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Geolagccal7ecicxlcslnc Page 10 Site Conceptual Model Report(2006 Revision) Valentine(Castle Motors) Project No 723 2 .lune 16,2006 ' 2.5 Contaminant Distribution ' 25.1 Vadose Zone Figures 7 & 8 illustrate the soil and April 17, 2006 groundwater contaminant distribution as ' superimposed on cross sections A - A' and B —B' In order to define the TPH-G distribution in soil, GTI divided the subsurface into two zones - ' shallow (0 - 25 feet bgs) and deep (25 - 60 feet bgs) The rationale for choosing 25 and 60 feet bgs as the zone limits is as follows ' . The only vadose zone samples to contain detectable TPH-G came from boring SB-B located adjacent to the former dispenser island The on-site wells contained contamination in the water table smear zone as groundwater fluctuated between 25 — ' 43 feet bgs in the 1995 — 2006 period (as depicted in Figure 9) The impacted shallow zone is that area between the detection points in SB-B and half the distance to wells MW-2 and MW-3 that are the closest non-detect points- see Figure 10 for the lateral extent of the shallow soil plume The zone is centered on the dispenser island that is the presumed source of the release • The highest concentration of detected TPH-G was in boring SB-B but below the water Mtable- 13000 mg/kg TPH-G at 31 feet bgs Adjacent wells MW-2 and MW-3 also contained high levels of contamination to depths of 55 feet bgs The high levels present in this zone appear to stop at the 44 mg/kg concentration detected in 67 feet ' bgs in boring MW-101 The TPH-G levels attenuate further in SB-201 and MW-201 below this depth It appears that as the contamination migrated downward toward the sandy zone present beneath the site at 60 — 75 feet it was diluted and transported ' laterally due to higher hydraulic conductivity of the sandy zone- see Figure 11 for the lateral extent of the deep soil plume ' a The vadose zone samples from off-site boreholes MW-7 & MW-9 were non-detect for TPH-G • The soil plume is not shown extending to the soil contamination in boring MW-8 at ' 30 feet bgs because the adjacent site has had a documented gasoline release • The vadose zone samples from down gradient off-site boreholes CPT-1, CPT-2 and CPT-3 were non-detect for TPH-G 2.5.2 Groundwater ' As stated above, the site's monitoring wells have been grouped according to the aquifer interval that the screened section intercepted (see Figure 5 for well screen intervals) ' Shallow Wells (screened 35 —55 feet bgs) MW-4, 5, 6, 7, 8 & 9 Intermediate Wells (screened 43 —70 feet bgs) MW-1, 2, 3, 107, 108 & 109 Deep Wells (screened 76— 80 feet bgs) MW-207, 208 & 209 Deepest Wells (screened 94—97 feet bgs) MW-101, 307, 308 & 309