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HydroGeo Consultants ' Based on the sediment types identified in the drilling logs, the aquifer properties calculated using the pumping well data are most likely the most representative of actual Site conditions Based on an average value obtained from the drawdown and recovery ' data, Site aquifer conditions are represented by T = 44 75 ft2/day, K= i 1 ft/day, V = 0 024 ft/day, and the stagnation point is 223 feet from the pumping well This stagnation point distance is in very good agreement with the distance obtained from the graphical solution obtained by plotting distance and maximum drawdown for each of the observation wells (240 feet) Based on the results of the pump test, it would appear that a sustained constantum in p p g rate of 1 6 gpm in MW-5 would be sufficient to effectively capture the dissolved phase plume surrounding this well This conclusion is based on the measurable drawdown t observed in all of the monitoring wells at the Site during this test at a constant flow rate of 1 50 to 1 60 gpm from MW-5, and the calculated stagnation distance of 223 to 240 feet from MW-5 (the pumping well) Based on these results, it is also concluded that pumping ' groundwater with onsite treatment is a feasible technology for remediating groundwater at the Site 7.0 EVALUATION OF REMEDIAL ALTERNATIVES HydroGeo Consultants evaluated several alternatives for remedial action at the Site in order to select the best, most cost-effective method for inclusion in this Remedial Action Plan A Corrective Action Plan was previously prepared and submitted by Thrifty Oil to the PHS-EHD dated September 26, 1995 This report entitled"Corrective Action Plan, Thrifty Service Station#171, 1250 North Wilson Way, Stockton, California", ' recommended vapor extraction and treatment combined with groundwater extraction and _ --=treatment-for-the preferred-remedial alternative-for the Site=A=vapor extraction-and= treatment system was subsequently installed in July of 1999 The system is still currently —`in eperati6n afthe Site and has-operated for approximately 15,002 hours and removed ___and_destroyed-approximately 62,941 pounds of TPH-g through March 26, 2002 7.1 Excavation ' Excavation of soil in the MTBE source areas would be a fast and easy solution for t remediation of TPHg, BTEX, and MTBE source areas at the Site However, the excessive cost, the impact on existing structures on-site, and significant safety issues associated with excavation to depths in excess of 25 feet below grade adjacent to a busy intersection ' and city streets makes this method impractical 7.2 Enhanced Bioremediation In recent years, a patented magnesium peroxide compound was designed to slowly ' release oxygen in groundwater to biodegrade BTEX compounds in situ The patented compound is designed to increase the dissolved oxygen content in groundwater, increasing aerobic biodegradation of hydrocarbons by naturally occurring ' ___ Page-12 — _ __ __ rOC 0171 Aquder"rest and RAP_Rpt _—__�