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01/19/98 14:34 '$510 842 8370 CHEVRON U.S.A. Z002/004 <br /> emo <br /> ';; Chevron <br /> Chevron Products Company <br /> January 16, 1998 6001 Bollinger Canyon Road <br /> Building L <br /> San Ramon,CA 94583 <br /> P.C.Box 8004 <br /> San Ramon.CA 94583-0904 <br /> Ms.Mary Mc ays Marketing—Sales West <br /> San Joaquin County Environmental Health Phone 510 842-9500 <br /> 304 E.Weber Ave.,Third Floor' <br /> Stockton,CA 95202 <br /> Re: Chevron Station#9-4275.2905 West Benjaarnin Holt Drive,Stockton,CA <br /> Attached Remediation Status Report(11/14/97) <br /> Dear Ms_Meays: <br /> I would like to acknowledge receipt of your letter dated December 29, 1997.Your letter provided <br /> comments to Geraghty&Miller's Corrective Action Plan,Blaine Tech's Third Quarter Monitoring <br /> Report,and discussed the need for further corrective action. <br /> Chevron was requested to submit an addendum to Geraghty&Miller's Corrective Action Plan by January <br /> 15, 1998.1t was stated that Chevron would be required to continue assessment of the evidenced plume.To <br /> that extent,additional groundwater monitoring wells would be required to monitor groundwater <br /> conditions cross and down gradient from well MW-4. <br /> The plume in the vicinity of MW-4 has been represented by detections of MTBE.The second and third <br /> quarter 1997 sampling events detected increases of 3,600 and 2,100 ppb,respectively_'These detections <br /> were not confirmed by GC/MS analytical methods.The fourth quarter(November) 1997 sampling event <br /> however,detected only 54 ppb MTBE.The fourth quarter measurements were cottfinned by GC/MS <br /> methods.The fourth quarter sample was collected at the same approximate groundwater elevation where <br /> 2,100 ppb were detected during the third quarter.It is not known whether the observed concentration <br /> decrease is a result of ongoing remediation effectiveness or whether.it simply rei3cets inaccuracies of the <br /> two prior measurements.In light of these uncertainties.Chevron feels it would be premature to act on any <br /> trends in groundwater quality that haven't been confirmed. As a result.we do not agree with the need for <br /> further assessment.Instead,Chevron recommends continuing GC/MS analysis on all groundwater <br /> samples.The decision to Ai rther assess groundwater plume limits(and submit a CAP addendum)will be <br /> based on more accurately established trends.Your concurrence and patience on this approach would be <br /> greatly appreciated. <br /> Your letter raised the issue regarding Geraghty&Miller's approach to calculating the current <br /> hydrocarbon mass in place. When compared to the total mass in place,the amount represented by <br /> hydrocarbons dissolved in groundwater is insignificant.knowing this_Chevron concurred with Geraghty <br /> &Miller's approach. As requested,Chevron will provide mass calculations in quarterly remediation state <br /> reports. <br /> Your agency also requested that.Chevron re-sample groundwater at all site-related air sparge wells.The <br /> initial sampling was performed on April I, 1997 in attempt to define the vertical extent of contamination <br />