Combining hydrodynamics, geochemical and multiple isotopic tracers to understand the hydrogeological functioning of karst groundwater system in Jinci, northern China

Karst groundwater is important for global sustainable water supply and ecological development. However, the lack of understanding of karst systems’ hydrogeological functioning increases the challenges of karst water resources management. Here, the natural responses of the main discharge sites (such as Jinci Spring, Pingquan Spring and so on) of Jinci karst water system were controlled by continuous monitoring of the discharge rate. Water samples were collected to constrain the chemical evolution of groundwater in Jinci karst water system through geochemical and isotopic tracers. The application of trend analysis tests (Mann-Kendall) shows the Jinci karst water system is highly dependent on natural recharge and influenced by human activities. Our research first timely revealed that old karst groundwater from the stagnation areas might potentially be one of the main recharge sources of karst aquifers. The recharge proportions and chemical effects of the old karst water were evaluated. The recharge proportions of old karst water increased gradually along the main flow path from the Jinci Spring to the Dongyu artesian well, where the highest estimated mixing ratio was up to 24.8 %. It was observed that the mixing recharge of the old karst water causes a significant elevation of total dissolved solids (TDS), sulfate (SO₄^2-), and sulfur isotope (δ³⁴S_-SO4) values in karst water from the southern area of Jinci Spring, which may threaten the quality and safety of drinking water supply. Our research provides a new view on groundwater circulation and poses new challenges for groundwater management in the karst areas.