Quantifying the transferability of hydraulic parameters using geostatistics: The Boom Clay Case

The Boom Clay is currently investigated as potential host formation for the deep disposal of high-level and/or long-lived radioactive waste in Belgium. As such, the formation is expected to play a role of natural barrier, to slow the migration of radionuclides towards the biosphere for a sufficiently long time when the man-made barriers are no longer effective. In this context, the Boom Clay aquitard requires to be precisely characterized in terms of hydrogeological parameters, to confirm its role of geological barrier between its surrounding aquifers. Therefore, hydraulic conductivity and migration parameters have been intensively measured over the years in a few boreholes in Belgium; the latter are mainly located in the Mol-Dessel area, assuming historically a good lateral continuity of the geology. Combining these measurements with more densely acquired geophysical information allows quantifying their spatial variability and consolidating the continuity assumption. From a methodological point of view, the modeling of hydrogeological parameters requires to solve several issues. First, it is required to find a consistent geo-reference coordinate system allowing to laterally correlate thin observations derived from boreholes separated by several tens of kilometers. Then, in order to provide a reliable 3D model, it is compulsory to integrate the correlation between the scarcely sampled target parameters (core measurements) and numerous geophysical logs (gamma ray, resistivity). Geostatistics provides a suitable framework to analyze and solve these issues. Finally, a 3D model of the target parameters is proposed, together with an uncertainty envelope. This uncertainty quantification is of significant added value to assess the efficiency of the geological barrier. Besides the actual modeling of target parameters, the paper also presents sampling recommendations for forthcoming boreholes.