High-level waste can be radiotoxic for thousands of years and should be carefully handled to prevent accidents. The research for geological disposal of radioactive waste focuses on the construction and durability of a geologic disposal facility in Dutch clays or salts. One of the challenges is assessing these host rocks’ capability to retard radionuclides and prevent them from entering the biosphere. This research focuses on disposal in clay. Because of clay’s low permeability, water movements are slow, and radionuclides transport is expected to occur predominantly by diffusion. The Peize and Waalre formations, situated on the interface between brackish and salt groundwater, serve as a natural analogue for targeted deeper, poorly indurated host rocks. The known disparity in chlorine levels between the aquifers adjacent to these clays qualify these formations for NaCl-tracer research. A cutting sampling study determined the conductivity of these clays’ porewater. Combining this with a one-dimensional modelling study, the saline history of this formation has been simulated. NaCl gradients were demonstrated at different drilling locations. The most manifested gradient is attempted to fit in the one-dimensional model. The model results suggest that this gradient originates from adjacent aquifer salinity, the clays’ physical properties and the difference in hydraulic head. The observed salinity discrepancy between aquifers and ion concentration gradient in the firstWaalre Clay confirm the assumption that the member can be a natural analogue. Uncertainty on the continuity of the total system prohibits concluding that diffusion-dominated transport in Dutch poorly indurated clays can be assumed. The best fitting scenario this research found fitting the empirical Waalre clay salinity curve is combined transport by diffusion and advection. The finding of diffusion-advection transport implies that the Waalre clay shows more complexity than initially expected.