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PUBLIC�HEALTH 'SERVICES <br /> SAN JOAQUIN COUNTY a.�Q <br /> ENVIRONMENTAL HEALTH DIVISION <br /> Karen Furst, M.D., M.P.H., Health Officer <br /> 304 East Weber Avenue, Third Floor • Stockton, CA 95202 e. <br /> 2091468-3420 FILE <br /> C <br /> ROBERT BOUST <br /> UNOCAL CORPORATION JUN 2 6 1998 <br /> 2121 N CALIFORNIA BLVD SUITE 250 <br /> WALNUT CREEK CA 94596 <br /> RE: UNOCAL SITE CODE: 1213 <br /> 1665 Pacific Avenue <br /> Stockton, CA <br /> San Joaquin County Public Health Services, Environmental Health Division (PHS/EHD) has <br /> reviewed the"Vertical Fuel Hydrocarbon Plume Delineation Hydropunch Investigation/Piezometer <br /> Installation"work plan dated May 26, 1998, prepared by ARCADIS Geraghty & Miller, Inc. for the <br /> above referenced site. <br /> PHS/EHD does not agree that the investigation proposed will complete the site investigation <br /> necessary to provide data for the preparation of a corrective action plan (CAP). <br /> UNOCAL originally leased the site in January 1947 and removed underground storage tanks <br /> (UST's) in March 1988. It was stated that the station was completely remodeled in 1967. <br /> According to PHS/EHD file records, the UST's removed in March 1988 were installed at that time. <br /> The location of the UST's operated prior to the 1967 station remodel has not been shown on site <br /> maps. The location of this former UST system should be shown so its potential as a source of the <br /> contamination can be evaluated with information gathered from the site investigation to date. <br /> in 1978, depth to groundwater in the area of this site was about 60 feet below surface grade (bsg)_ <br /> PHSIEHD has not reviewed records on depth to groundwater prior to 1978. In 1986 and currently, <br /> depth to groundwater at the site was/is about 25 feet bsg. It is not known when the unauthorized <br /> release of petroleum hydrocarbons occurred, Therefore, the depth to groundwater at the time of <br /> the release(s) is not known. <br /> Soil contamination was documented at a depth of 51 feet bsg by a soil sample collected at that <br /> depth during the boring for the installation of vapor well VW2. The vertical extent of soil <br /> contamination has not been defined and the proposed work will not provide that definition. <br /> Petroleum hydrocarbons that have absorbed to the soil and have not dissolved into the <br /> groundwater yet are a continuing source of groundwater contamination. The investigation should <br /> not be limited to current dissolved-phase hydrocarbons. The potential for future releases to the <br /> groundwater from contaminated soil should also be investigated by sampling of the soil above <br /> and below current groundwater until the vertical extent of soil contamination has been <br /> demonstrated. <br /> The results of the soil samples collected from vapor extraction well VW2 revealed soil <br /> contamination from 6 feet to 51 feet bsg. The results of soil samples collected from monitoring <br /> wells MW4 and MW5 and soil boring 86 revealed non detectable soil contamination, although the <br /> detection limits were higher than are currently acceptable. Although, the groundwater gradient <br /> has been to the northeast, groundwater samples from MW4 have shown petroleum hydrocarbon <br /> A Division of San Joaquin County Health Care Services <br />