Dual-tracer analysis of stable isotopes and thermal signals to quantify groundwater residence times in karst rhythmic spring systems

As a hydrogeological manifestation of siphon conduit-cavity coupling mechanisms in karst systems, the regulatory capacity of rhythmic spring structures on groundwater storage dynamics and their heterogeneous controls on residence time distributions remain insufficiently quantified. Focusing on the Chaoshuidong (CSD) karst rhythmic spring system in southern China, this study employs a dual-tracer framework integrating stable hydrogen–oxygen isotopes and thermal signatures with sinusoidal transfer functions and linear reservoir modeling. Hydrological time-series data encompassing precipitation, surface water, and groundwater phases were systematically collected to compute mean residence times (MRT) across distinct flow regimes, thereby elucidating structural controls imposed by siphon-conduit networks on aquifer response characteristics. Key findings demonstrate: (1) Methodological coherence in MRT estimates derived from three independent approaches, revealing shallow circulation pathways (29.5–60 d) deep circulation reservoirs with MRT greater than 198–213 days; (2) The observed approximately 40 day phase lag between thermal tracer peaks and stable isotope signatures demonstrates differential advective-conductive heat transfer mechanisms along hierarchically structured groundwater flow paths; (3) Through analysis of the peak temperatures during intermittent discharges at the CSD spring and the corresponding groundwater temperatures upstream of the flow path, the peak discharge during the siphon conduit-cavity controlled intermittent outflow was estimated to consist of approximately 10 % base flow and 90 % deep-circulating groundwater. Furthermore, the water residence time within the siphon conduit-cavity structure was determined to be approximately 279 days. This observation indicates that siphon cavities function as temporary storage and mixing reservoirs for groundwater, thereby prolonging flow duration. Furthermore, this finding underscores the efficacy of temperature tracing in quantifying the mixing ratios of diverse groundwater sources. This study contributes to an enhanced understanding of water cycling within complex karst basins and extends the application of environmental isotopes to groundwater research.