Simplifying Dynamic River Water Quality Modelling: a Case Study of Inorganic Nitrogen Dynamics in the Crocodile River (south Africa)

Increasing water scarcity in many countries provides a strong impetus for investments in water quality remediation at the basin or sub-basin scale as a means to increase water availability. The application of mathematical river water quality modelling, as a support tool to evaluate remediation options, is often limited by the availability of reliable data. Current proposals to use the River Water Quality Model number 1 (RWQM1) are hampered by its relative complexity and large data requirements, with the result that use of such model is often not economically feasible, especially in developing countries. However, by simplifying this model to suit specific applications, the general water quality situation of a river can still be predicted with limited data. The ultimate goal of this study is to develop a simple dynamic river water quality model that is compatible with activated sludge models and which can be used for integrated water quality modelling studies in the future. During the present study, a simplified river water quality model was derived from the available RWQM1 model and used to investigate the daily dynamics of nitrate and ammonia nitrogen concentrations in the Crocodile River in South Africa. Using a modelling concept based on continuously stirred tanks in series, the simulation results for two years agree well with the measured seasonal dynamics of nitrate and ammonia concentrations in the river system. The model is most sensitive to the concentration of nitrifiers and to hydraulic parameters.