Laboratory tests are performed to characterise the soaking and retention properties of a clay barrier (Fo-Ca) under controlled homogeneous conditions. A physical model that allows calculating the retention properties and kinetics observed during the test is then detailed, and lastly simulations are examined using various soaking conditions. The results show the model predictions of cumulative cation concentrations in leachates (i.e. copper retention by clay). These curves are accurately fit by the model with the experimental data for the leaching phenomena. Clay retention capacity until reaching a physical limit in the exchanges is evidenced as well, with this limit being input into the model as the clay CEC.