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Gealo ual Techntcs Inc � Page$ Valentine(Castle Motors) Report—Additional Site Characterization Project No 723 2 January 21,2004 5B The mass of contamination in these zones is calculated as presented in Table 3, Appendix A The rationale for choosing 25 and 60 feet bgs as the zone limits are 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 — 2003 period (as depleted in Figure 313) 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 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 table- 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 bonng 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 ' • The terminal samples from all three new off-site boreholes were non-detect • 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 It is not clear that this gasoline originated from the Valentine site The possibility of co- mingled water plumes will be discussed below For the shallow zone, the soil samples' TPH-G values in borehole SB-B were averaged within the vertical profile to compute a value for the zone A value of one-half the detection limit was used for non-detect samples (0 5 mg/kg) ' For the deep zone, the soil samples' TPH-G values in boreholes SB-B, and MW-1 through MW-6 were averaged within the vertical profile to compute a value for the borehole The average borehole values were then used to complete the contour map presented in Figure 5B (utilizing computer software-SURFERTm) The average values between the contours (i e- 500 mg/kg, 1500 mg/kg, etc ) were then used in Table 3, Appendix A to approximate the mass of the zone intervals ' 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 and it is not clear that this gasoline originated from the Valentine site The possibility of co-mingled water plumes will be discussed below A mass balance of 11,545 kg (25,457 lbs or 4,106 gal) of TPH-G contamination in soil was calculated The calculated results are interpretive of site contaminant distribution conditions and the results are considered reasonable given the following factors • A limited number of data points at the site and aerially restricted placement of the data points yields contour boundary of the plume as rough interpretation of site conditions • Highly variable lithology in which TPH-G would be expected to sorb to silt/clays but not be retained in the sands