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• 41 Laboratory Quality Control Kiff Laboratory operates a quality assurance and quality control program The laboratory reports showed an irregular occurrence of methylene chloride in some soil sample data This contaminant had previously never been observed at the site Wright requested that the laboratory perform an internal quality control (QC) check The laboratory reported the internal QC revealed that the methylene chloride was attributed to internal use and not present in the samples and were contaminated after they arrived at the laboratory (see attached Kaff letter dated April 16, 2003 in Appendix B) 5.0 Subsurface Observations The excavation revealed interbedded clayey and sandy strata similar to those observed in the previous subsurface studies The contaminants were present at depths of about 4 to 6 feet below the surface grade and locally extended into the aquifer strata at depths of about 11 to 15 feet(see Figure 1 and 2) Groundwater occurred in the pits at depths of about 10 to 11 feet below grade Stained soil interpreted as "smear" within the capillary fringe in the 6- to 10-foot interval Historic capillary rise and fall is associated with aquifer recharge and discharge, and if petroleum contaminant is present a "smear" of contaminants may occur The rise and fall of the capillary water levels cause petroleum to move into pores and it may then become absorbed onto soil When water levels fall contaminants may remain in the pores creating a residual and relatively immobile portion of the contaminant. This discolored sediment (suggestive of "smear") in the aquifer strata was removed of field indications of contaminants were observed during excavation (see Site Photographs) Light sheen was intermittently observed in localized areas near sample locations near the locations of MW-3 and MW-9 Coarse-gramed sediment strata at these locations appeared to have contaminant-coated grains, and were stained Several other areas of coarse sediment continued within finer strata appeared to have higher contaminant content than other strata This sheen decreased with tame as the contaminated soil was removed and water was removed from the excavation The soil samples collected near the southern limit of excavation nearest the former tank pit showed relatively high TPHG and Benzene contaminants The samples near the assumed edge wall of the former tank pit displayed staining and petroleum odor This area would correspond to the area of suspected initial underground tank leakage that then moved down gradient(north) Page 7 of 13