Calcium and Multi‐Isotope Constraints on Cation Exchange in Groundwater

Abstract Cation exchange is one of the most important processes in water‐rock interaction in the presence of clay minerals, with a common Ca‐Mg‐Na exchange system. However, systematic verification of Ca isotopes for tracing cation exchange in natural environments is lacking due to overlapping factors, such as dissolution‐precipitation of Ca‐minerals. In this study, Ca isotope fractionation factors following adsorption were measured in the laboratory and then used in a field study (a loess tableland) to trace Ca‐related cation exchange. The results showed that light Ca isotopes are preferentially adsorbed onto clay minerals, and montmorillonite exhibits the greatest fractionation extent, followed by illite and kaolinite. There is a general trend that the lower the initial Ca concentration, the smaller the particle size, and the stronger the fractionation of Ca isotopes. Results from multiple tracers ( δ 44/40 Ca, 87 Sr/ 86 Sr, δ 13 C, δ 15 N, 3 H, 14 C, and water chemistry) in the loess tableland have shown that groundwater Ca and Mg were derived mainly from the dissolution of continental carbonates in loess, and that Na was derived from cation exchange beyond atmospheric input. The decrease in Ca and Mg concentration and increase in Na concentration were mainly due to cation exchange, leading to an increase in δ 44/40 Ca. The changes in Ca concentration and δ 44/40 Ca conformed to the Rayleigh model. This study systematically validates Ca isotopes as a robust tracer of cation exchange in natural systems, bridging laboratory‐measured fractionation factors with field observations to accurately decipher the evolution of water chemistry.