Vertical distribution of HCO3- transport parameters in Boom clay in the Mol-1 borehole (Mol, Belgium)

Boom Clay is presently studied as a reference host formation for the disposal of high-level radioactive waste in Belgium. This layer, situated between about -190 m and -290 m deep below the Mol site in the northeast of Belgium, presents various characteristics favourable to the final disposal of radioactive waste. Apart from a very low hydraulic conductivity (around 2.5 × 10-12 m s-1), it also has a strong sorption capacity for many radionuclides as well as self-sealing properties. A safety analysis of the radiological risks in the biosphere due to the release of radionuclides from the waste requires to calculate the transport rate of radionuclides in Boom Clay. Transport in the Boom Clay is mainly due to diffusion. Because of the very low value of the hydraulic conductivity K and the very low vertical hydraulic gradient (about 2 %), advection is negligible and diffusion is dominant in the overall transport. The diffusive transport is determined by (1) the product ηR of the diffusion accessible porosity η and the retardation factor R, and (2) the apparent dispersion coefficient D. Initially, these parameters have been determined on clay cores taken from the underground research facility in Mol, at a depth of -223 m. In 1997, a borehole (Mol-1) was drilled at the Mol site in order to cover the complete profile of the Boom Formation and to analyze the detailed vertical distribution of the transport parameters of non-sorbed tracers as a function of depth and stratigraphy. By performing migration experiments on clay cores from Mol-1, the hydraulic conductivity in the direction perpendicular to the bedding plane, the ηR product and the apparent dispersion coefficient of iodide and tritiated water have been determined as a function of depth (Aertsens et al., 2004). This study is completed by doing the same for H14CO3-. Other cored boreholes were drilled in northeast Belgium to evaluate the spatial distribution of some of these parameters at a larger scale.