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FARR ASSOCIATES 5 2 2 Fate and Transport of Contaminants of Concern i Comparison of the soil to groundwater data indicates that the low level of residual hydrocarbons in soils at the site have not adversely impacted groundwater Contaminant Concentrations in groundwater have decreased over time at the Site due to both soil removals and natural attenuation, with no chemicals of concern detected in the latest data The most recent data show that no residual benzene was detected in site soil or soil-gas samples I6.0 REMEDIAL ALTERNATIVE EVALUATION 1 61 Mass of Contaminants Removed and Remaining on Site Approximately 550 cubic yards (420 m3) of soils were removed from the vicinity of Tank 3, and i1200 cubic yards (918 m) were removed from the vicinity of Tank 4 An estimate of the average concentration of contaminants in the removed soil was estimated based on sampling of the excavated soil and soil boring sampling within the excavated area, as summarized in Table 4 The total mass of removed contaminants was estimated using the following equation Total Est Mass = Cs, pwa,s Vs where, CS, = average chemical concentration in soil [mg/kg] bulk wet density [kg/'M'l VSa = volume of soil affected by average concentration (CS...) [in The bulk wet density of 1,992 kg/m3 was calculated usin; the mean concentration of six samples collected on September 10, 1996 and analyzed for \A et Inilk density by Method RP-40 (Appendix E) Total contaminant mass removed was estimated using the above formula, as summarized in Table 4 As shown on Table 4, it is estimated that a total mass of over 2,100 kg of contaminants was removed from the Site The estimated total mass of contaminants in residual soil at the Site was estimated based on soil sampling conducted on the side walls and base of the excavation, as well as from borings in the near vicinity of the excavation As pointed out earlier in this report, most of the data used to estimate residual soil contamination at the Site is seven to nine years old Significant biodegradation has no doubt occurred over this time period Figure 9 depicts the estimated lateral extent of residual soil contamination in the vicinity of Tanks 3 and 4 based on data from the early 1990s Figures 10-12 provide a depiction of the vertical extent of contamination surrounding these two tanks The total mass of contaminants residing at the Site was estimated using the above equation based on the average residual chemical concentrations in soil, as summarized in Table 6 C\MyFdes\AnnelwpwmlprojectslumonlClnsureRpt wpd 10