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'RISKPRO'S SESOIL for Windows User's Guide s MCEC=a • CEC • MWTJVAL (14) ' s where. ' MCEC = maximum pollutant cation exchanged by the soil (µg/g soil) a , = 10.0 (units coefficient) ' CEC = cation exchange capacity of the soil (meq/100 g of dry wt. soil) _ MWT = molecular weight of the-pollutant cation (g/mol) - VAL = valence of the cation 0 With--clays,the-exchanged-ion-is-oftemcalcium;and-clay-soils-tend-to-have-the--highest-cation— exchange capacity Note,that the CEC value of a soil increases with increase in pH, but pH is not included in the CEC algorithm in SESOIL. The CEC value must be adjusted manually to include effects due to pH In SESOIL; cation exchange computed by Eq (14) is assumed to occur instantaneously, and irreversibly. Once maximum adsorption via exchange has been reached, no additional adsorption will be calculated The process is also assumed to take precedence over all other soil processes ' in competition for the pollutant cation The use of the cation exchange subroutine is optional If it is used, Eq (8) should not be us4M (i.e., model inputs for Y...d and I{a should be 0 0) unless the user has selected the model inputs in such a way as to avoid double accounting. It is up to the user to be sure that cation exchange is the predominant adsorption mechanism at the modeled site. This determination i includes-considerations of leachate characteristics such-as pH,ionic strength--and the presence - and_concentration of_other cations __The other cations, often found in landfill,leachate and_ aqueous industrial wastes, may have higher affinity for exchange with soil cations, and may, effectively block exchange between the pollutant and the soil cations--,In addition, the speciation of the pollutant should be considered (Bonazountas and Wagner, 1984). 3.5.5 Degradation: Biodegradation and Hydrolysis The pollutant cycle of SESOIL contains two transformation routines, that can be used to estimate ' pollutant degradation in the soil _ Biodegradation is the biologic breakdown of organic chemicals, most often by microorganisms Hydrolysis is a chemical reaction of the pollutant with water Both processes result in the loss of the original pollutant and the creation of new chemicals The SESOIL model accounts for the mass of original pollutant lost via degradation but does not keep track of any degradation products The user is responsible for knowing what ' the degradation products will be and their potential significance 1 Page 26 ' I '