High-frequency dual-tracer approach to identify contaminant transport pathways and quantify migration behaviors in karst underground river system

Karst aquifers heterogeneity presents significant challenges in deciphering pollutant transport mechanisms. To better understand the multi-pathway transport of pollutants and their lag effects in karst underground river systems, we propose a dual-tracer approach that integrates the dynamics of natural pollutants and artificial tracer experiments. Here, we couple the continuous input characteristics of acid mine drainage (AMD) with the pulse injection of dye tracers at the Qingxisi (QXS) underground river system. By using high-frequency hydrogeochemical data and dual-tracer quantitative analysis, this study systematically investigates the different transport behaviors of pollutants in the complex karst system. Results show that: (i) Three hydraulically distinct conduit pathways are newly revealed in the study sites, two of which are found to transport the AMD pollutants for more than 10 km. The migration velocities of the two conduits differ significantly by 3 to 6 times, resulting in a two-stage lagged response in the pollutant peaks during storm events; (ii) The transient pulses shown in the breakthrough curves (BTC) of the artificial tracers strongly contrast to the sustained characteristics of AMD pollutants. We found that the combined use of artificial tracers and AMD enables a comprehensive characterization of the transport dynamics in karst systems; (iii) A conceptual model of pollutant migration is proposed. This model captures a sequence of interconnected processes including intermittent pollutant input driven by stormflow, preferential migration through fast and slow conduits, transient storage within fractures and matrix, and subsequent secondary release. This study underscores the complementarity and applicability of adopting a high-frequency dual-tracer framework that contributes to improved understanding of the dynamic pollutant transport processes and mechanisms in karst systems.