Differentiation of groundwater flow patterns in multiple karst systems in a covered karst basin in China through hydrochemistry, stable isotopes, and 222Rn

Groundwater in covered karst basins is abundant, but the thick soil cover and the lack of distinct surface karst features complicate the understanding of subsurface flow. This study integrates physicochemical parameters, stable hydrogen and oxygen isotopes, and the radon isotope (²²²Rn) to characterize groundwater flow within local, intermediate, and regional karst water systems in a covered basin in South China. The aims are to distinguish differences among these systems, examine radon concentration variations at their outlets, and assess the potential of ²²²Rn in identifying groundwater flow characteristics. Groundwater in the regional karst system is characterized by the most extensive water-rock interaction, extended flow paths and residence time, minimal anthropogenic influence, and slow, fracture-dominated flow in the final stages of flow paths. In contrast, local and intermediate karst systems are more responsive to external changes, with rapid conduit flow primarily occurring in their terminal sections. Owing to the short residence time and flow paths, radon is rapidly transported to the discharge points, resulting in higher concentrations in groundwater from local karst systems compared to intermediate and regional systems, which have longer residence time. Similar to hydrochemical and isotopic trends, radon concentrations in local and intermediate systems fluctuate more than in regional systems, suggesting rapid flow in the terminal sections of both. Radon activity can qualitatively identify flow characteristics in karst aquifers. The integration of hydrogeochemical, stable isotopes, and ²²²Rn data provides a comprehensive understanding of flow patterns linked to aquifer structures and insights into radon behavior in karst water systems.