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1 <br /> UNOCAL Corporation <br /> Page 2 <br /> As discussed in letters regarding other UNOCAL site in San Joaquin County and as shown by <br /> depth to groundwater at this site since 1990, when the first monitoring wells were installed, there <br /> have been significant variations in depth to groundwater in this area. <br /> The dates of operation of the underground storage tank(UST) system that was removed in June <br /> 1988 has not been discussed in reports. In addition, any UST system that was operated at this <br /> site prior to the system removed in June 1988 has not been discussed. This information as well <br /> as depth to groundwater information during the dates of UST operations should be provided since <br /> it is not known when the unauthorized release of petroleum occurred. According to San Joaquin <br /> County Flood Control maps in 1979 the depth to groundwater at that time was about 60 feet bsg. <br /> At this time PHS/EHD directs you to submit a work plan to define the vertical extent of the soil and <br /> groundwater contamination and evaluate the effectiveness of the vapor extraction system. Soil <br /> borings should be drilled in the area of VW2, MW2, and MWS. Soil and groundwater samples <br /> should be collected and analyzed from these borings. if the vertical extent of the contamination is <br /> deeper than the currently screened monitoring wells, a well casing appropriately screened should <br /> be placed in the bore hole(s). At least one bore hole downgradient and in the area of MW2 and <br /> VW4 should have a monitoring well constructed in it with the screened interval intersecting the <br /> current groundwater level. The contingency for installation of an air sparge well, if field <br /> observations indicate significantly contaminated soil remains below groundwater, should also be <br /> included in the work plan. <br /> In addition, PHSIEHD directs you to collect and analyze water samples from PTW1 semi- <br /> annually. <br /> After the data from the above directed work is reviewed, PHS/EHD will comment on any additional <br /> investigation or remediation required for this site. <br /> If you have any questions contact me at(209)468-3449. <br /> Donna Heran, REHS, Director <br /> Environmental Health Division <br /> Margaret Lagorio, Supervising REHS <br /> Site Mitigation Unit <br /> c: SJCADA-David Irey <br /> SWRCB-Jim Munch <br /> TOSCO - Ed Ralston <br /> Gettler Ryan -Stephen Carter <br /> CVRWQCB - Mark List <br /> Dayton Hudson Corp - Dave Luick <br />