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GETTLER - RYAN INC. <br /> February 17, 1999 <br /> Mr. Ed Ralston <br /> Tosco Marketing Company <br /> Environmental Compliance Department <br /> 1380 Lead Hill Road, Suite 120 <br /> Roseville, CA 95661 <br /> Subject: Addendum to Work Plan For Soil Boring And Monitoring Well Installation at Tosco <br /> (Former Unocal) Station#6981,4707 Pacific Avenue, Stockton,CA <br /> Mr. Ralston: <br /> GeoStrategies(GSI)previously proposed the installation of one groundwater monitoring well downgradient <br /> of the subject site between former wells MW-12 and MW-13, and one soil boring adjacent to vapor <br /> extraction well VW-2 (GeoStrategies Report No. 7855.21-1, Work Plan For Soil Boring And Monitoring <br /> Well Installation,dated September 12, 1997). This proposed scope of work was never approved by the San <br /> Joaquin County Public Health Services-Environmental Health Division (SJCPHS-EHD). In response to a <br /> letter from SJCPHS-EHD to Unocal Corporation dated January 8, 1998,Tosco Marketing Company(Tosco) <br /> requested that Gettler-Ryan Inc. (GR) prepare an addendum to the subject Work Plan. <br /> In their letter, SJCPHS-EHD directed that soil boring be drilled in area of wells VW-2, MW-2 and MW-5, <br /> and that appropriately screened well casings be installed in these borings if evidence indicates that <br /> hydrocarbon impact extends below the depth of the currently installed groundwater monitoring wells. <br /> SJCPHS-EHD also directed that a groundwater monitoring well screened across the current water table be <br /> constructed in a boring drilled in the area of wells MW-2 and VW-4. Finally, SJCPHS-EHD directed that <br /> a well be installed downgradient of existing wells MW-2 and VW-4 that is screened across the current water <br /> table. <br /> The most recent monitoring data(October 14, 1998)for groundwater at the subject site is measured between <br /> 27 and 30 feet below ground surface(bgs). Existing groundwater monitoring well screens are drowned as <br /> groundwater at this site has risen more than 20 feet since April 1993. Groundwater flow is toward the east <br /> and northeast. The most recent (October 14, 1998) chemical analytical data indicate the dissolved <br /> hydrocarbon plume is partially defined. Total Petroleum Hydrocarbons as gasoline(TPHg)and benzene are <br /> detected only in wells MW-2 and VW-4. Methyl tert-butyl ether is detected in wells MW-2,MW-4,MW-5, <br /> MW-7 and VW-4. <br /> 140124.04 <br /> 6747 Sierra Court , Suite J Dublin , California 94568 ( 925 ) 551 - 7555 <br />