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GETTLE' R - RYAN IVC <br /> CD <br /> May 31,2000 - <br /> Mr. Bob Boust ry -W . <br /> Unocal - DBG/AMG <br /> 2121 North California Boulevard, Suite 250 -k- <br /> r <br /> Walnut Creek, CA 94596 <br /> w <br /> Subject: Review of Quarterly Monitoring Data from Former Unocal Service Station 40123,1034 Central <br /> Avenue,Tracy, CA. <br /> Mr. Boust: - <br /> I have reviewed the data from the first quarter 2000 monitoring and sampling event for the subject site. Groundwater i' <br /> levels rose an average of 0.38 feet from levels measured during the fourth quarter 1999. These levels are within the i <br /> historical fluctuation range. During the first quarter 2000 groundwater flow was toward the north-northwest at a 5 <br /> gradient of 0.004. These groundwater data are consistent with the historical record. <br /> I <br /> Low concentrations of Total Petroleum Hydrocarbons as gasoline(TPHg),benzene,or methyl tert-butyl ether(MtBE) <br /> continue to be detected in on-site monitoring wells M W-1,MW-2 and MW-4. However,these concentrations are very <br /> low. The TPH-concentration consists of unidentified hydrocarbons. Benzene is only detected in well MW-4,situated <br /> in the source area. MtBE is detected only in wells MW-I and MW-2, and the concentrations are below the current <br /> primary maximum contaminant level(MCL)of 13 parts per billion(ppb)and secondary MCL of 5 ppb. Hydrocarbons <br /> were not detected in downgradient well MW-6. Total Petroleum Hydrocarbons as diesel(TPHd)were riot detected in <br /> any of the wells this quarter. Hydrocarbon concentrations and distribution detected during the first quarter 2000 are <br /> consistent with historical data. <br /> The dissolved plume is defined to the north(well MW-6,downgradient of the site). Lateral delineation of benzene has <br /> also been achieved(wells MW-1 and MW-2). The detectable conelentrations of TPHg and TPHd at this site are not high <br /> enough to warrant additional investigation. Since this station ceased operation in the early 1960s,the presence of MtBE j <br /> suggests at least some of the dissolved hydrocarbons detected in the groundwater beneath the site may due to an off-site <br /> source. Based on these data and conclusions,additional wort-at tl a site is riot warranted. The site appears to be of low I <br /> risk, and should be considered for closure. <br /> Please call us at 916.631.1300 if you have quesM. <br /> r- <br /> Sincerely, Qtr `QEµ <br /> Gettler-Ryan Inc. <br /> Stephen J. Carter No.5577 <br /> Senior Geologist LP <br /> R.G. 5577 OF C���F�QI <br /> Attachment: First Quarter 2000 Groundwater om oring 8 Sampling Report, G-R Job 11280047 <br /> dated April 24,2000. <br /> cc: Mr Mike Infurna, San Joaquin County Department of Environmental Health <br /> 6747 Sierra Court , Suite J Dublin , California 94568 { 925 ) 551 - 7555 <br />