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6ETTLER- - RYAN I _ <br /> November 10,2000 �' f <br /> Mr.Nick Nickerson <br /> Unocal- DBG/AMG <br /> 8788 Elk Grove Boulevard , <br /> Building 3, Suite 15 <br /> Elk Grove,CA 95624 <br /> Subject: Review of Monitoring Data from Former Unocal Service Station#0123, 1034 Central Avenue, <br /> Tracy,CA. <br /> Mr.Nickerson: <br /> L have reviewed the data from the third quarter 2000 and previous monitoring and sampling events for the subject site. <br /> Groundwater levels dropped an average of 2.71 feet from levels measured during the second quarter 2000. These levels <br /> are below the historical fluctuation range. During the second third quarter 2000 groundwater flow was toward the north- <br /> northwest at a gradient of 0.006. These groundwater data are consistent with the historical record. <br /> Low concentrations of benzene and Total Petroleum Hydrocarbons las gasoline(TPHg)and as diesel(TPHd)continue <br /> to be detected in on-site monitoring well U-4,situated in the source area. However,these concentrations are very low, <br /> and continue to show a declining trend. Total Petroleum Hydrocarbons as diesel(TPHd)were detected in upgradient <br /> well MW-2. Low concentrations of TPHg,toluene, ethylbenzene and xylenes were also detected in well U-1. MtBE <br /> was not detected in any of the monitoring wells this quarter. Hydrocarbons were not detected in downgradient well <br /> MW-6.Hydrocarbon concentrations and distribution detected during the third quarter 2000 are consistent with historical <br /> data. <br /> Since this station ceased operation in the early 1960s, the historical presence of MtBE suggests at least some of the <br /> dissolved hydrocarbons detected in the groundwater beneath the site is due to an'off-site source. The continued <br /> historical presence of TPHd in upgradient well U-5 and chlorinated hydrocarbons in all site wells also indicate the <br /> presence beneath the subject site of hydrocarbons from an off-site source. Based on these data and conclusions,the site <br /> appears to be of low risk. <br /> At the request of San Joaquin County Environmental Health Division(SJCEHD), GR installed two additional wells at <br /> this site to further evaluate the lateral extent of hydrocarbon plume impact. This work was proposed in GR's Work Plan <br /> For Monitoring Well Installation(Report No.240073.04,dated October 12,2000),and approved by SJCEHD in a letter <br /> dated October 19,2000. The wells were installed on November 13,2000. <br /> Please call us at 916.631.1300 if you haveGEO`O <br /> Sincerely, ��, ��`c!a J.C0 <br /> Gett r-Ryan Inc. CID ` 1 <br /> Stephen J. Carter <br /> No. 5577 'k <br /> Senior Geologist �- <br /> R.G. 55777T•. <br /> tOFCA.OL <br /> Attachment: Third Quarter 2000 Groundwater onitoring&Sampling Report,G-R Job#280047 <br /> dated November 3,2000. <br /> cc: 'Mr. Mike Infurna, San Joaquin County Department of Environmental Health , <br /> Kimberly Ayers and Leesa Ward,Ayers&Ward Real EsIItate Investment Services, 1034 Central <br /> Avenue,Tracy, CA 95376(without attachment) <br /> 6747 Sierra Court , Suite J • Dublin , California 94568 • ( 925 ) 551 - 7555 <br /> 240073.01 <br />