Hydrological Controls on Drip Water δ18O Heterogeneity: A 4-Year Monitoring Study in Remi Cave, Southern China

Understanding how stable water isotopes are modified during percolation through the soil-epikarst-bedrock continuum is a fundamental challenge in karst hydrology, particularly in subtropical monsoon regions with heterogeneous infiltration dynamics. This study presents a four-year (December 2020–December 2024) isotopic monitoring dataset from Remi Cave (southern China), including three precipitation sites (P1–P3), two soil water depths (50 and 95 cm), six drip sites (D1–D6), and two surface karst springs (R1 and R2). Our results show that: (1) Soil water, drip water, and spring water all plot close to the Local Meteoric Water Line but with lower slopes and intercepts, indicating evaporative fractionation during infiltration, with shallower soil water (50 cm) exhibiting stronger evaporation than deeper soil water (95 cm). (2) Considerable spatial heterogeneity exists among drip sites: D1, D2 and D5 preserve seasonal precipitation signals with the mean residence time (MRT) of approximately 1 year (~345–366 days), modulated by cave ventilation and drip-rate variability. D3 and D4 show the most depleted δ¹⁸O values (−7.8‰ and −7.6‰, respectively) and negligible seasonal amplitudes, indicating that they are recharged primarily by wet-season rainfall. D6 exhibits an MRT of ~73 days and shares similar isotopic characteristics with the two karst springs (MRT ~60–76 days), demonstrating rapid hydraulic connectivity through preferential fracture flow. (3) The marked offsets in δ¹⁸O among coeval drip sites within the same cave reveal that hydrological pathways, rather than climatic inputs alone, determine the isotopic composition of cave drip waters. These findings provide a quantitative framework for assessing water transit and recharge in subtropical karst. They also imply that site-specific hydrology must be characterized before using speleothem δ¹⁸O for paleoclimate, and that ‘non-replicated’ records may encode past recharge dynamics.