The progress and prospects of potassium stable isotopes in hydrogeochemistry

Chemical weathering of silicate minerals is an important source for Ca, Mg, Na and K. However, in comparison with other major elements (e.g., Ca, Mg and Na) in waters, how K behaves during water-rock interaction remains poorly understood. Recent studies have shown that large K isotopic fractionation could occur during various processes of low-temperature water-rock interaction such as chemical weathering and adsorption, making K isotopes gradually become a powerful tracer for the sources, migration and transformation of K cycling in the subsurface. This overview summarizes K isotopic compositions of major reservoirs at the Earth’s surface, including upper continental crust, hydrosphere and other reservoirs (plants and fertilizers). We conclude that ⁴¹K is enriched in hydrosphere than upper continental crust, providing an opportunity to identify the K source in groundwater.The magnitudes and mechanisms of K isotope fractionation during common water-rock interaction processes are also summarized (i.e., silicate dissolution, secondary mineral formation, adsorption, cation exchange), demonstrating that limited K isotope fractionation occurs during silicate dissolution, while clay formation results in enrichment of ⁴¹K in waters and adsorption and cation exchange leads to depletion of ⁴¹K occurring in waters. These different behaviors of K during these water-rock interactions provide an opportunity for tracing the migration and transformation process of K in groundwater. This paper presents the latest research that applied K isotopes to trace silicate weathering and water pollution. Since K isotopes are an excellent tracer for silicate weathering, they can be used to reveal the sources, migration and transformation of K cycling in aquifers with abundant CO₂. Additionally, the distinguishable behavior of K isotopes during chemical weathering, clay adsorption and cation exchange can be used to identify various water-rock interactions. Future K isotopic studies in the field of hydrogeochemistry should focus on: (1) constraining the contribution of multi-endmember control on sources of potassium in groundwater; (2) quantifying K behavior during long-term groundwater circulation; and (3) using multiple isotopes to trace carbon cycle-related processes.