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7 , J PUBLIC HEALTH SERVICPS oPou,N SAN JOAQUIN COUNTY Q' ENVIRONMENTAL HEALTH DIVISION W Karen Furst, M.D , M PH, Health Officer 304 East Weber Avenue, Third Floor • Stockton, CA 95202 2091468-3420 ED RALSTON TOSCO MARKETING COMPANY SEP 2 B 1997 ENVIRONMENTAL COMPLIANCE DEPARTMENT 2000 CROW CANYON PLACE STE 400 RECEIVED SAN RAMON CA 94583 OCT 0 2 1997 Re Unocal Station#6981 Site Code 1217123 4707 Pacific Avenue BY. Stockton CA 95207 San Joaquin County Public Health Services,Environmental Health Division(PHS/EHD)has completed review of the Tosco correspondence dated September 12, 1997 and the GeoStrategies"Work Plan for Soil Boring and Monitoring Well Installation"dated September 12, 1997 Thank you for your efforts to comply with the August 13, 1997 PHS/EHD regulatory directive PHS/EHD has prepared the following comments for your consideration and response The work plan proposed to drill two soil borings One soil boring was proposed to be used for the installation of one monitoring well with thirty feet of screened casing to replace the monntormg wells destroyed to accommodate the City of Stockton's interchange improvement project. One soil boring was proposed to investigate the effectiveness of the vapor extraction system to remediate soil contamination This boring is proposed to be drilled to 40 feet below ground surface(bgs)and to be backfilled from 40 to 33 feet bgs with OCR slurry and from 33 feet to surface grade with neat cement using a tremmie pipe and pump ORC slurry is not currently an approved backfill material pursuant to the San Joaquin County Well Ordinance Standards Additional description of this material will be required prior to its use The proposed bormg to evaluate the effectiveness of the vapor extraction system was proposed to be located approximately 5 feet from VW2 and drilled to a total depth of 40 feet Sol] contamination of up to 4,300 ppm TPH- gas, 17 ppm benzene, 130 ppm toluene,42 ppm ethylbenzene,and 200 ppm xylene was evidenced to the maximum depth of BI I-VW2 which was 50 feet bas While this proposed boring will provide some information,additional soil borings are required to demonstrate the effectiveness of the vapor extraction system,particularly given the reported condition of the wells used for vapor extraction The work plan proposed also to install ORC(PHS/EHD assumes this is a series of ORC socks)m MW2 to enhance natural attenuation of the dissolved hydrocarbon plume Please note that MW2 actually has only approximately 2 6 feet of screened interval based on the last measured total depth of the well and this interval is located approximately 17 feet below the most recently reported static water level While there likely will be no direct harm from the installation of ORC socks, its effectiveness is certainly questionable It would seem that Tosco did not have available the most recent monitoring report which included the well depths of the vapor extraction wells when comments were included in the above referenced correspondence such as "the bottoms of the vapor extraction wells, originally installed in the unsaturated zone,now extend below the water table and groundwater samples are also collected quarterly from these wells" For example,a sample could not be collected from VW2 because although it was screened between 25 5 and 50 5 feet bgs,it was reportedly dry when sample collection was most recently attempted PHS/EHD has directed Unocal to install monitoring wells to replace those destroyed and to supplement existing monitonng wells so that the groundwater plume maybe accurately characterized PHS/EHD cannot agree with the Tosco assertion that the lateral dissolved hydrocarbon plume has stabilized and is apparently shrinking PHS/EHD A Division of San Toaauin County Health Carp 4prvirps