Effect of ground water high in Mg on cation exchange of Na-illite in the presence of CaCO3

Ground waters with electrical conductivity of 4.9–7.4 mmhos/cm, a pH of 7.7–8.8 and a Mg/Ca ratio of 0.8–41.5 were equilibrated with a Na-illite in the presence and absence of CaCO₃. In the presence of CaCO₃, exchangeable Ca, exchangeable $\text{Ca+Mg}$ and the exchangeable Ca/Mg of the equilibrated clay increased but exchangeable Mg and Na decreased. The pH values of the ground waters were positively correlated with (1) exchangeable Ca, (2) exchangeable $\text{Ca+Mg}$, and (3) exchangeable Ca/Mg ratio, both in the presence and absence of CaCO₃.