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RISKPRO'S SESOIL for Windows User's Guide The sorption process may be defined as the adhesion of pollutant molecules or ions to the surface of soil solids. Most sorption processes are reversible, adsorption describing the movement of pollutant onto soil solids and desorption being the partitioning of the chemical from solid into the liquid or gas phase (Lyman et al , 1982) Adsorption and desorption are usually assumed to occur in equilibrium and are therefore modeled as a single process (Bonazountas et al., 1984). Adsorption is assumed to occur rapidly relative to the migration of the pollutant in soil moisture, it can drastically retard pollutant migration through the soil column SESOIL employs the general Freundlich equation (see Eq [8] above) to model soil sorption processes. The equation correlates adsorbed concentration with the dissolved concentration of the pollutant, by means of an adsorption coefficient and the Freundlich parameter. This equation has been found to most nearly approximate the adsorption of many pollutants, especially organic chemicals, and a Iarge amount of data have been generated in support of this assumption and is available in the references (see Bonazountas and Wagner, 1984, Fairbndge and Finke, 1979, Lyman et al., 1982) I For most organic chemicals, adsorption occurs mainly on the organic carbon particles within the soil (Lyman et al , 1982) The organic carbon partition coefficient (K') for organic chemicals Ican be measured or estimated (Lyman et al , 1982) Ka is converted to thepartition coefficient (Kd) by multiplying by the fraction of organic carbon in the soil. Values for the Freundlich exponent can be found in the literature They generally range between 0 7 and 1 1, although values can be found as low as 0.3 and as high as 1.7. In the ' absence of data, a value of 10 is recommended since no estimation techniques for this parameter have yet been developed. Note that using 1 0 for the Freundlich exponent assumes a linear model for sorption (see Eq [8]) The user is cautioned about indiscriminately using literature values for the partition coefficient Kd, or the Freundlich exponent n, or estimation methods for Kd There can be much variability in the values that are estimated or found in the literature compared to actual measurements for a site For examples, refer to the study of Melancon et al (1986) ' Another option for modeling adsorption in SESOIL uses cation exchange capacity (CEC) Cation exchange occurs when positively charged atoms or molecules (cations such as heavy metals) are exchanged with the cations of minerals and other soil constituents CEC is a ' measure amount of cations per unit of soil that are available for exchange with the pollutant The cation exchange algorithm in SESOIL is very simple and estimates the maximum amount ' of pollutant that can be adsorbed The calculation of the pollutant immobilized by cation exchange is given by (from Bonazountas and Wagner, 1984) ' Page 25 I