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Cieological7"ecbtks lite Page 13 Additional Site Characterization Report Project No 662 2 • February 20,2004 ' (MW-2) In order to calculate the mass of MTBE in the soil the average concentration of the detection points in the circle were used (one-half the reporting limit was used for non- detect points) Table 2 includes an estimate of the MTBE mass contained in a cylinder defined by the circle in Figure 7 and extending from 14 to 26 feet in depth These ' elevations were selected to represent the bottom of the former UST excavation and the depth of the first sand unit below the excavation The rationale for this decision is that less contaminant absorption would be expected in the saturated sand units than in the overlying clay/silt units As shown in Table 2, there is approximately 0 12 kg or 0 04 gallons of MTBE remaining in ' the soils at the site Water ' In order to calculate the contaminant mass in groundwater, GTI divided the aquifer into three zones- shallow, intermediate and deep The shallow zone extends from 26 to 38 feet ' bgs, the intennediate zone extends from 38 -- 48 feet bgs, and the deep zone extends from 48 —70 feet bgs (see Figure 8) The rational for these zones is as follows M • The shallow zone includes the fluctuating water table (from 20 —29 feet bgs during the investigation) and extends to the top of the extensive sand units at 38 feet bgs described as continuous across the site in section 4 1 above • The intennediate zone comprising the sand units extending from 38 — 48 feet bgs across the site (with variable thickness) ' • The deep zone is from the bottom of the sand units extending to the lower sand unit at 70 feet bgs in CPT-4 location (dilution will occur in this unit and only trace MTBE was detected below it 0 92 ug/1 CPT-3 @ 73') With these horizons established, GTI then used the sample point data to develop contour diagrams of MTBE and TPH-G for each aquifer zone Since no TPH-G was detected in the ' intermediate zone an attempt to quantify the amount was not attempted Figures 9 - 13 include the contour diagrams for the MTBE and TPH-G concentrations in the aquifer zones with contaminant data indicated next to the sample points The median value between ' contours and the porosity values from the Cooper Testing Laboratory data were used in the mass balance calculations that are included in Table 3 ' As shown in Table 3, there is approximately 10 kg or 4 gallons of MTBE, and I 1 kg or 4 gallons of TPH-G remaining in groundwater at the site