Identification of hydrologic response and contaminant transport processes in karst underground river systems using acid mine drainage and artificial dye tracer

Due to the high heterogeneity of karst aquifers, understanding the transport of contaminants within karst underground river systems remains challenging. Moreover, there is limited knowledge regarding the risks posed by pollutants in karst aquifers and their attenuation potential. To characterize the differential transport and release of various contaminants through different recharge pathways in a karst underground river system. This study integrates intermittent inputs of acid mine drainage (AMD) and conservative dye tracers. High-frequency monitoring of discharge and water quality, along with breakthrough curve (BTC) analysis of the tracers, was conducted to perform both qualitative and quantitative assessments. The research was carried out in a typical karst underground river system (Qingxisi) located in western Hubei, China. The results revealed three distinct, non-intersecting karst conduits that converged at the underground river outlet. Among them, Conduit 1 was unaffected by AMD pollution, while Conduits 2 and 3 exhibited contaminant transport distances exceeding 10 km. Notably, the arrival of AMD pollutants was significantly delayed compared to the arrival of Conduit  1, with Conduit 2 demonstrating a faster transport velocity than Conduit 3. The pollution pattern in the underground river system suggests intermittent leakage of AMD pollutants, leading to periodic water quality responses upon contaminant release. In contrast to the rapid attenuation of the conservative dye tracers, AMD pollutants exhibited slower and more persistent attenuation processes. The rates and extents of contaminant attenuation varied among the conduits, depending on the degree of conduit development. Sulfate (SO₄²⁻), a characteristic pollutant of AMD, showed the fastest attenuation rate, while several heavy metal elements displayed negative attenuation rates, indicating secondary pollution during transport , potentially related to adsorption-desorption processes with sediments. The storage and release of contaminants, driven by hydraulic gradient changes between karst conduits and fracture media, were found to delay the natural attenuation of pollutants, suggesting potential limitations in the long-term natural attenuation capacity of the underground river system. This study enhances the understanding of contamination processes and the mechanisms of water quality change and natural attenuation in vulnerable karst groundwater systems, contributing to the management and prevention of groundwater pollution in karst environments.