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• Cieologi al rechws l/tc Page 4 In Situ Remediation Feasibility Study Project No 507 2 September 20,2002 ' • If the lateral distance between the well � is air eater than a few feet, then calculations p must be made to correct the down-gradient piezometric head to account for the sloping ' water table between the wells The calculation considers the slope of the water table and the distance in a down-gradient direction between the two wells used in the calculations Figure 2 shows the location of the well cluster used for calculating the vertical groundwater gradient in this report, MW-9 and MW-109 Table 1 shows Summary of Water Level and Gradients Slope and Bearing, and Table 2 shows the calculated vertical gradients The information used in the calculations is shown below Vertical gradient calculation formulas are as follows • Vertical correction for gradient [(gw gradient slope) x (distance) = vertical correction] • Vertical head [(head of deep well) - (head of shallow+ correction) = vertical head] I • Vertical gradient [(vertical head) / (vertical distance) =vertical gradient] ' Vertical gradients were calculated for the MW-9/MW-109 pair for all five events in the investigation The vertical gradients were negative (a downward direction) for all five of the events M2.3 Contaminant Distribution ' Figures 5 and 7 illustrate the cross section profiles with soil sample contaminant concentrations superimposed on the profile The diagrams also include the groundwater TPH-G data from the June 20, 2002 monitoring event placed adjacent to the wells' screened ' intervals Tables 3 and 4 contain the historical groundwater and soil contaminant concentrations, respectively The Wm J Hunter & Associates analytical data is included in Appendix B plume is limited in lateral extent and appears to be localized as a The soil contaminant p u e pp "core" in the vicinity of MW-9/MW-109 It is defined laterally (clockwise- from north) by soil borings MW-6, MW-2, MW-8, MW-1, MW-7, SB-6 and MW-10 The highest levels of soil contamination are present in soil boring MW-9 that is located just east of the foriner UST excavation The vertical distribution of the contamination is concentrated in the -- silt/clay units at 10 — 30 feet bgs The TPH-G concentrations detected in soil attenuate vertically and laterally with distance from the core location The groundwater plume is also localized in the core area described above Figure 8 illustrates the groundwater TPH-G concentrations for the June 20, 2002 event The lateral extent of the contamination plume is defined to the east, northeast and southeast by wells MW-3, MW-2 and MW-1 respectively The plume is defined to the northwest and southwest by wells MW-10 and MW-7 respectively