Stable isotope and aquatic geochemistry of a typical subtropical karst subterranean stream in southwest China

A hydrogeochemical and isotopic study was conducted on the subterranean karst stream, namely the Guancun subterranean stream (GSS). The hydrogeochemical processes of the GSS were controlled through calcite dissolution and precipitation and were driven by the concentration of CO2, which controlled changes in the pH and of PCO2 in the water. The delta O-18 and delta D values of the GSS were within the global meteoric water line and the local meteoric water line, thereby indicating that the water of the GSS comes from precipitation. Certain abnormal delta O-18 and delta D values suggest the effect of evaporation on the GSS given its use in a particular irrigation system, wherein the GSS in transformed into a surface stream and flows for a relatively long time on the surface during the wet season. The delta C-13(DIC) values of the GSS range from -13.5 to -11.3aEuro degrees in the dry season and from -13.9 to -9.5aEuro degrees in the wet season, thereby indicating that the GSS belongs to a semi-open system. The delta C-13(DIC) values in the GSS were formed by the delta C-13(DIC) values of the soil CO2 and carbonate dissolution at different proportions. According to the simplified mass balance formula, the contributions of carbonate dissolution to the dissolved inorganic carbon (DIC) of the GSS were calculated to be 50.2-58.3% and 48.7-64.7% in the dry and wet seasons, respectively, thereby indicating a less than 50% carbonate dissolution contribution during the formation of DIC in karst groundwater. Moreover, sulfuric acid and nitric acid were observed to participate in karst processes.