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]3 <br />Working To Restore Nature <br />FATE OF CONTAMINANTS <br />As stated in the Soil Excavation Report Addendum prepared by RESNA (April 1, 1994), the <br />concentration of a substance in groundwater (Cw) can be predicted using the concentration in <br />soil (CS) with which it is in contact. This is accomplished by use of the soil distribution <br />coefficient (Kd) (Hounslow, 1990). Distribution coefficients are substance specific, unitless <br />relationships defined as: <br />Kd = Cs/Cw (Equation 1) <br />The larger the value of Kd the greater the tendency of a substance to remain within the soil. <br />Kd values are expected to increase with increasing organic carbon and clay content in the soil <br />One study (EPA, 1978) directly determined soil distribution coefficients for eleven organic <br />compounds in sediment from Coyote Creek in Santa Clara County, California The organic <br />carbon content of the sediment was 1.4%, similar to that in the Jack -tone Clay at the Corrigan <br />site. Utilizing the experimental data, a relationship between the soil distribution coefficients <br />for organic substances and their respective solubilities was derived <br />log Kd = 5.35 - 0.66 log sol (ppb) (Equation 2) <br />Equation 2 indicates that the distnbution coefficient decreases with increasing solubility The <br />expected concentration of a contaminant in groundwater at the site can be estimated using the <br />average concentration remaining in the soil (mass of contaminant divided by the mass of soil) <br />by the equation: <br />CW = Cs/Kd (Equation 3) <br />Soil distribution coefficients were calculated for the residual BTEX, diesel and gasoline <br />constituents. Potential groundwater concentrations were determined using the average <br />remaining soil concentrations: <br />Benzene <br />Kd = 17 <br />Toluene: <br />Kd = 38 <br />Ethylbenzene <br />Kd = 86 <br />Xylenes. <br />Kd = 86 <br />TPHd. <br />Kd = 1970 <br />TPHg <br />Kd = 127 <br />Csavg = 0 029 ppm <br />Csavg = 0 025 ppm <br />Csavg = 0 031 ppm <br />Csavg = 0 042 ppm <br />Csavg = 40 ppm <br />Csavg = 6 5 ppm <br />Cwavg = 1 7 ppb <br />Cwavg = 0 66 ppb <br />Cwavg = 0 36 ppb <br />Cwavg = 0 49 ppb <br />Cwavg = 20 ppb <br />Cwavg = 51 ppb <br />With the exception of benzene, toluene, and TPHg, all the calculated values are below their <br />respective laboratory method detection limits (MDLs). The calculated values for benzene, <br />toluene, and TPHg approach their respective MDLs and are representative of calculated <br />1500541RAD2-994 DOC 2 <br />