Seasonal, diurnal and storm-scale hydrochemical variations of typical epikarst springs in subtropical karst areas of SW China: Soil CO2 and dilution effects

SummaryTwo-year continuous pH, conductivity, temperature and water stage of the two typical epikarst springs, Nongla spring and Maolan spring (about 200 km apart) in subtropical karst areas of SW China were presented. Our primary study objective was to understand how karst systems respond hydrochemically to recharge at different time scales, and what the biogeochemical processes and controlling factors in the SW China epikarst environment are. A thermodynamic model was used to link the continuous data to monthly water quality data allowing the calculation of CO2 partial pressures and calcite/dolomite saturation levels on a continuous basis. Marked seasonal, diurnal and storm-scale variations were observed for pH, conductivity, CO2 partial pressures and calcite/dolomite saturation indexes of the springs, indicating that both springs are dynamic and variable systems. However, the coefficients of variation of these hydrogeochemical features tends to be in the order of seasonal ⩾ storm-scale > diurnal. The seasonal and diurnal variations of these features (higher conductivity and lower pH in summer and at daytime; lower conductivity and higher pH in winter and at nighttime) tend to co-vary with temperature which influences the production of CO2 in soils, thus being the driving force for the variations (soil CO2 effect). The storm-scale fluctuations occur during the spring-summer rainy days due to the storm-events. Depending on the rainfall intensities, however, there are differences in magnitudes and direction of the variations of these features. At very high rainfall intensity, the dilution effect dominates the variations, characterized by the decrease in both conductivity and calcite/dolomite saturation of the springs, while soil CO2 effect determines the variations at lower rainfall intensity, characterized by increase in CO2 partial pressure and conductivity but decrease in pH and calcite/dolomite saturation. In a word, the hydrodynamic aspects together with hydrobiogeochemical characteristics need to be taken into account to correctly explain the hydrochemical variations of the epikarst springs. Results from the study demonstrate the need to redesign hydrogeochemical sampling strategies for epikarst springs in karst areas with monsoon climate like SW China (i.e., with remarkable seasonal matching fluctuations in temperature, rainfall and vegetation).