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Ms. Margaret Lagorio, SJCEHD February 2,2005 Interim Remedial Action Plan Joe's Truck Plaza,Lathrop,California Page 2 proposed in the Work Plan should provide additional constraints on the lateral plume configuration. Impact in Sand B (encountered at approximately 25 to 35 feet bgs, approximately 7 to 18 feet thick) appears to have been adequately characterized. Cross- sections depicting subsurface stratigraphic conditions are shown in Figures 3 through 5. The approximate lateral extent of the dissolved plume is shown in Figure 6. Using quarterly monitoring data and an assumed hydraulic conductivity for silt and sandy silt of 1 meter/day (Driscoll, 1995), Stratus calculated mass flux in Sand A of approximately 0.05 grams/day passing through a line drawn perpendicular to groundwater flow at well MW-3. Assuming an extraction rate of approximately 7.5 gallons per minute (gpm), the initial mass extraction rate along this transect would be approximately 3.8 grams/day (Appendix A). Possible technologies available to address mitigation of dissolved phase MTBE in groundwater include dual-phase extraction (DPE), groundwater extraction, vapor extraction in conjunction with conventional air sparging, and ozone injection. Stratus performed a DPE feasibility test in June 2004. The test sustained an average groundwater extraction rate of 7.2 gpm, with a radius of influence (ROI) for groundwater extraction of approximately 30 feet. The influent soil vapor extraction rate (under an applied wellhead vacuum of 23 to 27.5 inches Hg [or approximately 312.8 to 374 inches WC]) ranged from 86 to 88 cubic feet per minute (cfm), but the vacuum ROI could not be established. The Dual Phase Extraction Test Report (dated September 28, 2004) concluded that DPE was a technically viable remedial option, but that it did not appear to be a cost effective remedial alternative. Vapor extraction and conventional air sparging technologies are not likely to be effective technologies for removing dissolved MTBE from groundwater. Ozone sparging has been demonstrated to be effective in breaking down dissolved MTBE, but additional chemical analyses, bench-scale testing, and pilot testing would be required by the RWQCB prior to implementation. As stated in the January 5, 2005 Work Plan, Stratus has proposed collection of soil and groundwater samples to evaluate the feasibility of ozone injection. Given the historical MTBE concentrations, current plume configuration, and hydrogeologic conditions at this site, groundwater extraction appears to be a viable remedial option. This IRAP proposes to initiate removal of impacted groundwater by installing a temporary groundwater extraction and treatment system (GETS) that will utilize electrical submersible pumps to extract impacted groundwater from existing wells MW-2, MW-3, and MWO-1X. The SJCEHD letter dated November 10, 2004, requested that proposed remedial activities target the areas of highest MTBE levels. Wells MW-2 and MW-3 are screened across Sand A, where the highest dissolved MTBE concentrations have been reported. These wells also appear to be situated near the center of the dissolved MTBE plume. Dissolved MTBE concentrations in well MWO-1X also appear to warrant groundwater extraction. This well is screened across the interval of Sand B. Grab groundwater samples from boring CPT-3 collected in this depth interval K:Voe's Truck Plaza\Work PlansUoes IRAP 020205.doc