Hydraulic conductivity variability of the Boom Clay in north-east Belgium based on four core drilled boreholes

The hydraulic conductivity of the Boom Clay is investigated since many years in Belgium in the framework of the site characterisation programme supporting the performance assessment of potential geological disposal of high-level radioactive waste in this formation. In this paper, the variability of the hydraulic conductivity is assessed in a large part (1100 km2) of the deposition area of the Boom Clay. It is based on numerous vertical and horizontal hydraulic conductivity measurements (Kv and Kh) performed with permeameter-type experiments on clay cores from four distant boreholes, i.e. Doel-2b, Zoersel, Mol-1 and Weelde-1. A thorough analysis of the results of the grain-size measurements is performed and its relationship with the hydraulic conductivities is examined through multiple regressions. This approach allows deriving site-specific empirical relationships to predict vertical hydraulic conductivities from the grain-sizes used as soft data. This increases our confidence in the global value of the vertical hydraulic conductivity at the scale of the formation and enhances the overall assessment of the variability of this parameter at the regional scale. Further efforts should be done to evaluate the variability of the hydraulic conductivity with relationships that are suitable for the whole depositional area. This would make the method more attractive for the overall assessment of a potential host-formation for geological disposal.