SITE INFORMATION AND CORRESPONDENCE FILE 1

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2315 1 3 5 E Dorado,Stockton Page 2 The two sets of monitoring wells proposed furthest from the site on Hampton and Adam Street: MW-9,MW-109,MW-209,and MW-11,MW-111,MW-2110,respectively,are not approved. These three proposed wells are located at 140 feet,310 feet,and 280 feet from MNV-3,which is the well I with the historically highest levels of contamination. EHD does not believe that sentinel wells at these distances are necessary at this time given the information currently available. i The inferred groundwater flow direction has been consistently to the east. EHD approves the proposed location for the installation of MW-10,MW-110,and MW-210 cluster east of the site,-and the locations for the installation of MW-7,MW-107,M%V-207 cluster north of the site,and MW-12, MW-112,MW-212 cluster south of the site. Instead of installing these 9 monitoring wells in 9 separate borings in 3 locations,EHD strongly recommends that you consider installing 3 multi- chambered wells (MCW)instead. The casing of an MCW is capable of housing up to 7 different discrete screen sections of varying lengths. The number of wells needed could be reduced by incorporating three screen sections equivalent to the three proposed monitoring wells per MCW. Since the screen sections of a MCW are typically built on site,EHD also recommends obtaining a continuous core from the boring in order to provide the exact geological information for discrete screen section construction. Submit a brief work plan addendum reducing the above-discussed 9 monitoring wells to 3 MCWs if you agree to these recommendations. EHD requires the following conditions be met: 1) EPA Method 8260 must be used to analyze groundwater samples from all new wells for 5 oxygenates, 1,2-DCA,EDB,and ethanol. 2) Collect additional VOAs of samples from all impacted monitoring wells. Effort must be made to lower the detection limits. Previous detection limits of analyses for MTBE and other oxygenates were often set at level as high as 250, 500,even 5,000µg/l. Primary Maximum Contaminant Level (MCL) for MTBE in drinking water ektablished by California Department of Health Services is 13 µg/1,and the Secondary MCL is 5 µg/1. 3) The geological/ hydrological model should be refined with the new acquired data. 4) Implement approved portion of the work plan by November 17,2002. Submit a report of findings to EHD by January 17,2003. 5) EHD strongly recommends that use of hollow stem auger to complete the installation of monitoring wells and collect soil samples be attempted before going to mud rotary. Portions of the work plan not approved maybe implemented at a later time if justified based on concentrations detected or migration of the contamination. A work plan addendum stating reasons and data sufficient to approve and implement these portions of the work plan may be submitted to EHD at such time. Donna Heran,REHS,Director i Environmental Health Department t Jeffrey REHS--J Nuel C. Henderson,Jr.,RG LOP/Site Mitigation Unit IV LOP/ Site Mitigation Unit IV Cc: CRWQCB,Central Valley Region—Marty Hartzell Cc: Geological Technics,Inc. -Raynold I.Kablanow i