Vertical distribution of HTO and 125I- transport parameters in Boom Clay in the Essen-1 borehole (Belgium)

In Belgium, Boom Clay is studied as reference host formation for nuclear waste disposal. For a safety analysis, it is important to know the transport rates of radionuclides in Boom Clay. Therefore one needs to know the value of (a) the product of the diffusion accessbile porosity and the retardation factor, and (b) the apparent dispersion coefficient, as well as the hydraulic conductivity K. Initially, the transport parameters have been determined on clay cores taken from the HADES Underground Research Facility in Mol. In 1997, the Mol-1 borehole was drilled to measure the distribution of the transport parameters as a function of depth and stratigraphy. To evaluate the transferability of these results over the whole Campine Basin, boreholes have been drilled at several locations and cores have been sampled over the entire thickness of the Boom Clay formation. The present study assesses the vertical variability of the transport parameters for the Essen-1 borehole. The transport parameter values at Essen-1 are compared with the values at other boreholes to get insight in the overall lateral variability. Special care is taken on the influence of the pore water.