Modelling trace metal in river and sediment compartments to assess water quality

Abstract This paper focuses on trace metal (TM) dynamics along 2 stretches of European Rivers: the French part of the Mosel River (upstream part of the watershed) and the Belgian part of the Meuse River (middle part of the Meuse watershed). A deterministic description of hydro-sedimentary processes and simulation of trace metal concentrations have been achieved. Trace metal particulate concentrations are strongly dependent on the particle sizes. Fine particles have a high adsorption capacity. A multi-class grain size representation has then been used. The dissolved and particulate trace metal phases have been calculated with specific partitioning coefficients associated to each suspended particulate matter sediment (SS) class. A model describing pollution transport and dilution at the watershed scalehas been enhanced with the implementation of the MicMod sub-model aiming to characterize the dynamics of trace metals in water column and bed sediment. Trace metals have been simulated by applying mass-balance relationships in aquatic ecosystem. Processes involve in trace metal fate have been calibrated in the MicMod sub-model: settling velocity, trace metal releases from the watershed (point and diffuse loads), etc. This sub-model also considers calibrated sorption processes and additionally describes trace metal behaviour and the interactions between water column and suspended particulate matter. The explicit description of SS leads to a better representation of the TM behaviour in river ecosystem. It was demonstrated that a description of at least one class of fine particles is necessary to get an adequate representation of TM concentrations. This paper establishes consistent pressure-impact relationships between the TM loads (urban, industrial, soils, etc.) discharging from the watersheds into the river. Considering the potential capacity of that type of models to be used for water quality management plans (in the scope of WFD), priority plans, etc. a special attention addresses validation phases, using measured data, to reinforce the modelling operationality.