The Boom Clay geochemistry: natural evidence

In Belgium, the Boom Clay is studied as the reference formation for geological disposal of high-level radioactive waste and spent fuel. As the Boom Clay is considered as the main barrier for radionuclide migration/retention, a thorough characterisation of the clay and its pore water is necessary. This enables to better understand the long-term geological processes and the distribution of the trace elements and radionuclides. From a mineralogical/geochemical point of view, the Boom Clay is considered as a rather homogeneous sediment, vertically as well as laterally. It is composed of detrital minerals, organic matter and fossils. Small variations in mineralogical/geochemical composition are related to granulometrical variations. The radiochemical study indicates that the Boom Clay is in a state of secular radioactive equilibrium, meaning that the Boom Clay has not been disturbed for a very long time. Pore water sampling is done in situ from various piezometers, or by the squeezing or leaching. These pore water sampling techniques are compared and evaluated. Boom Clay pore water is a NaHCO3 solution of 15 mM, containing 115mg/l of dissolved natural organic carbon. Some slight variations in pore water composition have been observed and can be explained by principles of chemical equilibrium and cation exchange.