Hydrogeochemical characteristics and associated mechanism based on groundwater dating in a karstic basin, Guizhou Province, China

Multiple tracers were used to assess the spatial distribution of hydrogeochemistry and its kinetic process in a typical karst basin in Guizhou Province, China. The groundwater apparent age in study area is from 26 to 37.5 years based on CFC-113 (trichlorotrifluoroethane, C2F3Cl3). Based on the characteristics of major ions and the SI values of minerals in groundwater, the rise in the Ca2+ and SO42- with the increasing groundwater age is mainly controlled by natural water-rock interaction; however, the Mg2+ and HCO3- of groundwater should be supplied more by contamination processes than the interaction between water and rocks. The change of major ions in the main stream compared with the groundwater was attributed to the change of the partial pressure of CO2 and varied ratio of groundwater discharge to the total stream flow, which was estimated to range from 35 to 80 %. Land use can be considered as the dominant factor that affects the temporal changes of NO3-, Cl-, Na+ and K+ in groundwater which have increased since the 1970s. From the viewpoint of residence time, the concentration of these ions in springs and groundwater will continue for decades even if the fertilizer application is under control in a reasonable level from now on.