In freshwater systems, trace metal pollutants are transferred into water and sedimentary columns under dissolved forms and/or fixed onto solid particles. Accumulated in the sedimentary areas, these latter ones can constitute important stocks of materials and associated pollutants and may impair water quality when environmental changes lead to increase their mobility. The mobility of the stocks of pollutants is mainly depending on the erosion, on the interstitial diffusion of the mobile phases (dissolved and colloidal) and on the bioturbation.
In this context, this study involves the analysis of the mobility by interstitial diffusion. This topic consists in studying trace metal fractionation between their mobile (dissolved and colloidal) and non mobile (fixed onto the particles) forms. This point is governed by sorption/desorption processes at the particle surfaces. These processes are regulated by physico-chemical parameters (pH, redox potential, ionic strength…) and are influenced by biogeochemical reactions resulting from the oxidation of the organic matter by the microbial activity. These reactions generate vertical profiles of nutrients and metal concentrations along the sedimentary column.
To understand these processes, this work is based on a mixed approach that combines in situ, analysis and modelling.
In situ experimental part consists in sampling natural sediments cores collected at 4 different sites (1 site in Durance and 3 sites on the Rhone). These samples are analyzed according to an analytical protocol that provides the vertical distribution of physicochemical parameters (pH, redox potential, size distribution, porosity), nutrients and solid - liquid forms of trace metals (cobalt, copper, nickel, lead, zinc).
The analysis and interpretation of these experimental results are based on a model that was developed during this study and that includes:
1) model of interstitial diffusion (Boudreau, 1997),
2) biogeochemical model (Wang and Van Cappellen, 1996),
3) model for calculating pH (Devallois et al., 2008) and
4) model of solid-liquid exchange with Kd specific to the main biogeochemical phases(L.molphase -1), for each trace metal.
The results highlight that the availability of trace metals is closely linked to the behaviour of main biogeochemical phases and that the interstitial diffusion mobility depends mainly on grain-size characteristics that determine both diffusion and distribution of biogeochemical phases (organic matter, metal oxides, carbonates, sulphides).