Site-specific management of nitrate contamination based on groundwater flow system characterization in two agricultural areas of Jeju volcanic island, South Korea

Understanding groundwater flow systems in relation to hydrogeological features is essential for evaluating NO₃⁻ responses in groundwater, particularly on volcanic islands vulnerable to contamination. We investigated two agricultural sites—Gosan and Pyoseon—on Jeju Island, South Korea, using hydrochemistry and environmental tracers [δ¹⁸O–δD of water, δ¹⁵N–δ¹⁸O of NO₃⁻, and chlorofluorocarbons (CFCs)] to identify nitrogen (N) sources, recharge elevations (RE), and groundwater ages. Pearson correlation analysis was also conducted to explore key hydrologic factors influencing groundwater flow systems. Despite similar land uses of agricultural lands in both study areas, the groundwater NO₃⁻ contamination differed significantly. Severe contamination was observed in Gosan (average NO₃⁻: 62.63 ± 40.29 mg/L), whereas Pyoseon experienced minimal influence from N sources (11.90 ± 9.10 mg/L). Variations in recharge rate (R) and hydraulic conductivity (K) determined water table depths leading to contrasting flow systems as topography-controlled system for Gosan (R = 544 mm/year; K = 3.69 m/day) and recharge-controlled system for Pyoseon (R = 1222 mm/year; K = 17.95 m/day). Gosan exhibited predominantly local flows with lower REs (406 ± 89 masl) and older CFC-12 ages (33 ± 5 years), resulting in higher and delayed responses of NO₃⁻ in recharge water due to facilitating inflows of fertilizer-derived N. In Pyoseon, intermediate flows with higher REs (1083 ± 90 masl) and younger CFC-12 ages (28 ± 2 years) led to limited NO₃⁻ input, as recharge primarily occurred in upland natural areas and higher R promoted dilution of N contaminants. Our findings emphasize the importance of characterizing site-specific groundwater flow systems in managing NO₃⁻ contamination under varying hydrogeologic settings.