Effect of different land use/land cover on karst hydrogeochemistry: A paired catchment study of Chenqi and Dengzhanhe, Puding, Guizhou, SW China

Rainfall, spring stage, water temperature, pH and conductivity in the paired karst spring catchments of Chenqi and Dengzhanhe, which shared the same climatic condition but different land use/land cover (LULC) at Puding, Guizhou Province, SW China, were monitored by two high-resolution multi-parameter auto-recordable instrument of CTDP300 during the hydrological year of September 2007-September 2008. Other monthly hydrogeochemical and carbon isotopic (δ ¹³C) variations in the paired karst catchments during the same hydrological year were also investigated. A thermodynamic model was used to link the continuous data to monthly hydrogeochemical data allowing the calculation of CO₂ partial pressure (pCO₂) and calcite saturation index (SIc) on a continuous basis. The primary study objective was to understand how the karst processes and karst hydrogeochemistry respond to different LULC, which is essential to assessing the karst-related carbon cycle. Marked seasonal and storm-scale variations were found for pH, conductivity, pCO₂, SIc and δ ¹³C of the two springs, indicating that both springs were dynamic and variable systems. However, there were differences in the magnitude and direction of the variations of these features between the two springs. The higher pCO₂ and [formula removed] concentration and lower pH, SIc and δ ¹³C in Chenqi spring than those in Dengzhanhe spring tend to be related to the difference in LULC between Chenqi and Dengzhanhe spring catchments: in the Chenqi spring catchment, there was larger soil cover and the paddy land was located in the discharge area, both of which produced and kept more CO₂ (a major driving agent for the karst processes) and lower δ ¹³C in the soil-aquifer system, while in the Dengzhanhe spring catchment area, there was larger bare carbonate rock occurrence and the paddy land was located mainly in the recharge area. Moreover, the pH increased and pCO₂ decreased generally in Chenqi spring after rainfall, possibly due to more carbonate dissolution in the larger soil cover rich in limestone fragments in the spring catchment, while the pH decreased and pCO₂ increased generally in Dengzhanhe spring after rainfall. All these differences show that soil cover and land use pattern played important roles in the karst processes. In other words, the karst hydrogeochemistry and the karst-related carbon cycle could be regulated effectively by different LULC. In addition, the higher concentrations of Ca²⁺, [formula removed], Mg²⁺ and conductivity of Dengzhanhe spring were due to the dissolution of more gypsum and dolomite minerals in the strata of Dengzhanhe spring catchment. Therefore, the karst hydrogeochemical parameters, including pH, conductivity, [formula removed], Ca²⁺, Mg²⁺, [formula removed], pCO₂, SIc, and δ ¹³CDIC, could serve as good indicators of different LULC and the other environmental changes.