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lww Geminl Petrographic Investigations B. FIELD OBSERVATIONS Field observation during the drilling phase supports the laboratory analytical results as hydrocarbon contamination was not detected teased on visual determinations(lack of soil discoloration or odor)or volatile constituents(PID readings were not observed)in the collected samples. V. CONCLUSIONS We present the following conclusion regarding possible sources of loll contamination and potential Impact on groundwater at Cozad Trailers,4907 East Waterloo Road,Stockton, A. SOIL CONTAMINATION Based on field observations and confirmatory laboratory analysis,the most probable source of low levels(ppb range) 8TX&E contamination confirmed in the lower three samples Is the result of petroleum residues left €n-sltu as groundwater level has lowered over the past several years.This Is supported by the following three observations: • Lack of detectable levels of TPN (as gasoline), Benzene and Ethylbenzene In soil sample 90481.1-2 through 90481-1-4,10.0 to 20.0',respectively. The near detection limit concentrations of Toulene and Xylene should be suspect, possibly as the results of contamination introduced by using duct tape to seal the sample tubes. If the former gasoline tank was the source of contamination,we should have seen elevated concentration of all constituents In the upper three samples,with a gradual depletion of contaminates with depth.Instead,the results Indicate a opposite trend,with the highest level toward the bottom,and gradually decreasing towards the surface. s The highest concentrations of low level contamination and corresponding significant increase In BTX&E concentrations at 45.0', cone€des with the approximate maximum historical ground Nater level. Chemically, BTX&E constituents are more soluble In water than gasoline hydrocarbons. Typically in a contaminated so]] profile, resulting contamination from an underground storage tank would show the highest 8TX&E concentrations Immediately below and adjacent to the tank. There would be a depletion of contamination ( zone of lowest concentrations)In the soil profile above the groundwater Interface,with concentrations sharply increasing at the interface, and the highest values within the upper zone of saturation. The laboratory data in relation to the subsurface horizons exhibRing contamination does not exhibit this type of contamination pattern. • Given the dense, clayey subsoil characteristic of the site, vertical migration of contaminates would be impeded or severely restricted due to the very low vertical hydraulic conductivity of the soil material. -------------- B. GROUNDWATER IMPACT Field and laboratory data indicate that soil contamination resulting from the unauthorized release of petroleum constituents at the former excavated gasoline tank site does not extend vertical or horizontally beyond those zones previously re-excavated earner this year. Low level contamination found In this Investigation Is most 6