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07 November 2004 AGE-NC Project No 96-0232 Page 4 of 11 Unless infection wells are properly located, channeling of air flow streams may occur and contamination pockets may be bypassed This problem is more common where fine-grained strata having low permeability are interbedded with coarse-grained strata in the saturated zone Air flow is retarded within fine-grained strata, and hydrocarbon recovery is poor 4 1 3 Monitoring Activities One of the potential problems associated with air sparging is lateral spreading of the contaminant plume This can be caused by either mounding of ground water above infection points or lateral air flow away from sparging wells To detect this problem,ground water elevations in monitoring wells and extraction wells (if appropriate) should be monitored on a monthly basis, at the minimum, to detect mounding and the presence of organic vapor 414 Feasibility In-situ air sparging of dissolved MTBE in ground water at the site has a low feasibility Soil in the saturated zone between 25-40 feet bsg has a moderate to high amount of sandy silt, sandy clay and silty/clayey content,making IAS ineffective through this zone(Figure 10) Based on the guide How to Evaluate Alternative Cleanup Technologies For UST Sates (October 1994) published by the United States Environmental Protection Agency(EPA)Office of USTs,IAS is generally ineffective in silty and clayey material and/or when permeability rates are greater than 10.9 cm' (see Exhibits VII-4 &11 in Appendix A) Another concern with IAS at the site would be the potential for off-site lateral spreading of MTBE and air flow channeling between the intermediate sand intervals (15-25 feet and 40-50 feet) and intermediate silt/clay intervals at depths between 25 and 40 feet bsg It may not be possible to effectively recover sparged air due to site geological characteristics (see Exhibit VII-6 in Appendix A) Since July 1996,ground water levels at the site have generally fluctuated between 3 feet and 18 feet below surface grade(bsg),however,ground water levels were most typical at depths between 10 feet and 14 feet bsg Based on the limited vadose zone consisting mostly of silts and clays (Figure 10) between surface grade and 15 feet bsg, recovery of sparged hydrocarbons utilizing SVE in conjunction with IAS would be considered ineffective as a remediation method Henry's law constant quantifies the relative tendency of a dissolved constituent to transfer to the vapor phase Based on the same above referenced EPA guide, constituents with Henry's law constants greater than 100 atmospheres are generally considered suitable to removal by air sparging Advanced GeoEnvironmental,Inc