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f L t ' f s i29 October 1998 • AGE-NC Protect No 98-0448 Page 8 of 9 , 1 40 SUMMARY AND CONCLUSIONS i d upon on da mer 1 8 AGE concludes p to collected between July and September 99 , i r • In August 1998, ground water was inferred to be flowing generally southwest at a calculated gradient of 0 009 ft/ft, or approximately 69 feet per mile (Figure 9), in September 1998, i ground water was inferred to be flowing generally southwest at a calculated gradient of 0 015 ft/ft Due to the Iow gradient at the site, slight variations in ground water measurements or changes of recharge/discharge of the local ground water could greatly modify ground water iflow direction The low gradient at the site implies that ground water is moving slowly • TPH-g was detected at concentrations ranging between 160 ppb and 140,000 ppb from ground water monitoring wells MW-1, MW-2, MW-4A, MW-4B and MW-6 (Table4) i Laboratory analysis bf ground water samples indicates that the greatest concentrations of dissolved petroleum hydrocarbons is located near wells MW-2 and MW-4 (Figure 11) i • Laboratory`analysis detected DIPE in a ground water sample from MW-2 (09 September • 1998) at a concentration of 51 ppb and from MW-6 (25 August 1998) at a concentration of i 0 94 ppb, MTBE was detected in a ground water sample from MW-2 at concentrations up to 690 ppb (EPA methods 8020 and 8260) in August and September 1998 sampling events, TBA was detected in a ground water sample from MW-6 at a concentration of 9 4 ppb (25 i August-1998), no other fuel oxygenating compounds were detected above laboratory reporting limits from selected soil or ground water samples (Tables 1 and 4) i • Field observations and laboratory data indicate that petroleum hydrocarbon-impacted soil is limited to depths between 20 and 55 feet bsg in the area containing MW-4 The estimated extent of petroleum hydrocarbon-impacted soil is illustrated in cross sectional view in iFigures 12 • Field observations and laboratory data suggest that the impacted ground water beneath the ' site appears to have taken a relatively narrow, elongated oval shape (Figure 11) • The dissolved TPH-g plume appears to be constrained north, east and west of the current network of ground water monitonng wells Laterally, the dissolved plume appears to extend islightly south of MW-6 • Based on the guide How to Evaluate Alternative Cleanup Technologies For UST Sites i (October 1994) published by the United States Environmental Protection Agency (EPA) Office of USTs, the two most importalit factors that determine the effectiveness of SVE are permeability of the soil and volatility of the contaminant The guide further states that iintrinsic permeability between the magnitude of 10 2 and 10-$ and general volatility for • gasolmes are generally adequate for Soil Vapor Extraction (SVE) technology (Ethrbit H-6 iAdianced GeoEn{tronmental,Inc "