钾稳定同位素在水文地球化学领域的研究进展与展望

The progress and prospects of potassium stable isotopes in hydrogeochemistry

地表硅酸盐岩矿物风化通常是水体中钙、镁、钠、钾等元素的重要来源,然而相比于水体中的钙、镁和钠,目前对钾的水文地球化学行为的认识仍十分有限。表生地球化学领域最新研究证明风化、吸附等多种水岩反应伴随着较大的钾同位素分馏,表明钾同位素技术可以用于示踪地下水中钾的来源及迁移转化。文章通过系统总结上地壳、水圈和其他地表储库(植物、肥料)的钾同位素组成,发现水圈普遍比大陆上地壳富集41K,为识别地下水的钾来源提供了基础;通过总结钾同位素在常见的水岩作用过程(硅酸盐岩矿物溶解、次生黏土形成、吸附作用、离子交换反应)中的分馏行为,发现硅酸盐岩矿物溶解分馏有限,次生黏土矿物形成引起水体富集41K,表面吸附和离子交换使水体富集39K,不同水岩反应中K同位素行为差异为示踪地下水中钾的迁移转化过程提供了基础;列举了应用钾同位素示踪硅酸盐岩风化和水体污染的最新研究成果。由于钾同位素是硅酸盐岩风化的良好示踪剂,可以利用钾同位素揭示CO_(2)较充足含水层中钾元素释放及迁移转化机理;由于表面吸附和离子交换控制的钾同位素分馏方向与风化控制的钾同位素分馏方向不同,可以利用钾同位素识别出地下水循环过程中多种水岩反应对钾迁移转化的共同控制。在此基础上,对钾同位素在水文地球化学领域的应用进行了展望:(1)开展研究区多端元控制下地下水钾来源贡献的研究;(2)开展地下水漫长循环过程中钾迁移转化的定量研究;(3)联合使用多种同位素示踪碳循环相关的过程。

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.