[Distributions and Driving Factors of Hydrochemical Compositions in Different Types of Karst Groundwater in Qingjiang River Basin].

Abstract

The karst groundwater resource is the key water supply for native residents and industrial and agricultural production in karst regions of southwest China and is even the only water supply for rural regions. Understanding the formation mechanism of hydrochemical compositions is practically imperative to the sustainable development and utilization of karst groundwater resources. The hydrochemical ions of 34 karst spring samples and 12 underground river water samples from the Qingjiang River Basin were measured and analyzed in this study. On the basis of investigating the distribution characteristics of hydrochemical components, methods such as hierarchical clustering analysis, Gibbs model, and relationships between hydrochemical components were used to comprehensively reveal the chemical genesis mechanism of karst groundwater from the perspectives of water-rock interaction and anthropogenic activities, especially in the disturbance mechanism of acidic pollutants on the natural dissolution of carbonate rocks. The results showed that the karst groundwater in the Qingjiang River Basin was weakly alkaline （7.12 ≤pH ≤ 8.69）, and the overall water quality was good. Only the maximum concentration of NO₃^- in the karst spring （80.2 mg·L^-1） exceeded the maximum acceptable limit for drinking purposes （10 mg·L^-1 as N）. The Ca²⁺ and HCO₃^- were identified as the dominant ions, associated with the main water phrase of HCO₃-Ca·Mg in approximately 97.1% of karst spring samples and all underground river water samples. The dominant ions were derived from the strong dissolution of calcite, whereas Mg²⁺、SO₄^2-, and F^- were controlled by the weak dissolution of dolomite, gypsum, and fluorite, respectively. Karst groundwater NO₃^- was derived from the agricultural fertilizers and rainwater input, whereas the H₂SO₄ acid rain input was identified as another important source of karst groundwater SO₄^2-. In addition, the carbonate rocks in the study area were mainly naturally dissolved； however, the input of exogenous acid by humans has changed the original karst hydrogeochemical evolution process and further decreased the carbonate sink flux of karst aquifers, accelerated the decrease rate of karst groundwater table, and increased the pollution risks of karst groundwater.