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i C?eolagfcal reclutics!hu Page 12 ' Additional Site Characterization Report Project No 662 2 February 20,2004 ' In Figure 6, up gradient borehole CPT-5 is seen on the west side of the cross section This borehole was non-detect for all samples except the water sample from 45 feet bgs This tsample contained 0 96 ug/1 MTBE or dust above the reporting limit of 0 5 ug/1 MTBE In the center of the cross section are boreholes MW-3, CPT-3, CPT-4, and MW-104 MTBE ' contamination is present in the soils in this area The MTBE detections in soil samples from 20 w-36 feet bgs ranged 0 013 — 1 2 mg/kg Soil samples from depths of greater than 36 feet contained <0 005 - 0 064 mg/kg MTBE The water samples in this area are similar to those ' in Figure 4 The water sample from CPT-3 at 31 feet bgs contained 12,000 ug/1 TPH-G and 10,000 ug/1 MTBE Below this depth, water samples from CPT-3 and CPT-4 contained 0 71 — 33 ug/1 MTBE On the right (east) side of the cross section CPT-7 samples were all ' non-detect except the water sample from 47 feet bgs This sample contained 100 ug/1 MTBE ' Off site CPT boreholes CPT-6 and CPT 8 are not shown on the sections but they were non- detect for all analyses Borehole CPT-1, placed adjacent to MW-1, contained trace to low levels of MTBE in soil samples from 30 — 60 feet bgs A groundwater sample from this ' borehole at 55 feet bgs contained 130 ug/1 TPH-G and 100 ug/I MTBE The 45 feet bgs sample was clean in this location and deeper samples contained trace (12 ug/1) to non-detect (<0 5 ug/1) MTBE From the cross sections it is apparent that soil contamination is localized in the vicinity of the foriner UST excavation This residual contamination continues to source a groundwater ' plume that is also localized in this same area Shallow water samples from GeoProbe borings GP-3 through GP-9, and from CPT boreholes CPT-6, CPT-7 and CPT-8 indicate that the water table plume is not extensive But the presence of a submerged plume in the location of CPT-7 at 45 feet bgs indicates that the sand layers at 38 —48 beneath the former excavation act as a preferential pathway for the MTBE plume to migrate eastward It is ' likely that the plume is diluted by up gradient fresh water recharge The sand layers noted at 38 —48 feet bgs form the boundaries for estimating the mass of the ' residual contamination The following section includes a methodology for quantifying the amount of residual contamination ' 4.3 Contaminant Mass Estimate Calculations The analytical data generated in the site investigation were reviewed in conjunction with the geological data to develop a site conceptual model for the soil and groundwater plumes Soul ' The soil analytical data is limited with those samples that did contain contamination only having MTBE- no BTEX or TPH-G was detected Figure 7 shows a circle between the plume core data points (GP-1, GP-2, MW-1, MW-2 and MW-3) and non-detect points 1