Stable isotope signatures of precipitation and implications for groundwater recharge on Jeju volcanic island, South Korea

Jeju Island, South Korea, is a volcanic island where the population depends entirely on groundwater for freshwater supply, and the island exhibits unique hydrogeological characteristics. This study examines groundwater recharge processes by analyzing the stable isotopes of oxygen (δ¹⁸O) and hydrogen (δ²H) in monthly precipitation and groundwater. Precipitation isotopes showed clear seasonal variability, characterized by a strong summer monsoon effect. Northern-slope precipitation was slightly more depleted in ¹ ⁸O and has lower d -excess values compared to other slopes. Groundwater in this area exhibited similarly depleted isotopic signatures, suggesting that high-elevation recharge moves downgradient along preferential flow pathways toward coastal areas. Mixing analysis indicates that high-elevation summer rainfall is the dominant source of groundwater recharge. These findings significantly enhance the understanding of the linkages between precipitation patterns and groundwater recharge dynamics on the island. • Summer rainfall is the primary source of recharge. • Isotope mixing analysis indicates high-altitude summer rainfall as the main source. • Low δ¹ ⁸O in northern-slope indicates rapid flow of high-altitude recharge. • Long-term isotope monitoring is vital for sustainable water management.