Quantitative assessment of natural purification capacity at the freshwater-saltwater interface against nitrogen loading

Key Findings

1. Confirmed broad distribution of denitrification zones at the freshwater-saltwater interface developing at the bottom and periphery of the freshwater lens.

2. Estimated that approximately one-third of anthropogenic nitrate is removed from groundwater through this natural purification process.

3. Proposed the importance of groundwater quality management and evaluation of nutrient outflow to the ocean, considering the effects of denitrification.

Overview

Groundwater contamination by nitrate and nitrogen outflow to the ocean are global issues. While denitrification—a phenomenon where nitrate is reduced to nitrogen gas and returned to the atmosphere—occurs in nature, quantitative evaluation of this natural purification has been lagging. A research group led by Kumamoto University utilized the freshwater lens on Tarama Island, Okinawa, as a model to conduct the world's first watershed-scale estimation of denitrification rates. Using stable isotope tracers, the group clarified that denitrification zones are widely distributed at the freshwater-saltwater interface. Based on water and nitrogen balance calculations, they showed that approximately one-third of anthropogenically loaded nitrate is removed by denitrification. These findings were published online in the international academic journal "Water Research" on March 12, 2026.

Conclusion and Future Outlook

While this evaluation is an estimate based on many assumptions, it proves that a non-negligible scale of nitrogen loading, primarily derived from agriculture, is removed by denitrification. This knowledge provides essential information for groundwater management and coastal ecosystem conservation in island regions. Future advancements in estimation accuracy are expected through the acquisition of high-resolution spatiotemporal data and verification using groundwater flow simulations incorporating physical models.