Mobility of trace metals in freshwater sediments: a modelling coupling solid-liquid exchanges, biogeochemical reactions and interstitial diffusion
Titre du congrès :International Conference on Radioecology and Environmental Radioactivity
Ville du congrès :Bergen
Date du congrès :15/06/2008
In freshwater systems, trace metal pollutants are transferred into water and sedimentary columns under dissolved forms and fixed onto solid particles. Accumulated in the sedimentary areas, these latter ones can constitute important stocks of matters and associated pollutants. 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 communication presents an analysis of the mobility by interstitial diffusion by comparing experimental results and modelling.
This topic involves two main points: 1) Trace metal fractionation between their mobile (dissolved and colloidal) and non mobile (fixed onto the particles) forms; 2) Interstitial diffusion of the mobile phases.
The first point is governed by sorption/desorption processes at the particle surfaces. In the sedimentary column, these processes are regulated by physico-chemical parameters as pH, potential redox, concentrations and nature of particles. In sediments, these parameters are mainly influenced by biochemical reactions resulting from the oxidation of the organic matter by the microbial activity: oxic respiration, denitrification, manganese hydroxides and iron hydroxides reduction, sulphate reduction, methanogenesis (Froelich, Klinkhammer et al. 1979). In this context, this work proposes a model coupling these biogeochemical reactions to sorption/desorption processes of trace metals and interstitial diffusion. In order to evaluate this model an experimental protocol has also been developed to obtain the vertical distributions of the predominant physico-chemical parameters and solid-liquid fractionations of metals (Fe, Mn, Co, Cu, Ni, Pb, Zn) in natural sediment cores. The first comparisons realised on cores collected in the Durance and Rhône rivers allows to identify the gaps and the efficiencies of the modelling approach.