Nitrate sources, timing, and pathways of a permeable volcanic aquifer system with mixed land use in Jeju Island, South Korea

The occurrence of various N-related human activities increases the difficulty in distinguishing the major sources of NO₃^- contamination in groundwater, especially in areas with mixed land uses. In addition, the estimation of the timing and pathways of NO₃^- is necessary to better understand the processes of NO₃^- contamination in the subsurface aquifer system. This study applied environmental tracers, such as stable isotopes and age tracers (δ¹⁵N and δ¹⁸O of NO₃^-, δ¹¹B, chlorofluorocarbons, and ³H), to elucidate the sources, timing, and pathways of NO₃^- contamination in the groundwaters of the Hanrim area, which has suffered from illegal disposal of livestock wastes since the 1980s, and also characterizes them based on mixed N-contaminant sources such as chemical fertilizers and sewage. The combined use of δ¹⁵N and δ¹¹B overcame the limitation of using only NO₃^- isotopes for the identification of overlapping sources of N and successfully identified the major source of N as livestock wastes. The lumped parameter model (LPM) estimated the binary mixing of the young (age: 23-40 years, NO₃-N: 2.55-15.10 mg/L) and old (age: >60 years, NO₃-N: <3 mg/L) groundwaters, and explained their age mixing behaviors. The young groundwater was highly affected by livestock-derived N loading during 1987-1998, which coincides with the period of improper dumping of livestock wastes. Furthermore, the young groundwater with elevated NO₃-N followed the historical NO₃-N curves with younger ages (6 and 16 years) than those derived from the LPM, suggesting the possibility of faster inflows of livestock wastes through the permeable volcanic structures. This study demonstrated that a comprehensive understanding of NO₃^- contamination processes can be achieved using environmental tracer methods, which enables the efficient management of groundwater resources in areas with multiple N sources.