SITE INFORMATION AND CORRESPONDENCE_FILE 1

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o / � r1 Chevron Chevron Chevron U.S.A.Products Company 2410 Camino Ramon July 5, 1994 San Ramon,CA 94583 P0.Box 5004 San Ramon,CA 94583-0804 Marketing Department Ms. Mary Meays Phone 510 842 9500 San Joaquin County Environmental Health P.O. Box 2009 Stockton, CA 95201 Re: Chevron Station #9-6171, 6633 Pacific Avenue, Stockton, CA Attached groundwater monitoring report(Sierra, 5/2/94) Dear Ms. Meays: Attached you will find a report dated May 2, 1994, which was prepared by Chevron's consultant, Sierra Environmental Services (Sierra), to describe groundwater monitoring performed at the subject site on March 24, 1994. The measured direction of groundwater flow during the March monitoring event was southeasterly. All six site-related wells were gauged and sampled by Sierra. Samples were analyzed for the presence of TPHGas and BTEX constituents. Detectable concentrations of petroleum hydrocarbons were measured at all wells. The measured concentrations were consistent with those detected during previous site monitoring events. I would also like to take this opportunity to address the issues raised in your letter dated, May 24, 1994. Since approximately December, 1991 groundwater contamination has been intermittently detected at all monitoring well locations. As was pointed out in your letter, the concentrations detected at MW-2 have been the highest and appear to be increasing. Two possible explanations exist for the apparent increase in dissolved hydrocarbon concentrations at MW-2. One would be that residual hydrocarbons derived from pre-1986 service station operations remain in soil and are currently impacting groundwater. This is probably not the case as groundwater contamination at MW-2 was not detected until late 1991. Also, there was no soil contamination detected from former tanks or lines or from monitoring well installations upgradient from MW-2. A second possible explanation would be that groundwater contamination at MW-2 is from a source associated with the current UST complex and/or south pump islands. It is this possibility that probably lead to your agency's request for additional investigation at these areas. Of the two possible explanations, the second seems most likely. A review of Chevron compliance and maintenance records for the subject site was made in attempt to recognize any source(s) of fuel release. There have been no known releases at this station as a result of recent operations. Piping and UST integrity has been demonstrated through interstitial tank monitoring and annual tightness tests. Maintenance records show however, that the spill containment boxes on the UST's have been worked on several times since 1991. Spillage has not been documented during any of the repairs but, considering the time frame which they have occurred (1991-1994) and the possibility for undetected leakage at these areas, the spill containment boxes may represent the most likely source for groundwater contamination.