Investigating the structure of a multiple karst aquifer system and its hydrological process response using high-resolution multi-tracer data

Understanding the hydrogeological structure and hydrogeochemical processes of karst aquifers is critical for groundwater management. This study innovatively integrates drilling, tracer tests, high-resolution monitoring, and multi-isotope techniques (H-3, C-14, delta D, delta O-18) to systematically characterize the spatial heterogeneity of a multiple karst aquifer system. Hydrochemical data was analyzed with the electrical conductivity decomposition method. Multivariate statistical analysis and the ions ratio are used to identify the sources and components of the spring water during storm events. The results indicate that: (1)The upper and lower aquifer systems are composed of limestone and dolomite, respectively, while the intermediate aquifer system is constituted by both limestone and dolomite. The upper and intermediate aquifer systems exhibit conduit flow regimes, whereas the lower aquifer is characterized by fracture flow. The lower aquifers (similar to 13,862-year residence time) show unique SO42-, Sr2+, F- enrichment and lighter delta D/delta O-18 signatures, reflecting prolonged water-rock interactions. (2) Comparative analyses of ion ratios during storm events in the BLD spring, demonstrate that both limestone and dolomite significantly influence the spring hydrochemistry. The hydrochemistry changes in the spring outlet are caused by dilution and mixing of recharge from limestone and dolomite aquifers, triggered by rainfall. During rainstorm events, the dolomite aquifer is the primary source of recharge. (3) The study establishes a "structure-process-evolution" conceptual model, demonstrating how multi-tracer synergy resolves hydrological complexity in heterogeneous karst systems. Our study highlights the importance of collecting and interpreting high-resolution multi-tracer datasets to unravel the dynamics of hydrological processes in complex multiple karst aquifer systems.