Vertical distribution of (HCO3-)-C-14 transport parameters in Boom Clay in the Mol-1 borehole (Mol, Belgium)

摘要

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 (HCO3-)-C-14) in Boom Clay and the upper lying sands of Eigenbilzen or Voort have been determined by pulse injection experiments on cores taken from different depths in the Mol-1 borehole (Belgium). The results for HTO and iodide have been reported previously and the present paper completes this study with (HCO3-)-C-14. In the range from 191 m to 281 m depth (Below Drilling Table), the Boom Clay Formation can be considered as homogeneous with respect to the hydraulic conductivity and the transport parameters of (HCO3-)-C-14. The value of the vertical hydraulic conductivity is around 2.5-3 x 10(-12) m/s. The average apparent diffusion coefficient D-app of (HCO3-)-C-14 in the direction perpendicular to the bedding plane is (6 +/- 3) x 10(-11) m(2)/s. The product eta R of the diffusion accessible porosity eta and the retardation factor R of (HCO3-)-C-14 is 0.26 +/- 0.03, lying between the value of iodide (eta R = 0.16) and of HTO (eta R = 0.37). Due to anion exclusion, iodide has a lower porosity than HTO. Because of the lower eta R value of an anion as iodide, a slight sorption of (HCO3-)-C-14 cannot be excluded (R <= 0.26/0.16 approximate to 1.6).