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Monitoring Well Installation Report—Former Unocal Service Station:0187, Stockton,CA August 2, 2000 Soil Anal ical Results TPHg, BTEX, or MtBE were not detected in soil samples from well boring U-12 except for minor amounts of benzene at 80 5, 90, 100 5, and 1 I I feet bgs at concentrations of 0 0242, 0 0502, 0 0136, and 0 00712 ppm, respectively Groundwater Analytical Results TPHg were not detected in wells U-10 or U-1 1, and BTEX or the fuel oxygenates were not detected in well U-I 1 TPHg were detected in wells U-12 (440 ppb) and U-13 (800 ppb) Benzene was detected in wells U-]0 (0 76 ppb), U-12 (64 ppb), and U-13 (430 ppb), and MtBE was detected in those wells at concentrations of 7 7, 3 5, and 3 9 ppb, respectively, by EPA Method 8260A The other fuel oxygenates, TBA, ETBE, DIPS, or TAME,were not detected in the wells WASTE DISPOSAL Soil cuttings generated during drilling activities .were placed on and covered with plastic sheeting at the sample SP-1(A-D) was collected (Table 1) Approximately subject site and a disposal confirmation 5 6 yards (7 0 tons) of soil cuttings were removed from the site on July 24, 2000 by Manley and Sons Trucking and transported to the Forward Landfill in Manteca, California, for disposal A copy of the disposal confirmation letter is in Appendix B DISCUSSION TPHg were detected in groundwater from wells U-12 and U-13 at concentration up to 800 ppb, with screen depth Benzene was detected in groundwater from wells concentrations increasing with increasing U-10, U-12, and U-13 at concentrations up to 430 ppb, and these concentrations also increase with screen depth Benzene was detected in soil from well boring U-12 at concentrations ranging from 0 00712 to 0 0502 over the interval from 80 5 to 11 I feet bgs MtBE NN as detected in low concentrations by EPA Method 8260A in groundwater samples from wells U-10, U-12, and U-13, \pith concentrations generally decreasing with screen depth Based on the data from this phase of the investigation, the Vertical and lateral extents of the dissolved hydrocarbon plume are not defined The highest concentrations of TPHg and benzene are detected in groundwater from the ~Neil NNith the intermediate screen interval The dissolved MtBE plume does appear to be adequately delineated Concentrations detected in N%ells U-12 and U-13 are below the current primary maximum contaminant level (MCL) of 5 0 ppb Based on these data, additional subsurface investigation may be required to further delineate the TPHg and benzene plumes However, before additional subsurface investigation is undertaken, GR recommend four quarters of monitoring and sampling to assess the stability of the plumes In addition, an interim remedial work plan will be submitted by September 29, 2000 24004205-1 6