Hydrological processes in multi-layered aquifers of a karst watershed with coal mining activity: Insights from hydrochemistry and isotopes

The Laochang Karst watershed (LCKW) is located in eastern Yunnan Province, southwestern China. It is the representative karst area affected by coal-mining activities in southwestern China. Identifying hydrological processes of multi-layered aquifers in karst watersheds is challenging due to complex natural and anthropogenic processes. This study attempts to clarify the hydrological conceptual model of the LCKW using hydrochemistry and D, O, Sr, S, and C isotopes. Surface water and multi-layered groundwater have the hydrochemical types of SO 4 -Ca·Mg, HCO 3 ·SO 4 -Ca, and HCO 3 -Ca. Meteoric water and condensate were the major recharge sources. The main processes dominating hydrochemical compositions consist of sulfide oxidative dissolution, carbonate dissolution, positive cation exchange, and agricultural activities. Elevated SO 4 2− concentration in the mine water, river water and shallow coalbed water mainly originated from the oxidation of pyrite in the coal-bearing strata of the Longtan Formation. whereas the deeper layers and groundwater away from the mines were hardly contaminated by SO 4 2− due to the presence of aquiclude. HCO 3 − concentrations of surface water and multi-layered groundwater were mainly derived from carbonate dissolution and soil CO 2 , and mine water was also influenced by atmospheric CO 2 . Positive cation exchange contributed to increasing Na + concentration. Agricultural activities contributed NO 3 − , Cl − , and K + ions in aquifers, especially near large karst fallout caves. A hydrological model of multi-layered aquifers in the LCKW was built based on the above results. These findings will provide valuable guidance for understanding the hydrological processes of complex karst watersheds worldwide. Hydrological processes in the multi-aquifers of the Laochang karst watershed • Water-rock interactions dominate the hydrochemistry of the karst watershed. • Elevated SO 4 2− concentrations originate from the oxidation of pyrite. • HCO 3 − concentrations are derived from carbonate dissolution and soil CO 2 . • The karst watershed is recharged by atmosphere precipitation. • A hydrological conceptual model is built for sustainable water management.