Modeling Pollutant Transport in Runoff and Sediment

A dynamic model was developed to simulate mixing of rainwater with soils containing agricultural nutrients and pesticides, entrainment of these pollutants to runoff, and their transport in runoff and sediment within a watershed and during a single storm event. The model is an advancement to the existing models in simulating chemical transport during individual storms. It routes infiltrating rainwater and solutes through the soil profile using concepts of complete mixing and piston displacement. When runoff begins, chemical exchange between runoff and a mixing soil layer of the soil profile, containing the chemicals in dissolved and adsorbed forms, are simulated using the concept of non-uniform mixing with respect to the soil layer depths. Entrained chemicals in runoff are routed along slope lengths in dissolved and adsorbed forms based on mass conservation. The basic model concepts and algorithms on chemical mixing and entrainment were tested for time varying concentrations of nitrate, phosphate, and cyanazine in runoff using a suitable laboratory data set found in the literature. A total of 15 model runs, 5 for each pollutant, were made. Predicted concentrations were compared with the respective observed data. The comparisons showed that the concepts and the algorithms were valid and promising.