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ERI 205503,RQ2 Tosco 76 Senwe-Station 5886, Stockts Califorma <br /> _ March_;. 1999 <br /> • Laboratory analyses of soil samples collected from borings B8 through B10 did not detect <br /> • concentrations of residual hydrocarbons and oxygenated compounds at or above laboratory <br /> method detection limits <br /> • Residual hydrocarbons in soil are delineated at the site and do not extend offsite <br /> • Laboratory analyses of groundwater samples collected from the borings detected the <br /> following <br /> Laboratory analyses of groundwater samples collected from boring B8 detected total <br /> xylenes (0 57 ppb)and MTBE (4 4 ppb)at 7 feet bgs, MTBE (2 6 ppb) at 17 feet bgs, and <br /> toluene (0 56 ppb), total xylenes (1 6 ppb), and MTBE (1 2 ppb)at 31 feet bgs <br /> Laboratory analyses of groundwater samples collected from boring B9 detected toluene <br /> (0 52 ppb) at 7 feet bgs and methanol (91 ppb) and ethanol (21 ppb) at 30 feet bgs <br /> Laboratory analyses of groundwater samples collected from boring B10 detected MTBE <br /> (3 3 ppb) at II feet bgs, MTBE (35 ppb) and TEPHd (58 ppb)at 16 feet bgs, and MTBE(90 <br /> ppb)and TEPHd (490 ppb) at 30 feet bgs <br /> RECOMMENDATIONS <br /> Based upon the results of this investigation, ERI recommends the following <br /> . • Continuing quarterly groundwater monitoring and sampling, and analyzing groundwater samples <br /> collected from on-site monitoring wells MW1 through MW7S & D for TPPHg, BTEX, TEPHd, and <br /> the seven oxygenated compounds <br /> • Installing three nested groundwater monitoring wells downgradient of the subject site These nested <br /> wells will be located in the general vicinity of borings B8 through 1310 Each nested well will <br /> consist of three individual well casings(shallow, intermediate, and deep) and each casing will be <br /> installed and slotted into one of the three more permeable water bearing zones identified in borings <br /> B8 through B 10 The purpose of this nested well design is to evaluate dissolved hydrocarbon <br /> concentrations and select oxygenated compounds at specific depths within zones of increased <br /> permeability and to evaluate the hydrogeologic communication between these more permeable <br /> zones ERI is prepared to implement the well installation upon written approval from the County to <br /> Tosco and ERI of the proposed well locations The location of the wells are shown on Plate 2 ERI <br /> has tentatively scheduled drilling for March 24 through 26, 1999 <br /> LIMITATIONS <br /> This report was prepared in accordance with generally accepted standards of environmental practice in <br /> California at the time this investigation was performed This investigation was conducted solely for the <br /> purpose of evaluating environmental conditions of the soil and groundwater with respect toP etroleum <br /> hydrocarbons and gasoline-related constituents No soil engineering or geotechnical references are <br /> implied or should be inferred Evaluation of the geologic conditions at the site for the purpose of this <br /> i <br />