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1 RISKPRO'S SESOIL for Windows User's Guide In SESOIL, each soil layer (sublayer) has a set volume and the total soil column is treated a a series of interconnected layers Each layer (sublayer) has its own mass balance equation (Eq 1 [61) and can,,receive and release pollutant to and from adjacent layers (sublayers) Again, the individual fate processes that compose the SESOIL mass balance equations (e.g., volatilization, ' degradation) are functions of the pollutant concentration in the soil water of each zone and a variety of first-order rate constants, partitioning coefficients, and other constants. An iterative solution procedure is used to solve the system (the iteration parameter is c) See Bonazountas ' and Wagner (1984) for the numerical solution procedure The pollutant cycleequationsare formulated on a monthly basis and results are given for each ' month simulated. However, to account for the dynamic processes in the model more accurately, - an-explicit-time-step-of-l- day-is-used-in-the-equations The-monthly output_represents-the— summation of results from each day ' In the event that the dissolved concentration exceeds the aqueous solubility of the pollutant, the dissolved concentration is assumed to equal the aqueous solubility That is, if during solution ' of the mass balance equation for any one layer, the dissolved concentration exceeds the solubility of the chemical, the iteration is stopped for that time step and the solubility is used as the dissolved concentration The adsorbed and soil-aur concentrations are calculated using the ' chemical partitioning equations as before (Eqs [7] and [81). To maintain the mass balance, the remaining pollutant is assumed to remain in a pure phase (undissolved). Transport of the pure phase is not considered, but the mass of the chemical in the pure phase is used as input to that/ same layer in the next time step Simulation continues until the pure phase eventually disappears The pure phase: capability was not part of the original model and was added to SESOIL by Hetrick et al (1989) The discussion in the subsections that follow introduces the user to major algorithms and -' processes simulated in the pollutant cycle of-SESOIL 3.5.2 The Pollutant Depth Algorithm ' The pollutanf cycle in SESOIL is based on the pollutant concentration in soil moisture. In theory, a non-reactive dissolved pollutant originating in any unsaturated soil layer will travel to ' another soil layer or to the groundwater at the same speed as the moisture mass onginatsng in the same soil layer. The movement of a reactive pollutant, however, will be retarded in relation to the movement of the bulk moisture mass due to vapor phase partitioning and the adsorpuon of the pollutant on the soil particles If it is assumed that no adsorption occurs, and the vapor phase is negligible, the pollutant will move at the same rate as water through the soil 1 1 Page 20 ' I