Natural tracer profiles across argillaceous formations

: Argillaceous formations generally act as aquitards because of their low hydraulic conductivities. Together with the large retention capacity of clays for cationic contaminants, argillaceous formations may be suitable as potential host rocks for the geological disposal of radioactive waste. In several countries, programmes are ongoing to characterise the detailed transport properties of such formations at depth. In this context, the interpretation of profiles of natural tracers in pore waters across the formations can give valuable information about the large-scale and long-term transport behaviour of these formations. Based on a careful evaluation of the palaeo-hydrogeological evolution at nine sites in central Europe, model scenarios are derived for initial and boundary pore-water compositions and an attempt is made to numerically reproduce the observed tracer distributions in a consistent way for all tracers and sites, using transport parameters derived from laboratory or in situ tests. In all cases, the shapes of the profiles can be explained by diffusion acting as the dominant transport process over periods of several thousands to several millions of years and at the length scales of the profiles.