Abstract: Flushing of marine sediments with fresh water will induce a chromatographic pattern of the cations which are displaced from the exchange complex. Upstream from the salt/fresh water boundary, a sequence of Na+, K+, Mg2+ and lastly Ca2+ dominated water is expected (Ca2+ is the displacing cation; HCO3− is the major anion). This sequence is present in the Aquia aquifer. Field data from this aquifer have been modeled with a 1D geochemical transport model to satisfactory agreement. The upper 30 miles of the aquifer must have been flushed about 8 times under the model conditions, which means that the now observed chromatographic pattern can have been established in 100 ka.