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f90(09ichtrschK csInc. Page 7 Site Conceptual Model Project No.989.2 Deccniber 17,2003 MtBE was not tested for during the initial site investigation. During the installation of MW-4 through MW-9 and MW-101 soil samples were analyzed for this constituent. MtBE was present in three of the six soil borings. MW-4 at a depth of 20 feet, MW-5 at a depth of 30 feet and MW-6 at depths between 10 and 25 feet bgs (Table 1). MtBE was absent in samples ' collected from MW-7, MW-9 and MW-101 (Table 1). The extent of soil impacted by MtBE is estimated in Figure 10. 3.2.2 Extent of Groundwater Contamination TPH-G and BTEX The plume boundaries of TPH-G contamination appear to be defined to the north by MW-5, east by MW-2, south by MW-3, west by GP-1 and vertically by MW-101. A map view of the lateral extent of the TPH-G plume in groundwater is shown as Figure 7. The following assumptions and observations were made prior to calculating the extent of TPH-G and BTEX ' impacted groundwater; o BTEX constituents closely follow the model of the TPH-G plume and will be included as in the "TPH-G" model. o During the last four quarters of groundwater monitoring at VWDC, TPH-G has only been detected in MW-1, MW-3 and. MW-4. o Concentration values for each well used in producing the TPH-G Groundwater Contour Map (Figure 7) are the average TPH-G levels in each well over the past 4 quarters, 1 o GTI feels that the mass estimate calculations are conservative. The summary of groundwater analytical data (Table 2) suggests TPH-G and BTEX ' constituents are limited to the area directly around the location of the former USTs. Figure 7 shows the approximate lateral extent of TPH-G soil contamination in map view. Chart 3 indicates the plume has not migrated any significant distance from the source. MtBE The MtBE plume appears to be defined by MW-2 to the south, MW-7 to the west, and MW- 101 vertically (Figure 11). MW-9 and MW-9 have low levels of MtBE and most likely represent the north and east plunge edges, respectively. MtBE was present in four of the 10 groundwater-monitoring wells at VWDC and the concentrations appear to be approaching MCLs (Table 2). ' 4.0 FATE AND TRANSPORT 4.1 Surface Features The land surrounding the site is relatively flat and is primarily used for commercial and light industrial purposes. The site is almost directly beneath 1-5 and approximately one tenth of a