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• 6.0 Discussion The excavation area was primarily located in the area of the historic groundwater plume The plume has been present with the highest dissolved concentrations routinely observed at monitoring wells MW-3 and MW-9 The excavation revealed contamjnant in the capillary fringe and aquifer strata in areas noted in previous subsurface studies Small coarse-grained strata packages (coarse sand with fine gravel and locally extensive sandy beds)interbedded in slightly finer grained strata(strata containing more dispersed silt and clay) appeared stained and are interpreted to have acted as residual sources within the vadose and aquifer strata as discussed in previous Wright reports Light sheen was observed on the water surface near the former locations of MW-3 and MW-9 under the pipelines and adjacent to these sediment packages The small coarse- grained strata packages were excavated to the maximum extent feasible in and around the pipelines to remove residual contaminant sources The light sheen diminished over a few days and excavation dewatering pumping continued on a discontinuous basis until pumping ended on April 1, 2004 The southern area of the excavation near the former tank pit appears to be another area of residual contaminant based on perimeter soil sample data The TPHG concentrations were most elevated in the area dust down gradient of the former tank pit The quantity and timing of fuel release is unknown however this area of vadoselcapillary and aquifer strata would have been initially affected. On the basis of the soil chemical data the most elevated residual contaminant concentrations occur near former tank pit (see Figures 2,3 and 4) The maximum depth of contaminant penetration on the basis of color stain into the aquifer appears to diminish between roughly the 10-foot and 15-foot depth interval This corresponds with the approximate working "aquitard contact" of underlying strata of silt, clayey sand and sandy clay While these aquitard strata are discontinuous in lateral and vertical extent, at least some vertical hydraulic conductivity decrease appears across this contact that tended to inhibit vertical movement below about 15 feet 6.1 Interpreted Contaminant Pathways and Soil Plume Location Soil isoconcentration maps show the soil plume between the pipelines and may have followed a preferential pathway between the pipelines. Residual TPHG isoconcentration maps (see Figure 3) appears to range from 100 to 1000 parts per mullion (ppm) in the vicinity of the excavated areas ,lust down gradient of the former tank pit Soil concentrations diminish to the north with localized areas of TPHG over 1000 ppm that correspond to the coarser sediment package locations Benzene soil isoconcentration map (see Figure 4) shows a similar trend with very low concentrations of Benzene primarily in the vicinity of the southern excavation near the former tank pit Most Benzene concentrations are below 0.10 ppm with one soil sample showing the highest level at 23 ppm Benzene. This chemical data is interpreted to Page 8 of 13