Anthropogenic impacts on the hydrochemistry of karst underground river system around the China Sky Eye

The Dajing and Xiaojing karst underground river system is the longest underground river in Guizhou Province, Southwest of China，located around the Five-hundred-meter Aperture Spherical radio Telescope (FAST). The water quality of underground rivers around the FAST being affected by multiple anthropogenic activities. The underground river system is an extremely non-homogeneous and anisotropic media with complex hydrodynamic and hydrochemical responses. Multi-tools (hydrochemistry, water isotope, sulfur isotope and statistical methods) were applied to accurate identification and quantification of the proportions, spatiotemporal evolution and mechanisms of karst aquifer impacted by different anthropogenic activities. The synchronous variations of δ 18 O, δD, d-excess and δ 34 SO 4 2- in the effluent and the underground rivers suggested that the underground rivers are influenced by anthropogenic activities. The relationships of ionic ratios revealed that agricultural activities, effluent from the sewage treatment plant, and water-rock interactions control the hydro-chemical characteristics of the underground river system with a distinct spatiotemporal differentiation. The response time for the water quality of Dajing and Xiaojing underground river to anthropogenic impacts were 24 and 27 days, respectively. Spatially, the PCA-ACPS-MLR and MixSIAR indicated that more impacts of anthropogenic activities on Xiaojing underground river than Dajing underground river system. A quantification of spatiotemporal response differentiation provides new insights into the precise identification and prevention of pollution in karst underground rivers. • The synchronous variations of δ 18 O, δD, d-excess, and δ 34 SO 4 2- in the effluent and the underground river system revealed that the underground rivers are influenced by tourism activities. • The impact of human activities on the water quality of the underground rivers showed distinct spatiotemporal differentiation. • The natural contribute and anthropogenic impactswere precisely quantified in the Dajing and Xiaojing underground river system.