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ameO encountered at depths between 25 and 66 feet overlying sandy gravel extending to the maximum depth drilled of 82 feet bgs. Vertical profiles of the Site subsurface are presented in the Site Conceptual Model(Geocon, 2009a) and are attached in Appendix C. The depth to first groundwater beneath the Site as observed during the Fourth Quarter 2008 was approximately 45 feet bgs. This groundwater is unconfined and represents the piezometric surface beneath the Site. Based on the April 2009 groundwater elevation data, the groundwater flow direction in both the deep-screened and shallow-screened wells is toward the southeast, both with an average gradient of 0.001 feet per foot. Historical groundwater flow direction at the Site has varied from east to southeast (Geocon, 2009b). 2.4 REMEDIAL APPROACH In August 1996, Geocon conducted an SVE pilot using VW-2 to evaluate the feasibility of this technology to remediate the petroleum hydrocarbons in the source area (Geocon, December 1996). Results of the pilot test indicated that a radius of influence (ROI) of 354feet could be v achieved at an extraction rate of approximately 50 standard cubic feet per minute (scfm) at approximately 50 inches of water vacuum. Based on these results, Geocon estimate that approximately 300 pounds per day of petroleum hydrocarbon mass could be removed by extracting vapors from VW-1 and VW-2. Although the results of the SVE pilot test appeared to support the use of SVE for remediation of the source area, a remedial design was not completed until June 2003. The remedial design included installation of an SVE/AS System and relied upon design parameters (e.g. radius of influence, vapor extraction rates, and air injections rates) measured during operation of an SVE/AS system located at the Caltrans Shop 10 Facility (Shop 10), located adjacent to the Site. The Shop 10 system has successfully reduced source area petroleum hydrocarbon concentrations and the SJCEHD has granted Caltrans permission to shut down the system. Caltrans anticipates moving the Shop 10 treatment system equipment to the Site pending agency closure of Shop 10. Following a review of the historical site data, AMEC believes that a combination of SVE and AS is likely the most feasible approach for remediation of the source area. Between March 2007 and May 2008, monitored natural attenuation parameters (sulfate, dissolved iron and manganese, methane, dissolved oxygen, oxygen reduction potential)were monitored in Site groundwater monitoring wells (Appendix B). These results indicate that reduced conditions are present within the area of the petroleum hydrocarbon plume and that the addition of oxygen below the water table will likely promote biodegradation. Further, the successful use AMEC Geomatrix, Inc. P:\Project\14000s\14443_Caltrans\TO 10\11000_PLANS\AS WP_March 201 OAS WP.doc 4 L