Sources, Spatial Distribution, and Seasonal Variation of Major Ions in the Caohai Wetland Catchment, Southwest China

The Caohai Wetland, Guizhou Province, China, is a nationally important nature reserve. In this study, we examined the major ion composition of the inflows to, and the water in, the Caohai Wetland. The main sources of major ions in the wetland water were the groundwater and river water inflows, the chemical compositions of which were controlled by the local geology, aquifer water–rock interactions, and human activities. The inflowing waters were the Ca–HCO3 type. The wetland water was classified as the Mg-HCO3, Mg-SO4, Ca-HCO3, and Ca-SO4 types during the high-flow season, and as the Ca-HCO3 type in the low-flow season. The physical and chemical properties of the wetland water varied widely from west to east. Concentrations of K+, Ca+, Mg2+, Cl−, and HCO3− in the wetland water were higher in the low-flow season than in the high-flow season; K+, Na+, Cl−, and Mg2+ concentrations were higher in the wetland than in the inflowing waters in both seasons, and HCO3− concentrations were lower in the wetland than in the inflowing waters. The chemical composition of the wetland water was mainly controlled by biogeochemical processes and evaporation in the high-flow season, and by evaporation in the low-flow season.