River water quality assessment: a hydrodynamic water quality model for propagation of pollutants

A hydrodynamic river water quality model is presented. The model consists of a quantity and a quality sub-model. The quantity sub-model is based on the Saint Venant equations. The solution of the Saint Venant equations is obtained by means of an explicit scheme based on space-time conservation. The method considers the unification of space and time and the enforcement of flux conservation in both space and time. On the other hand the quality sub-model is based on the advection dispersion equation. Particularly, the principle of upstream weighting applied to finite difference methods is employed. Such a method enables us to reduce the numerical dispersion, avoiding oscillation phenomenon. The optimal weighting coefficient has been calculated on the basis of the mesh Peclet number. With regards to the quality processes the model takes into account the main physical/chemical processes: degradation of dissolved carbonaceous substances; ammonium oxidation; algal uptake and denitrification; dissolved oxygen balance, including depletion by degradation processes and supply by physical reaeration and photosynthetic production. To properly simulate the river water quality four state variables have been considered: DO, BOD, NH4, and NO. The model was applied to the Savena River (Italy), which is the focus of a European-financed project in which quantity and quality data were gathered. A sensitivity analysis of the model output to the model input or parameters has been carried out.