SITE INFORMATION AND CORRESPONDENCE_FILE 2

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Site Conceptual Model February 15,2008 Hang's Auto Mart Page 9 of 20 Groundwater Contamination Table 8 summarizes the screen intervals used to designate shallow, intermediate and deep zones. The shallow wells are screened at 50 feet bgs or shallower and are often dry(the average depth-to-water at the Site is 51.61 feet bgs). The intermediate zone used in the mass estimates is defined as having a screen interval that intersects the groundwater table between the average depth-to-water and 64.5 feet bgs. Some of the intermediate wells have deeper screen intervals but must come in contact with groundwater at 64.5 feet bgs or shallower. The deep zone wells as used in the mass calculations must not come in contact with groundwater above 65 feet bgs. The following assumptions are used to calculate the contaminants dissolved in the groundwater beneath the site: TPHg—Groundwater Mass Estimates • The TPHg plume is divided into two areas:the Core and the Outer Ring o The Core is approximately 2,275 ft2 and is represented by the area within the 10,000 ppb contour interval(Figure 9) o The Outer Ring is approximately 23,500 ft2 and occupies the area between the estimated extent of the plume and the 10,000 ppb contour interval(Figure 9) o The concentrations used in the calculations are an average of the highest concentration found in each well within the given area over the last four monitoring events o MWS and MW7 are outside the estimated extent of the plume and were not included in the calculations o If monitoring well was ND for the last four monitoring events, fifty percent of the reporting limit was used in the calculations • The vertical thickness of the impacted zone is assumed to be 44-feet o Wells are screened in this area between 35 and 84.5 feet below grade o Highest water table measurement was 40.90 feet below grade(MW7) o The assumption uses bottom of screen (84.5 feet bgs) less the highest water table (40.90 feet bags) • The deep zone is defined by MW1D (Figure 10) and has not shown the presence of any analyzed constituent in the last seven sampling events(Table 5) • Porosity is assumed to be 35% Pounds of contaminant= Volume * porosity * av& contaminant concentration(mg/L) * 2.2E-6 lb/mg Pounds of TPHg Core = (2,834,832 L) * (0.35) * (100 mg/L)* (2.2E-6 lb/mg) 218 lbs Outer Ring = (29,282,880 L) * (0.35) *(2 mg/L) * (2.2E-6 Ib/mg) 45 lbs Total TPHg 263 lbs Benzene—Groundwater Mass Estimates • The benzene plume is divided into two areas:the Core and the Outer Ring o The Core is approximately 2,125 ft7* and is represented by the area within the 1,000 ppb contour interval(Figure 11) o The Outer Ring is approximately 48,000 ft2 and occupies the area between the estimated extent of the plume and the 1,000 ppb contour interval(Figure 11)