Hydrological and hydrochemical regime of a typical subterraneous river in a deep canyon karst area: A case study in the Santang underground river, Guizhou

The Pingzhai Reservoir, a water source project of the Central Guizhou Province hydro-junction project, is located in the deep canyon karst area of the Sancha river in the Guizhou Plateau. It is an important water conservancy infrastructure to ensure the safety of water resources in central Guizhou. In this paper, the hydrological and hydrochemical automatic monitoring technology is used to study the hydrological and hydrochemical regime of the Santang underground river system, an important tributary in the interval basin of the Pingzhai Reservoir, which lacks the early basic research of hydrogeology. On this basis, the karst development and conduit model of the deep canyon karst subterraneous river system are preliminarily discussed. The results show that the hydrochemical regimes are mainly controlled by the cover CO₂ effect, effective rainfall dilution effect and opening effect of runoff-discharge channels. In different time scales and atmospheric precipitation conditions, the variation characteristics of hydrochemical regime are different, and the corresponding dominant effect is also different. The annual variation of water temperature is high in summer and low in winter, and the daily variation is high in day and low at night. The annual variations of electrical conductivity and partial pressure of CO₂ of the groundwater is high in normal-water level period and low in high-water level period because of the combined effects of land cover CO₂ and rainfall dilution. On the monthly scale, the cover CO₂ effect can be observed during the initial stage of rainfall, but after rainfall the dilution effect control the variation of electrical conductivity and Pco₂. On the daily scale in the dry season, the daily hydrochemical regime controlled by the opening effect of the runoff-discharge channels can be observed. The identified opening effect of the runoff-discharge channels can provide a hydrochemical basis for judging the pressure state of karst conduit in the modeling study of the karst water system. This study provides a hydrogeological basis for the joint scheduling of surface and underground reservoirs and the reasonable utilization of water resources in this area in the future.