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PUBLIC HEALTH SERVICES Uro SAN]OAQUIN COUNTY ENVIRONMENTAL HEALTH DIVISIONKaren Furst, M D , M PH , Health Officer304 East Weber Avenue, Third Floor • Stockton, CA 95202 209/468-3420 BOB BOUST UNOCAL CORPORATION DEC, 10 N7 2121 N CALIFORNIA BLVD STE 250 WALNUT CREEK CA 94596 RE 437 East Miner Ave ,Stockton CA 95202 SITE CODE I I&I Stockton CA 95202 San Joaquin County Public Health Services, Environmental Health Division (PHS/EHD) has completed review of the GeoStrategies "Work Plan and Feasibility Study"dated October 9, 1997 and the "Well Installation Report"dated October 21, 1997 PHS/EHD has prepared the following comments for your information and response The work plan proposed to install one monitoring well to delineate the lateral extent of hydrocarbon- impacted groundwater The monitoring well was proposed to be located approximately 120 feet downgradient of the newly installed well,U-9 The well was proposed to be installed within an 8 inch diameter borehole that will be drilled to a total depth of approximately 5 5 feet below ground surface (bgs) The work plan proposed that the well be constructed with 30 feet of screened interval The work plan proposed that at minimum two soil samples would be submitted for laboratory analysis utilizing EPA Method 8015 Modified and 8020 The well will subsequently be surveyed by a California licensed surveyor,developed and sampled The monitoring well, U-9,which was installed on August 19, 1997,is located approximately 180 feet downgradient of one of the former tank complexes which evidenced petroleum hydrocarbon contamination This well was installed with 30 feet of screened interval (25-55 feet bgs) which PHS/EHD has specifically disapproved in prior correspondence The reported depth to water prior to well development and sampling on August 26, 1997 was approximately 33 feet bgs, indicating a water column of 22 feet within the screened interval The sample collected evidenced 990 ppb TPH-gas, 130 ppb benzene,and 30 ppb xylene The sample had been diluted and the detection limits elevated The report incorrectly indicated that 12 ppb methyl tertiary butyl ether had been detected when the accompanying lab report stated that none was detected at the detection limit of 12 ppb AIso,the report failed to include appropriate tracking documentation for the soil cuttings disposed of at Forward Inc The feasibility study proposed to evaluate natural attenuation as a remedial alternative,because "site conditions do not appear to warrant implementation of the soil vapor extraction feasibility study at this time"and "The highest concentrations of residual hydrocarbons are below water which indicates that vapor extraction is not an effective remedial alternative at this time " The proposed feasibility study cited rising water levels and decreasing hydrocarbon concentrations over the last three years as the basis for the conclusion that the "groundwater plume does not appear to be migrating at this time (stable plume)" The proposed feasibility study includes quarterly groundwater monitoring for a minimum period of one year This monitoring data,including TPH-gas,BTEX,and methyl tertiary butyl ether analysis,will be used to evaluate natural attenuation and stability of the dissolved plume A Division of San Joaquin County Health Care Services