to agree quite well with the subsurface <br /> picture depicted by both existing cross sections. As such.Chevron and Geraghty&Miller see no <br /> incremental value to be gained by constructing more cross sections at this time. <br /> As requested in your letter. Chevron will begin submitting quarterly updates on the status of our <br /> remediation efforts. As mentioned above,these updates will include hydrocarbon mass calculations.For <br /> your information.I have included an informal copy of the most recent remediation operation update. A <br /> formal update will be submitted shortly after each calendar quarter. <br /> If you have any questions or conunents.I can be reached at(510)842-8695. <br /> Sincerely, <br /> /S 4 1 <br /> Brett L.Hunter <br /> Environmental Engineer <br /> Site Assessment and Remediation <br /> Attachment <br /> cc: Elizabeth Thayer,Central Valley RWQCB,Sacramento.CA <br /> Ed Formosa. City of Stockton.Department of Municipal Utilities.2500 Navy Dr., <br /> Stockton, CA 95206 <br /> Paul Supple.ARCO Products Co.,P.O.Boa 6549,Moraga.CA 94570 <br /> Steve Misner.Brown and Caldwell,9616 Micron Ave..Suite.600, Sacramento.CA 95827 <br /> Aaron O'Brien,Geraghty&Miller. 1050 Marina Way South,Richmond.CA 94804 <br />