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CLEARWAI tR ' G R O U P Enwronmenta[ services the late spring Groundwater flow direction is typically toward the southeast at a gradient on the order of 0 005 ft/ft 3.2 Contaminants of Concern Cumulative soil and groundwater sample analytical data indicate that gasoline hydrocarbons (i e TPHg and BTEX) are the contaminants of concern beneath the site The gasoline additive MTBE has not been detected in soil samples collected to date MTBE has only been detected once at trace concentrations in water samples collected throughout the course of the investigation Considering that it has not been detected with confirmation at or above laboratory method detection concentrations (EPA Method 8020 and confirmation by EPA Method 8260 have been used) MTBE is not a contaminant of concern at this site ' 3.3 Sorbed-Phase Contamination Historical investigation data indicate the presence of one relatively large area of soil contamination beneath the site The most contaminated area is located in the vicinity of MW-5B lie and extends up-gradient to the eastern end the former UST pit (source area) and probably just beyond MW-5B in the down-gradient direction Soil contamination in the area of the former UST pit, as defined by concentrations of TPHg ' greater than 1 mg/kg, appears to extend from approximately 19 to 29 feet bgs with an areal extent of approximately 10,881 ft2 and estimated thickness of 7 to 10 ft The resulting volume of soil ' affected in this area is estimated to be 108,810 ft3 (4,030 cubic yards) (in place) The average concentration of TPHg contamination in this area was calculated to be approximately 264 mg/kg (0 000264 unitless) The mass of residual sorbed-phase gasoline contaminants in this area was ' calculated to be approximately 2,872 6 lbs (or approximately 463 gallons of gasoline) An important consideration is that the sorbed-phase contamination at this site is indicated to reside ' completely within the saturated zone Thus, the proposed oxygen-infusion system would be expected to enhance degradation of soil-adsorbed hydrocarbons in addition to those of the ' dissolved phase ' Based on the mass calculations presented above, the most impacted soil occurs beneath the . former UST pit and in the southeast down-gradient direction This conclusion is corroborated by the resulting distribution of dissolved-phase contamination The core of the dissolved-phase I ZB178C RAP 14 May 18,2004