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Reactive transport models were developed to explore the evolution in groundwater chemistry along the ow path in three aquifers; the Triassic East Midlands aquifer (UK), the Miocene aquifer at Valréas (France) and the Cretaceous aquifer near Aveiro (Portugal). All three aquifers contain very old groundwaters and variations in water chemistry that are caused by large-scale geochemical processes taking place at the timescale of thousands of years. The most important geochemical processes are ion exchange (Valreas and Aveiro) where freshwater solutes are displacing marine ions from the sediment surface, and carbonate dissolution (East Midlands, Valréas and Aveiro). Reactive transport models, employing the modelling code PHREEQC (Parkhurst and Appelo 1999; Appelo and Postma 2005), which included these geochemical processes and one-dimensional solute transport were able to reproduce the observed patterns in water quality. These models may provide a quantitative understanding of the evolution in natural baseline properties in groundwater.