<p>Numerous databases and models have been developed to predict the behaviour of radio-contaminants (such as ⁹⁰Sr and ¹³⁷Cs) in the environment. Carbonate minerals found in surface environments, such as soils, sediments, and aquifers, significantly contribute to the chemical composition of pore water. The objective of this study is to examine the potential influence of calcium (in solution) and carbonate (in solution or crystallized form) on the retention of Sr and Cs on a multi-component alluvium. The isotherms acquired on the carbonate-free clay fraction (less than 2 µm) were used to validate the ion-exchange process involved in the retention of Sr and Cs on clay minerals. The concentration sorption isotherms of Sr and Cs at two Ca concentrations in solution clearly show that there is a significant reduction of Cs and Sr retention as Ca increases in the background solution. Both aforementioned conditions applied to the carbonated clay fraction indicate that the crystallized carbonates do not limit accessibility to the solid sorption site for Sr and Cs. However, at the highest Sr and Ca concentrations, the Sr retention increase suggests the formation of SrHCO₃⁺ and precipitation of strontianite (SrCO₃).</p>