SITE INFORMATION AND CORRESPONDENCE FILE 1

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`s rnerroe Chevron MAR 0 5 1997 Chevron Products Company March 3, 1997 6001 Bollinger Canyon Road Building L ENVIRONMENTAL HEALTH San Ramon,CA 94583 PERMITISERVICES P.O.Box 5004 San Ramon,CA 94583-0904 Mr. Michael Infurna San Joaquin County Environmental Health Marketing-Sales West P.O. Box 388 Phone 510 842-9500 Stockton, CA 95201-0388 Re: Former Chevron Station#94054. 2103 Country Club Blvd., Stockton CA Attached Groundwater Monitoring Report(Blaine Tech, 12/4/96) Dear Mr. Infirma: Attached you will find a report dated December 4, 1996,which was prepared by Chevron's consultant, Blaine Tech Services, Inc. (Blaine Tech),to describe the results of groundwater monitoring that was performed at the subject site on November 5, 1996. In November Blaine Tech gauged and sampled ten of the twelve site-related wells.Both,MW4 and MW- 9 were inaccessible. The measured direction of groundwater flow was toward the north-northeast under a hydraulic gradient of D.005 ft/8. This flow direction is consistent with historic trends. All groundwater samples were analyzed for the presence of TPHGas,BTEX constituents,and MTBE. Samples were also analyzed for various ions to evaluate the potential for intrinsic bioremediation. Detectable concentrations of dissolved hydrocarbons were measured at seven of the ten wells sampled. The measured concentrations were consistent with those detected during previous site monitoring events. The corresponding levels of MTBE detected at these wells ranged between 7.2 ppb and 770 ppb. There were no dissolved hydrocarbons or MTBE detected at wells MW-10,MW-11,or MW-12. A review of the historic groundwater data reveals several noticeable trends. First of all,there are presently six monitoring wells that have submerged well screens. These are wells MW-4 through MW-9. The depths of submergence vary between 0.3 and 1.9 feet. These conditions have occurred since November, 1995 and the results collected since then may not be entirely representative of groundwater conditions. However,when pre-submergence data is evaluated from all wells it is apparent that a significant source of hydrocarbon contamination remains in the vicinity of wells MW-I,MW-2, and MW-3. This is apparent by the consistently high concentrations that have been measured at these wells. The contaminant plume appears to be migrating;This is evidenced by the increasing trend of dissolved hydrocarbon concentrations measured at downgradient wells MW-6,MW-7,and MW-9. The prominent direction of groundwater flow has been toward the east-northeast. Based on the non-detect conditions of wells MW-10 through MW-12, the plume extent hay nc ro rhe northeast. The full extent of the contaminant plume has not been defined beyond wells MW-6 and MW-7. It cannot be determined what, if any, effect hydrocarbon releases on nearby properties have had on the contaminant plume characteristics. Considering these established trends, Chevron proposes the following actions and modifications to the current groundwater monitoring program: -Gauge and sample wells MW4,MW-6,MW-7,and MW-9 semi-annually(2ndQ&4thQ) analyze all samples for TPHG,BTEX,MTBE, and bio-parameters