Key points:

• Porewater investigations allowed derivation of profiles of natural tracers (including δ²H, δ¹⁸O, Cl, Br, ⁴He, ³He/⁴He) across the Mesozoic sedimentary column.
• These tracer profiles provide insight into the palaeohydrogeological evolution and dominant transport processes, as well as independent evidence for the hydraulic barrier effect of the host rock and the confining units.
• Transport modelling shows that the curved segments of the profiles result from diffusion-dominated exchange between the porewater and groundwater in aquifers, which were activated in the Pleistocene (variable times depending on the aquifer).
• The highest stable isotope values of the porewater are found in the central part of the Opalinus Clay host rock in NL and are interpreted as the oldest.
• Concentrations of the dissolved noble gas isotope ⁴He are more than 4 orders of magnitude higher than for air-saturated water. More than 65% of the He produced in Opalinus Clay since deposition is still present, which demonstrates the strong barrier efficiency of the system.