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RISKPRO'S SESOIL for Windows User's Guide 3.5.6 Metal Complexation Complexation, also called chelation, is defined here as a transformation process In SESOIL, complexation incorporates the pollutant as part of a larger molecule and results in the binding of the pollutant to the soil. For example, metal cations (e g copper, lead, iron, zinc, cadmium) combine with organic or other nonmetallic molecules (ligands) to form stable complexes. The complex that is formed will generally prevent the metal from undergoing other reactions or interactions of the free ion The pollutant fate cycle incorporates a simplified representation of the complexation process as a removal process. It is only available for scenarios in which the pollutant is a heavy metal. The model assumes a reversible process in which a metal ion is complexed by a specified soluble organic ligand to form a complex which is soluble, non-adsorbable, and non-migrating Possible ligands are humic acid, fulvic acid, and low molecular weight carboxylic acids, which are commonly found in landfill leachate (Bonazountas and Wagner, 1984) It is the responsibility iof the user to determine whether this process is likely to occur in the scenario being modeled, and to supply the appropriate information ' The complexation subroutine employs a nonlinear equation which must be solved numerically. It uses the same iterative procedure as the general pollutant cycle for monthly simulations Required data include the stability (or dissociation) constant for the specific complex, and the mole ratio of ligand to metal. Also required are the molecular weights of the pollutant metal and the organic ligand. Equations used by this subroutine are based on the work of Giesy and ' Alberts (1984), Brinkman and Bellama (1978), and Sposito (1981) The model does not consider competition with metal ions in the soil which may have higher affinity for the ligand Note that if the user chooses to model both cation exchange and metal complexation, the cation exchange ' process is assumed to occur first, ions involved in cation exchange are then unavailable for complexation. The general adsorption processes are modeled as being competitive with the complexation process (Bonazountas and Wagner, 1984) ' 3.5.7 Pollutant in Surface Runoff and Washload ' Pollutants can be removed from the soil area being simulated by SESOIL via surface runoff and washload. The pollutant in surface runoff is simply the surface runoff computed in the hydrologic cycle (for each month) multiplied by the pollutant concentration in the soil moisture ' of the surface layer (for each time step) The result of this calculation is multiplied by another user-supplied parameter called ISRM, which controls the amount of chemical partitioned into runoff. There is no basis for estimating ISRM a pnon, it can be set to 0 0 to "turn off' the ' pollutant participation in runoff, or it can be used essentially as a fitting parameter if data are available. In a calibration/validation exercise used to predict atrazine runoff at a site in Watkinsville, Georgia, the parameter ISRM was found to be 0 06 (see Hetrick et al., 1989) ' Page 29