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Feasibility Study/Corrective Action Plan 7 <br /> The Drake Property 26485 Thornton Road Thornton California November 26 1996 <br /> (MCLS) for drinking water The possibility of a second source (on- or off-site) contributing to <br /> hydrocarbons at the site is a moot point and need not be addressed here because site conditions meet <br /> criteria set forth by the CWQCB-CVR for"low-risk" conditions qualifying the site for"No Further Action" <br /> status (CWQCB-CVR, 1996) <br /> iv) The total pollutant mass remaining in the groundwater is decreasing at predicted rates and neither <br /> creates,nor threatens to create, a risk to human health and safety or future beneficial use(s) of the <br /> aquifer <br /> To verify natural attenuation of hydrocarbon concentrations on site, calculations were performed to <br /> estimate maximum theoretical hydrocarbon concentrations in groundwater (Table 5) Theoretical <br /> values can then be compared to real data obtained from the site <br /> Site specific soil data and leaching models or partitioning coefficients can be used to calculate the <br /> maximum theoretical groundwater concentrations for the site The maximum soil impacts at the site <br /> are from excavation samples at a depth of 9 feet BGS Depth to groundwater at the site varies from 6 <br /> to 11 feet BGS Since the soil with maximum impacts is periodically saturated with groundwater, the <br /> most conservative estimation of maximum concentrations is to use partitioning coefficients and assume <br /> that impacted soils are saturated year-round This is done with the following equation <br /> C„= C <br /> K <br /> In which <br /> C.= Maximum Theoretical Groundwater Concentration (milligrams per Inter, or mg/L) <br /> C. = Maximum Site Specific Soil Concentration (mglkg) <br /> K,= Distribution Coefficient (liters per kilogram, or Llkg), a product of Koc (partitioning coefficient) <br /> and foc (fraction organic carbon content) <br /> Calculations for site-specific soil concentrations are shown in Table 5 These calculations indicate that <br /> the maximum toluene, ethylbenzene, and xylenes concentrations that would be expected at the site are <br /> 110 ugA, 420 ugA and 1830 ugA, respectively These concentrations have not been detected at the site <br /> in monrtonng wells or water from the excavation (Tables 2-3), therefore these calculations are an over- <br /> estimation This is likely because of the following assumptions (which all result in aver-esbmabon) that <br /> are made when conducting these calculations <br /> 0191FEA RPT(Mrsc 15) <br /> FLUOR DANIEL GTI <br />