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Feasibility Study/Corrective Action Plan 7 The Drake Property 26485 Thornton Road Thornton California November 26 1996 (MCLS) for drinking water The possibility of a second source (on-or off-site) contributing to hydrocarbons at the site is a moot point and need not be addressed here because site conditions meet criteria set forth by the CWQCB-CVR for"low-risk" conditions qualifying the site for"No Further Action" status (CWQCB-CVR, 1996) IV) The total pollutant mass remaining in the groundwater is decreasing at predicted rates and neither creates,nor threatens to create, a risk to human health and safety or future beneficial use(s) of the aquifer To verify natural attenuation of hydrocarbon concentrations on site, calculations were performed to estimate maximum theoretical hydrocarbon concentrations in groundwater (Table 5) Theoretical values can then be compared to real data obtained from the site Site specific soil data and or nd leaching models partitioning coefficients can be used to calculate the (� maximum theoretical groundwater concentrations for the site The maximum soil Impacts at the site are from excavation samples at a depth of 9 feet BGS Depth to groundwater at the site varies from 6 to 11 feet BGS Since the soil with maximum Impacts is periodically saturated with groundwater, the . most conservative estimation of maximum concentrations ,s to use partitioning coefficients and assume pacted soils are saturated year-round This is done with the following equation dr N� 4 In which C.= Maximum Theoretical Groundwater Concentration (milligrams per liter, or mg/L) C, = Maximum Site Specific Soil Concentration (mg/kg) K,= Distribution Coefficient(liters per kilogram, or Ukg), a product of Koc (partitioning coefficient) and foc (fraction organic carbon content) Calculations for site-specific soil concentrations are shown In Table 5 These calculations indicate that the maximum toluene, ethylbenzene, and xylenes concentrations that would be expected at the site are 110 ug/l,420 ugM, and 1830 ug/l, respectively These concentrations have not been detected at the site in monitoring wells or water from the excavation (Tables 2-3),therefore these calculations are an over- estimation This is likely because of the following assumptions (which all result in over-eshmatlon) that are made when conducting these calculations 41 0181 F EA RPT(Mist 15) FLUOR DANIEL GTI �5_