Recent advances in the understanding of the Boom Clay porewater chemistry

The investigation of the Boom Clay porewater geochemistry over the past 40 years led to improved sampling methodology, systematic monitoring and overall better understanding of the underlying processes shaping the porewater composition of the Boom Clay. This research made it possible to compare and highlight the pros and cons of variable porewater sampling techniques. The experience showed that in situ sampling outperforms the other sampling techniques and yields porewaters least prone to experimental artifacts. The implementation of operational procedures for porewater sampling, systematic monitoring of the porewater composition including microbial analyses and data management system resulted in a larger and more reliable dataset. This dataset served as a base to derive reference porewater composition and develop the geochemical model. The porewater and gas sampling under in situ conditions made it possible not only to define porewater composition representative of natural conditions, but also characterise porewater chemistries affected by repository-induced disturbances, e.g. oxidation and temperature increase. The latter disturbance will be extensively examined as part of the PRACLAY experiment.