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r <br /> _PU_ BLIC hEALTH SERV110ES <br /> SAN JOAQUIN COUNTY '' <br /> ENVIRONMENTAL HEALTH DIVISIONh <br /> Karen Furst, M.D., M.P.H., Health Officer <br /> 304 East Weber Avenue, Third Floor • Stockton, CA 95202 6-ii� <br /> 209/468-3420 <br /> g <br /> ALEX PEREZ l OCT 14 lel <br /> SHELL OIL PRODUCTS COMPANY <br /> PO BOX 4023 <br /> CONCORD CA 94524 <br /> RE: SHELL STATION SITE CODE: 1058124 <br /> 620 W CHARTER WAY <br /> STOCKTON CA <br /> San Joaquin County Public Health Services Environmental Health Division (PHS/EHD) has reviewed the <br /> 3rd Quarter 1997 Groundwater Sampling Report prepared by Enviros Inc. and provides the following <br /> comments and directives. <br /> Data from cumulative tables indicate that ground water in monitor wells MW-1, 5, 7, 8, 9, and 10 exceed <br /> the screened intervals. Overall ground water elevation changes have likely contributed to dilution of <br /> contaminant concentrations over time. In addition, the effectiveness of the soil vapor extraction (SVE) <br /> system has also been decreased markedly as a result of diminishing vadose zones. <br /> PHS/EHD staff have reviewed data from 1989 pre-tank removal soil boring sampling. Without exception <br /> 1989 soil boring samples SB-2-3, 2-5, SB-3-3, 3-5, and SB-44, 4-5 provide evidence of increasing <br /> contaminant loading with depth near the former tank cavity. Maximum,reported investigation depth in <br /> these boreholes is 25 feet below surface grade (bgs). All 1989 soil samples analyzed were confirmed by <br /> laboratory analyses to contain petroleum hydrocarbon contaminants including the former dispenser islands, <br /> piping runs, and tank footprint areas. This data can be found in the Soil Management Plan For Tank <br /> Replacement report prepared by Delta Environmental Consultants September 6, 1989. Information in this <br /> report includes the age of the Underground Storage Tanks (USTs) at 17 years in ground/installed in 1972. <br /> Historic low groundwater elevations in this area in fall of 1978 recorded by San Joaquin County Flood <br /> Control was approximately 60' bgs. At this time PHS/EHD has no information which indicates that <br /> vertical or lateral definition is complete at this site. <br /> To the east beginning approximately 250 feet down gradient from this parcel are six contaminated sites. <br /> PHS/EHD has classified these sites as petroleum hydrocarbon co-mingled plume sites. PHS/EHD records <br /> indicate the existence of petroleum hydrocarbon contaminants in up gradient wells at sites down gradient <br /> from this site. MW-5 at this site is a critical down gradient positioned well since it lies directly east of the <br /> former tank pit and is the final monitor point onsite. Ground water has exceeded the screened interval <br /> height since spring of 1996. Contaminants detected in this well in July of 1992 (TPH-g 25,000, and <br /> Benzene 9,200 ug/1) while ground water was observed at 34.19' bgs decreased dramatically as ground <br /> water rose to within 0.41 feet from the top of the screened interval (MW-5 top of screen is-25.0' bgs) by <br /> May of 1993 (TPH-g <50, Benzene 0.86 ug/1). Concentrations after May of 1993 have been exceptionally <br /> low given the past history in this well. All wells for this site display similar trends. Most noticeable is the <br /> high concentrations of contaminants in ground water when water was deeper than 30' bgs. Given these <br /> trends it is likely that significant contaminant mass resides below ground water(deeper than 30'bgs). <br /> 1 <br /> A Division of San Joaquin County Health Care Services <br />