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RISKPRO'S SESOIL for Windows User's Guide employs a stochastic approach for the hydrologic and washload cycles, and a deterministic approach for the pollutant transport cycle 3.1 The Soil Compartment In SESOIL, the soil compartment (or column) is a cell extending from the surface through the unsaturated zone to the upper level of the saturated soil zone, also referred to as the aquifer or I groundwater table While SESOIL estimates the pollutant mass added to the groundwater, the saturated-zone is not modeled _The output from SESOIL can be used-for generating input values for groundwater transport models (In RISKPRO, the Analytic Transient 1-2-3 Dimensional Model, AT123D (Yeh, 1981), has been adapted to use SESOIL results for groundwater runoff [recharge]to simulate chemical movement-in the saturated-zone.-) In SESOIL the soil compartment is treated different) by the hydrologic cycle and the pollutant ' p Y cycle In the hydrologic cycle, the whole soil column is treated as a single homogeneous compartment extending from the land surface to the water table The pollutant cycle breaks the ' soil column into several corffpartments, also called layers.' The layers in the pollutant cycle can be further broken-up into,sublayers Each soil layer (sublayer) is considered as a compartment with a set volume and the total soil column is treated as a series of interconnected layers ' (sublayers) Each layer (sublayer) can receive and release pollutant to and from adjacent layers (sublayers) 4 The dimensions of the soil compartment are defined by the user The width and length of the column are defined as the area of application of pollutant released to the soil, and the depth to _the groundwater-is-determined-from-the thickness of user-defined soil layers that are used in the -' pollutant cyclo.- The soil column can be-represented Ch 2, 3, of-4 distinct layers Up to 10 sublayers can-be-specified-for-each-layer,-each having the same soil properties as the layer in _I which they reside There is no optimal size for the soil layers (sublayers); the dimensions of the soil column can ' be specified to cover any area from one square centimeter to several square kilometers The area of the compartments is important for mass balance, but in terms of pollutant concentration the area of application is irrelevant since it is constant for all layers (sublayers) Note that the ' equations in SESOIL have been normalized to an area of one square centimeter It is suggested that the minimum depth of a layer be one centimeter. Depending on the t application, layer depths can range from a shallow root zone of 5-25 centimeters to a deep layer of more than 10 meters When the pollutant enters a layer (sublayer), the model assumes instantaneous and uniform distribution of the chemical throughout that layer (sublayer) The model performs mass balance calculations over each entire soil layer (sublayer), there is no concentration gradient within a layer (sublayer) For a given amount of chemical releasers, the I Page 6 I �1