Long-term (> 5 years), high resolution multi-tracer-based analysis of groundwater recharge-flow regimes in a complex volcanic aquifer

초록

Volcanic aquifers are among the most complex groundwater systems due to their highly heterogeneous flow structures and recharge dynamics. Despite previous studies on Jeju Island, South Korea, few have integrated multi-year environmental tracer data with residence time modeling to resolve temporal and spatial variability in groundwater flow regimes. This study presents the first high-resolution, multi-tracer dataset (delta O-18, delta D, CFCs, H-3) collected over two project phases (2016-2020, 2022-2023) and applies Lumped Parameter Models (LPMs) to estimate mean residence times (MRTs) and flow regime structure. Recharge elevations were delineated using isotope-altitude relationships, and an optimal threshold of 1296 m a.s.l. was identified through ROC analysis (AUC = 0.936, p < 0.05), separating wells into Group 1 (highland recharge) and Group 2 (lowland recharge). Group 1 exhibited longer MRTs (34.6 +/- 8.1 yr) and low solute concentrations, indicating regional flow from pristine high-altitude recharge zones. Group 2 showed shorter MRTs (30.1 +/- 8.5 yr) and higher nitrate and chloride, reflecting localized recharge under anthropogenic influence. LPM simulations reduced uncertainties in age estimation compared to single-tracer methods and revealed intra-well MRT variation of up to 15 years across sampling periods. Our results demonstrate that recharge elevation, land use, and flow path heterogeneity jointly control groundwater quality and vulnerability. The elevation threshold offers a physically meaningful criterion for delineating protection zones in volcanic terrains. This multi-tracer framework provides a transferable tool for sustainable groundwater management in complex volcanic aquifer systems.

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