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 path- ways 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 (815N and 818O of NO3-, 811B, chlorofluorocar- bons, 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 815N and 811B 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 groundwa- ter 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 environ- mental tracer methods, which enables the efficient management of groundwater resources in areas with multiple N sources.

Automated summary

This study used environmental tracers to investigate the sources, timing, and pathways of NO3- contamination in groundwater. Livestock waste was identified as the major source of N, and younger groundwater showed a higher influence from livestock waste. Additionally, historical groundwater with elevated NO3-N concentrations may have been affected by faster inflows of livestock waste through permeable volcanic structures. Comprehensive understanding of NO3- contamination processes can be achieved using environmental tracer methods for efficient groundwater resource management in areas with multiple N sources.