Geochemical models are excellent tools for the understanding of trace elements dynamics, in particular in complex environments like estuaries. Their aim is to reproduce numerically the biogeochemical processes observed in the field or laboratory. The development and the use of this kind of tool requires:

- in-situ data on the distribution in space and time of the elements studied and relationship with the environmental variables.

- experimental data (mainly based on the use of radioactive analogues of the studied elements) for evaluation of model parameters and the validation of model assumptions.

The model must be capable not only of reproducing the distribution of metals between the dissolved and particulate phases, but also of evaluating the concentrations of various chemical species (especially those which are most bioavailable). The approach presented treats adsorption processes as a formation of complexes with functional surface groups (surface complexation model). The calculation of chemical species takes into account the presence of dissolved ligands. The model can integrate several types of "natural" particles (oxydes, particulate organic matter) and different types of surface sites. This approach requires the evaluation of various parameters (especially surface properties of natural particles). The usefulness and limits of this kind of approach are illustrated via an application with cadmium, cobalt and caesium using data collected in the Seine, Loire and Gironde estuaries.