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M <br /> GEORGE DEUKMEJIAN,Governor <br /> f ESTATE OF CALIFORNIA i <br /> CALIFORNIA REGIONAL WATER QUALITY CONTROL BOARD= <br /> CENTRAL VALLEY REGION. tl� <br /> 3443 ROUTiER ROAD <br /> SACRAMENTO,CA 95827-3098 <br /> 20 September 1989 SEP 2 2 1989 <br /> ENVIRONMEWAL.HEALTH <br /> pERM1I/SERVICES <br /> Ms ' -Diane Hinson <br /> Registered Environmental. Health Specialist <br /> Environmental' Health Division <br /> San Joaquin County Department of Public Health Services <br /> P.O. Box 2009 <br /> Stockton,CA ..952.01 �,: �" -- --. _ -�--�•- - _ � , <br /> ARCO SERVICE STATION NO.2093, 3425- TRACY BOULEVARD, TRACY, SITE INVESTIGATION <br /> REPORT <br /> We have reviewed the-25 July 1989 "Site Investigation Report, " and the 28 <br /> August 1989 "Quarterly Ground Water. Sampling Report" for- this .site. Both <br /> reports were .prepared by. Arco'•s consultant, Brawn &. Cal-dwell. Consulting <br /> Engineers. The' reports include a summary. .of.:past�activities at the site, and <br /> the results from two .rounds of sam.pl:-iog; of three newly •.installed ground water <br /> monitoring .wells. Low levels of hydrocarbon; contaminati.on were detected in <br /> water samples from two of:.the newly insta-1.led wells: (ATR-6 and ATR-7) . The <br /> consultants -recommend' continued quarterly sampling of the existing .mon.itoring <br /> wells. <br /> Although water samples from munitaring '•wells ATR-6 and ATR-7 were found to <br /> contain low levels of- hydrocarbon contamination, higher levels of hydrocarbon <br /> contamination were detected in soil samples- obtained during installation of <br /> well ATR-7.. Because thi,s '..well is the farthest,-downgradient well , contaminant <br /> concentrations in ATR--_7 will need to. be carefully evaluated during the-:next <br /> ' rounds- of sampling. 'In addition, the •chlorinated hydrocarbon, 1,-2- <br /> dichloroethane (1,2-DCA) was :detected .-in water samples from three. of the <br /> monitoring wells during a- previous phase of investigation. <br /> We are concerned about the de'te' ct-ion of chl,orinated,..hydrocarbons in ground <br /> water at this site; 1;2-DCA has an establishedMaximum Contaminant Level of <br /> 0.5 parts per billion. The next round of sampling should .include analysis to <br /> confirm the 'presente of this chemical . If 1,2-DCA is confirmed in .ground <br /> water, determining the extent of this contami-nation: will be an 'important <br /> f <br /> component of- the next phase =of irivesti.gation. <br /> The next phase of investigation should .bb designed. to determine the lateral <br /> extent of soil' and ground water contamination .both. on-site and off-site. <br /> r Concurrent with the next- phase -of investigation-, the operator Ishould also <br /> begin developing an appropriate soil and ground water remediation plan for <br /> this site. - <br />{ <br />