Vertical distribution of H14CO_3 transport parameters in Boom Clay in the Mol-1 borehole (Mol, Belgium)

An important question for the performance assessment of a potential geological waste disposal site for radioactive waste in Boom Clay is: can the Boom Clay be considered as a homogeneous clay layer from the point of view of its migration properties? Therefore, the hydraulic conductivity and the transport parameters of tritiated water (HTO) and two critical radionuclides (129I- and H14CO3-) in Boom Clay and the upper lying sands of Eigenbilzen or Voort have been determined on cores taken from different depths in the Mol-1 borehole. In the range from 191 m to 281 m depth, the Boom Clay Formation can be considered as homogeneous with respect to the hydraulic conductivity and the transport parameters of H14CO3-. The value of the vertical hydraulic conductivity is around 2.5 - 3 10-12 m/s. The average apparent diffusion coefficient Dapp of H14CO3- in the direction perpendicular to the bedding plane is (6  3) 10-11 m2/s. The product R of the diffusion accessible porosity  and the retardation factor R of H14CO3- is 0.26  0.03, lying between the value of iodide (R = 0.16) and of HTO (R = 0.37). Due to anion exclusion, iodide has a lower porosity than HTO.