An Integrated Media, Integrated Processes Watershed Model – WASH123D: Part 7 – Sediment and Reactive Chemical Transport in Surface Runoff

Abstract

A watershed system includes river/stream networks, overland regime, and subsurface media. This paper presents a numerical model of sediment and reactive chemical transport in surface runoff of watershed systems. The distribution of mobile suspended sediments and immobile bed sediments is controlled through hydrological transport as well as erosion and deposition processes. Transport of chemical species with a variety of chemical and physical processes is mathematically described by system of M advective-dispersive-reactive transport equations (where M is the number of species). Decomposition via Gauss-Jordan column reduction of the reaction network transforms M species-transport equations into three sets of equations: a set of thermodynamic equilibrium equations representing NE equilibrium reactions, a set of reactive transport equations of NKI kinetic-variables involving no equilibrium reactions (where NKI is the number of linearly independent kinetic reactions), and a sent of NC component transport equations (where NC is the number of components). The elimination of fast reactions from reactive transport equations allows robust and efficient numerical integration. The model solves the PDEs of kinetic-variables and components rather than individual chemical species, which reduces the number of reactive transport equations and simplifies the reaction terms in the equations. A hypothetical example was used to demonstrate the capability of the model in simulating sediment and reactive chemical transport subject to a complex reaction network involving both slow and fast reactions, under the effect of temperature. Based on the application of an eutrophication example, the deficiency of current practices in the water quality modeling is discussed and potential improvements over current practices using this model are addressed.