Hydrochemical evolution of karst groundwater under the mining influence in Beiya Mine, Northwest Yunnan Province

The Beiya gold mine in Yunnan Province, which is a karst water-filled mine, is a typical skarn-porphyry type deposits in China. Its groundwater has been unbalanced for a long time caused by mining activities, which has affected or damaged the aquifer to varying degrees, resulting in constant change of groundwater dynamic field and chemical field, and forming a typical human activities influenced groundwater system. In order to reveal the impact of mining activities on chemical evolution of groundwater and the water sources change of mine pit filling, this paper analyze the water chemical evolution under long-term mining, and the implication of conventional components of the karst groundwater chemistry in Beiya mining area to the water filling sources change in mine pit, by Piper three-line graph and ion combination ratio methods and according to water chemical data of different mining stages. Research shows that: (1) Influenced by mining activities, the overall concentrations of TDS, Na⁺, ${\rm{SO}}_4^{2-}$, and ${\rm{NO}}_3^{-}$ increased, and the concentrations of Na⁺ and ${\rm{SO}}_4^{2-}$ increased significantly in deep mining. The water chemical type evolves from calcium-carbonate water to calcium sulfate and sodium sulfate water, while the karst spring water chemistry is less affected by mining activities. (2) Obvious mixing occurred in mine pit water. It was initially determined that the shallow groundwater was HCO₃-Ca·Mg water or HCO₃-Mg·Ca water, while the groundwater in the structural zone and deep layers was SO₄-Na·Ca water or HCO₃-Na water. The karst water in the contact zone between limestone and porphyritic rock belongs to HCO₃·SO₄-Na and HCO₃-Na water. The water gushing point at 1,614m on the southwest side and 1,564m on the southeast side of the mining pit have similar hydrochemical types, belonging to HCO₃-Ca·Mg water or HCO₃-Mg·Ca water, which have a close hydraulic connection, and from the same water source. (3) The karst water chemistry in the study area is mainly controlled by carbonate water-rock reaction. Ca²⁺, Mg²⁺ and ${\rm{HCO}}_3^{-}$ in groundwater and surface water are mainly from the dissolution of carbonates. Na⁺ in the shallow part come from weathering and dissolution of porphyry, while in the deep part mainly come from dissolution of sodium-containing feldspar sandstone and underground low-temperature hot water. ${\rm{SO}}_4^{2-}$ is mainly affected by mining activities and originates from the oxidation of metal sulfides. ${\rm{NO}}_3^{-}$ in the shallow quaternary pore water is mainly affected by agricultural production and mining blasting operations. (4) The karst water chemistry is controlled by mining activities, mixing and water-rock interaction. Mining activities are the main influencing factor to the water chemical changes in the karst water system. Accompanied by gradual mining activities, the sealing degree of the aquifer is damaged, and the alternating rate of groundwater increases, which will affect the occurrence degree of water-rock interaction. It is suggested that optimized mining plans is necessary to enhance the comprehensive utilization rate of water resources, and establish and improve the monitoring system for surface water and groundwater to protect the water environment in karst areas.