Hydrochemical characteristics, evolution, and controlling factors of a karstic river with reservoirs: Insights from spatial-temporal analysis

Understanding water chemistry in karst regions is crucial for improving global water resource management and deepening our knowledge of the biogeochemical cycles shaping these sensitive environments. Despite advancements in karst hydrology, significant gaps remain in long-term trends, underlying processes, and quantitative effects of environmental changes. This is especially true in areas like the Wujiang River (WJ) in China, where human activities such as reservoir construction and land use/cover changes have accelerated hydrochemical changes. We combined recent and historical monitoring data to provide a detailed analysis of the spatial and temporal characteristics, evolution, and controlling factors of major ions in WJ. These findings are important for local water management and contribute to global efforts to manage similar karst systems facing human-induced pressures. Our research shows clear seasonal differences in solute concentrations, with higher levels during the dry season. WJ's water is rich in calcium, with Ca-HCO₃ ion pairs being the most common. Reservoir monitoring stations show much higher levels of NO₃⁻ and SO₄²⁻ compared to river-type stations, likely due to longer hydraulic retention time and increased acid deposition. The study confirms the significant role of pH and water temperature in rock weathering processes. Land use/cover changes were identified as the primary drivers of solute variations (46.37 %), followed by lithology (13.92 %) and temperature (8.35 %). Over the past two decades, intense carbonate weathering has been observed, especially during wet seasons. Among karstic provinces, Guizhou Province stands out with the highest ion concentrations, indicative of its extensive karst coverage and heightened weathering processes.