Hydrochemical and PCO2 variations of a cave stream in a subtropical karst area, Chongqing, SW China: piston effects, dilution effects, soil CO2 and buffer effects

Abstract

The role of CO2 in karst has been of interest for decades, and emphasized by IGCP 379, International Geoscience Programme, UNESCO started in 1995. There are still open questions about the dynamics of carbon in karst systems, particularly the flux of carbon between the surface and subsurface and between different components in the karst subsurface. This research report focuses on the variations of hydrochemistry and PCO2 (partial pressures of carbon dioxide) in subtropical karst groundwater, using high-resolution auto-monitoring hydrochemical data (15-min intervals). The aim of this study was to understand how hydrochemistry and PCO2 in karst systems respond to recharge over different time scales and what the controlling factors are. An auto-monitoring hydrochemistry station was installed about 300 m upstream from the exit in the active stream channel of Xueyu Cave, a typical subtropical karst cave. Four years of high-resolution continuous pH, specific conductivity (Spc), temperature and water-level data were collected. A thermodynamic model was used to link the continuous data to monthly water quality data, allowing the calculation of CO2 partial pressures and calcite saturation (SIc) levels on a continuous basis. Seasonal, diurnal and storm-scale variations were captured in the hydrochemistry and calculated PCO2 records, indicating that the cave stream is a dynamic and variable system. Seasonal features (higher specific conductivity and lower pH in summer; lower specific conductivity and higher pH in winter) tend to covary with temperature which influences the production of CO2 in soils, thus being the driving force for the variations (the soil CO2 effect). Due to the buffer effect of a thick vadose zone and large void cave space, diurnal variations are not obvious compared with epikarst springs in SW China. Storm-scale fluctuations due to storm events occur during the summer rainy season. Piston flow effects, dilution and soil CO2 effects determine the variations in different storm events. At the beginning of the rains, the piston effect drives the variations, characterized by increase in Spc, SIc and pH in the cave stream and decrease in PCO2. With heavy rainfall, decrease in Spc shows control by the dilution effect, while decrease in SIc and pH and increase in PCO2 indicates the greater influence of soil CO2. These results imply that the soil and cave voids are important factors influencing the hydrochemical evolution of karst groundwater. Future works need to use such high-resolution technology widely for tracing the PCO2 and hydrochemical variations in different karst aquifers.