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1 20 March 1996 AGE-NC Project No 95-0130 Page 17 of 23 ' ground water 7 1 3 MONITORING ACTIVITIES ' One potential problem associated with air sparging is inadvertent lateral spreading of the contaminant plume This could occur due either to mounding of ground water above injection points or lateral air ' flow away from sparging wells To detect this problem, ground water depths in monitoring wells and extraction wells (if appropriate) should be monitored on a monthly basis (minimum) to detect mounding and the presence of organic vapors 714 FEASIBILITY ' Remediation of large ground water plumes requires the installation of many injection and extraction wells Air injection and extraction rates must be balanced to prevent the forced migration of contaminants Extensive piping must be installed for the injection and extraction wells As in vapor extraction, air sparging is more effective for the remediation of shorter-chain hydrocarbons Factors to consider when evaluating IAS include the contaminant type (long-chain or short-chain hydrocarbons), soil characteristics (most importantly intrinsic permeability and stratification), and dissolved iron concentration in ground water IAS is not effective on longer-chained hydrocarbons or constituents with low Henry's Law constants(Iess than 100 atm) IAS effectiveness decreases with decreasing soil permeability, i e IAS is more effective in sandy soil types and less effective in clayey- rich soil Finally, if dissolved iron concentrations in ground water exceed 20 mg/1 (ppm), the addition of oxygen to the subsurface can precipitate out the iron and clog pore space Gasoline is the primary contaminant in ground water at the TFS site, and therefore, is amenable to IAS Based on soil samples collected during drilling procedures at the site, intrinsic permeability of soil at the site is estimated at 10-6 to 10-12 cm2, within the range of effectiveness for IAS No ground water samples have been analyzed for dissolved iron Based upon present site conditions, it is likely that IAS, combined with SVE, would be effective for reducing petroleum hydrocarbon concentrations in ground water at the site However, ground water samples collected during the next scheduled quarterly monitoring should be analyzed for dissolved iron 715 DURATION Air sparging in highly permeable aquifers generally results in rapid reduction of hydrocarbon