Modelling trace metal transfer in large rivers under dynamic hydrology: a coupled hydrodynamic and chemical equilibrium model

Trace metals (TMs) in rivers experience a complex chemistry that affects their partitioning between the dissolved and particulate fractions. Predictive modelling of this partitioning is challenging, particularly under variable hydrodynamic conditions in rivers. This paper presents a coupled hydrodynamic–geochemical model for trace metal dynamics in rivers, to highlight key parameters and set a basis for developing simplified models. A coupled model based on the hydrodynamic model MOHID and the chemical speciation model WHAM was applied to the middle part of the Garonne River (SW-France). The predictions of the coupled model are compared to those of a fixed Kd partitioning model. The model accurately simulates trace metal behaviour under varying hydrological conditions. The importance of the riverbed geomorphology and the diurnal variations in pH on TM concentrations in the water column are underlined. The sensitivity analysis ranked all physico-chemical parameters according to their influence on TM behaviour.