Modelling of pollutant behaviour in estuaries: Application to cadmium in the Loire estuary

A one-dimensional multivariate mathematical model was used to study the behaviour of cadmium in a macrotidal estuary. Dissolved and particulate Cd were computed using a complexation model to solve the thermodynamic equations governing balances between free ion Cd2+ and various ligands (chloride, sulfate and hydroxide ions). Suspended matter is also considered as a ligand, characterised by an adsorption constant linking free ion Cd2+ activity to concentrations of sorbed cadmium on suspended matter. The modelʹs sensitivity study, which was applied to the Loire estuary, shows that results can be highly dependent on: (1) the adsorption constant; (2) the initial conditions determining cadmium concentrations in particulate matter at the start of each simulation; (3) the longitudinal distribution of salinity and suspended matter. The above data is often badly evaluated, introducing many uncertainties as to the results and their interpretation. The number of degrees of freedom becomes too high to permit a proper validation. However, this type of approach does allow for a rapid evaluation of the variables controlling the evolution of a given pollutant in an estuary. The results obtained show that the application and validation of a model simulating pollutant behaviour require: (1) validation of the distribution of master variables (salinity and suspended matter (S.M.) in the case of Cd); (2) relatively precise evaluation of: either the initial conditions if the simulation time scale is short, or of boundary conditions and various inputs into the estuary if the model is capable of simulating the systemʹs evolution over a long period of time (several months).