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Châtenay-Malabry, France

Redon P.-O.,Andra Research and Development Division | Redon P.-O.,Ecole des Mines de Nantes | Abdelouas A.,Ecole des Mines de Nantes | Bastviken D.,Linkoping University | And 3 more authors.
Environmental Science and Technology | Year: 2011

Recent studies have shown that extensive chlorination of natural organic matter significantly affects chlorine (Cl) residence time in soils. This natural biogeochemical process must be considered when developing the conceptual models used as the basis for safety assessments regarding the potential health impacts of 36-chlorine released from present and planned radioactive waste disposal facilities. In this study, we surveyed 51 French forested areas to determine the variability in chlorine speciation and storage in soils. Concentrations of total chlorine (Cl tot) and organic chlorine (Cl org) were determined in litterfall, forest floor and mineral soil samples. Cl org constituted 11-100% of Cl tot, with the highest concentrations being found in the humus layer (34-689 mg Cl org kg -1). In terms of areal storage (53 - 400 kg Cl org ha -1) the mineral soil dominated due to its greater thickness (40 cm). Cl org concentrations and estimated retention of organochlorine in the humus layer were correlated with Cl input, total Cl concentration, organic carbon content, soil pH and the dominant tree species. Cl org concentration in mineral soil was not significantly influenced by the studied environmental factors, however increasing Cl:C ratios with depth could indicate selective preservation of chlorinated organic molecules. Litterfall contributions of Cl were significant but generally minor compared to other fluxes and stocks. Assuming steady-state conditions, known annual wet deposition and measured inventories in soil, the theoretical average residence time calculated for total chlorine (inorganic (Cl in) and organic) was 5-fold higher than that estimated for Cl in alone. Consideration of the Cl org pool is therefore clearly important in studies of overall Cl cycling in terrestrial ecosystems. © 2011 American Chemical Society. Source

Robinet J.-C.,Andra Research and Development Division | Robinet J.-C.,French National Center for Scientific Research | Sardini P.,French National Center for Scientific Research | Coelho D.,Andra Research and Development Division | And 5 more authors.
Water Resources Research | Year: 2012

The mesostructure (millimeter to micrometer scale) of clay-rich sedimentary rocks is generally characterized by a connected fine-grained clay matrix embedding coarser nonclay minerals. We use the Callovo-Oxfordian clay-rich rock formation (France) to illustrate how mesostructure influences solute transfer in clay-rich rocks at larger scales. Using micrometer resolution imaging techniques (SEM and micro-CT) major mineral phases (clay matrix, carbonates, tectosilicates, and heavy minerals) were mapped both in two dimensional (2-D) and three dimensional (3-D) at the mesoscale. Orientation and elongation distributions of carbonate and tectosilicate grains measured on mineral maps reveal an anisotropic mesostructure relative to the bedding plane, in agreement with the geological history of the sedimentary rock. Diffusion simulations were performed based on the 3-D mineral maps using a random walk method thus allowing direct computation of mesoscopic scale-related diffusion anisotropy and tortuosity. Considering an isotropic clay matrix, simulated diffusion anisotropy (1.11-1.26) was found lower than the one experimentally measured on macroscopic samples (1.5 to 2), due to the anisotropy feature of pores within the clay matrix. The effects of the mineral content variations on diffusion properties were then investigated by numerical modifications of a mineral map combined with diffusion simulations. Evolution of the tortuosity and diffusion anisotropy with the clay matrix content were successfully interpreted by the Koponen percolation/diffusion model, whereas the Archie approach fails to reproduce diffusion properties at low clay contents. A comparison of fitting parameters with those obtained experimentally indicates that diffusion coefficient variations observed at a large scale could be mainly controlled by the mesostructure. © 2012. American Geophysical Union. Source

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