Three-Dimensional Numerical Simulation of Water Quality Constituents in a Natural Lake in the Mississippi Delta

A three-dimensional numerical model has been developed to simulate water quality constituents in natural water bodies. In this model, four interacting systems are considered, including the phosphorus cycle, nitrogen cycle, phytoplankton kinetics and dissolved oxygen balance. Phosphate (PO4), organic phosphorus (OP), and organic nitrogen (ON) are each partitioned into particulate and dissolved forms that are simulated separately. The processes of adsorption/desorption, bed release, and exchange due to sediment deposition/resuspension are also incorporated. The concentrations of the particulate and dissolved nutrients, as well as the total nutrients can be obtained. Those approaches will improve the model accuracy for nutrient simulations. The developed model was applied to simulate the concentrations of water quality constituents in Beasley Lake in the Mississippi Delta. The effects of upland watershed land use as well as the chemical/biochemical processes in the lake were considered. The major force for flow circulations in the lake is wind-driven stress. The concentrations of phytoplankton, ammonia, nitrate, and the particulate and dissolved forms of nutrients, such as PO4, OP and ON in the lake were obtained. This lake is monitored by the USDA National Sedimentation Laboratory, and weekly or biweekly water quality samples have been collected at three stations for the last twenty years. These data were used for model calibration and validation and the commonly used statistical analysis were performed for model evaluation. The simulated results were generally in agreement with measured data. This model provides useful information to understand water quality distributions in small lakes under wind-driven circulations.