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zt GETTLER - RYAN INc. February 1,2001 Mr.Nick Nickerson i Unocal-DBG/AMG 8788 Elk Grove Boulevard Building 3,Suite 15 Elk Grove,CA 95624 Subject: Site Status and Review of Monitoring Data from Former Unocal Service Station#0123, 1034 Central Avenue,Tracy,CA. Mr.Nickerson: This letter was prepared by Gettler-Ryan Inc.(GR)at the request of Unocal Corporation. During the fourth quarter 2000,GR installed two off-site groundwater monitoring wells and monitored and sampled all site wells. Wells U-7 and U-8 were installed to the northwest and west of the site to further delineate the dissolved plume. Petroleum hydrocarbons were not detected in soil samples from either well boring. Installation of the wells was previously reported in GR's Report No.240073.05-1, Monitoring Well Installation Report(dated January 17,2001). The fourth quarter 2000 monitoring and sampling event included the newly installed wells, and was conducted on December 6, 2000. The report for this event is attached. I have reviewed the data from the fourth quarter 2000 and previous monitoring and sampling events for the subject site. Groundwater levels rose an average of 2.61 feet from levels measured during the third quarter 2000. These levels are within the historical fluctuation range. During the fourth quarter 2000 groundwater flow was toward the north at a gradient of 0.003. These groundwater data are consistent with the historical record. Well U-1 was inaccessible,and could not be monitored or sampled. P&_LE &AA. Low concentrations of benzene and Total Petroleum Hydrocarbo is as gasoline(TPHg)were detected only in on-site monitoring well U-4, situated in the source area. However,these concentrations are very low,and continue to show a declining trend.Total Petroleum Hydrocarbons as diesel(TPHd)were detected in all wells,at concentrations between 290 and 430 parts per billion(ppb). These TPHd concentrations are inconsistent with historical data for all wells except U-4. The laboratory report noted these concentrations were due to "discrete peaks." Further discussion with the laboratory indicated that the peaks are not caused by diesel,nor did any other type of petroleum hydrocarbons cause them(refer to GR's Well Installation Report). We therefore suspect the presence of the reported TPHd in these samples is due to errors in sampling,handling or analysis, or contamination by a non-petroleum compound. MtBE was not detected in any of the monitoring wells this quarter. TPHg,benzene or MtBE were not detected in downgradient wells U-6,U-7 or U-8.Except for TPHd,hydrocarbon concentrations and distribution detected during the fourth quarter 2000 are consistent with historical data. Since this station ceased operation in the early 1960s,the historical presence of MtBE suggests at least some of the dissolved hydrocarbons detected in the groundwater beneath the site are due to an off-site source. The historical presence of TPHd in upgradient well U-5 and chlorinated hydrocarbons in all site wells also indicate the presence beneath the subject site of hydrocarbons from an off-site source. The low level residual hydrocarbons from the operation of the fo er Unocal station appear restricted to the immediate vicinity of well U-4. The dissolved hydrocarbon plume is defined the plume configuration is stable,and concentrations in the source area(well U-4)continue a declining trend and appear to be naturally attenuating. Based on these data and conclusions,the site appears to be of low risk. Additional work at this site is not wan-anted, and the site should be closed. 6747 Sierra Court , Suite J - Dublin , California 94568 - ( 925) 551 -7555