Quick clay may be stabilised by increased magnesium concentrations in the pore water. Weathering has so far been cited as the source of magnesium, but is an unlikely process at 15 m depth in clays, were it was found at a test site in the south of Sweden. This study investigates the explanatory power of ion exchange as the source of magnesium. A rough PHREEQC model incorporating ion exchange, but not weathering, is used to model geochemical and transport processes since the last ice age for a borehole in the Göta River valley. Rain water, or rain water with added calcium or magnesium carbonate, is used as infiltrating solution. Calcium or magnesium could come from weathering in the dry crust. Advection, diffusion and ion exchange are sufficient to approximate pore water concentrations, if the infiltrating water contains calcium. Weathering at depths below 5 m is not included in the model, but observations of magnesium are reproduced nevertheless. The magnesium comes from the sea water at the end of the last ice age and has been stored on the ion exchange sites in the clay. The magnesium is displaced into the pore water when calcium enters the soil, is transported downwards, and partly reenters the exchange sites. The rough model supports the importance of ion exchange for the restabilisation of quick clay by natural magnesium at this site.