Models that are simple to use have the advantage of being used by most professionals while complex models require trained professionals to set up, operate and interpret the the results. When possible, models should be simple to use and at the same time detailed enough to include most relevant processes. Thus, a user friendly model was developed for predicting fate and transport of nutrients in Soil Treatment Units (STUs) in the unsaturated zone. The input parameters include operational parameters (effluent concentrations, hydraulic loading rates). The output is expected performance such as constituent concentrations and mass flux crossing a boundary.
An analytical solution is used to calculate profile of pressure and moisture distribution based on Darcy’s law. The chemical transport component is based on simplification of the general advection dispersion equation (ADE), which is based on mass balance theory. The model is relatively simple to use but detailed enough to account for important fate and transport processes such as sorption, nitrification and denitrification. The model calculates change in moisture content with depth; thus, the effect of water filled porosity on nitrification and denitrification rates can be determined. It also has provisions to account for the effect of temperature and carbon content.
A sensitivity analysis on model parameters showed that nitrate removal is sensitive to nutrient removal parameters such as denitrifcation rate, a threshold value for soil moisture content for denitrification, ammonium sorption isotherm and hydraulic parameters such as saturated hydraulic conductivity, hydraulic loading rate and other calibration parameters in the soil water retention function.