Investigation on Tarama Island reveals surface nitrogen pollution purified by underground freshwater layer

A research group including Professor Takahiro Hosono of the Faculty of Advanced Science and Technology at Kumamoto University, along with the Research Institute for Humanity and Nature (RIHN), Kitasato University, the University of the Ryukyus, and the University of California Santa Cruz (U.S.), has succeeded in estimating the denitrification rate at a watershed scale. They achieved this by conducting an island-wide survey using the freshwater lens (an underground lens-shaped freshwater layer formed on islands) of Tarama Island in the Miyako Islands of Okinawa Prefecture as a model region.

Through direct measurements using monitoring wells and others, they confirmed that the denitrification zone is extensively distributed along the freshwater-saltwater interface that develops at the bottom and periphery of the freshwater lens. Calculations of the water and nitrogen balances revealed that approximately one-third of the nitrogen is removed. This finding is expected to be instrumental for groundwater management and the conservation of coastal ecosystems. The results were published in the journal Water Research on March 12.

Nitrogen pollution triggers groundwater contamination and the eutrophication of aquatic environments, representing a global challenge. In the natural world, microbes in aquifers convert nitrate nitrogen into harmless nitrogen gas (denitrification). This natural purification process partially suppresses the runoff of nitrogen from land into marine areas.

On the other hand, quantitatively evaluating this natural purification capacity has been difficult. This assessment required understanding of the three-dimensional distribution of the underground denitrification zone and analyzing it in combination with nitrogen load data within the watershed.

Therefore, the research group focused on a freshwater lens, which is surrounded by a freshwater-saltwater interface, at the periphery of which denitrification is considered highly likely to occur. A freshwater lens is a freshwater body floating in a lens shape on top of a seawater-bearing aquifer due to density differences. It is found on islands in regions like the Caribbean and the Pacific, serving as a vital water resource.

Tarama Island serves as the base for freshwater lens research in Japan. It possesses a high-density monitoring well network that is rare worldwide, and data on active agricultural and livestock industry-derived nitrogen loads are readily available. The island is nearly circular and flat, measuring approximately 6 km east-to-west and 4 km north-to-south with an area of 19.75 km². Its geology consists of Quaternary limestone of coral reef origin, and a typical freshwater lens formed by the infiltration of rainwater with a thickness of several meters develops underground.

Most of the land is used as sugarcane fields and pastures. Because the islanders depend on the groundwater of the freshwater lens for drinking water, there were concerns regarding the impact of agricultural activities on nitrate nitrogen concentrations in the groundwater. In recent years, increases in nutrient concentrations in coastal areas and declines in coral cover have been reported.

In the study, field surveys were conducted a total of three times, November 25-29, 2018, July 12-14, 2021, and April 27-30, 2022. The team measured water levels and collected samples at various depths from monitoring wells, dug wells, natural springs, surface soils, and submarine groundwater distributed across the entire island. Multiple isotope tracer methods were utilized to estimate the three-dimensional denitrification zone.

Furthermore, they estimated denitrification efficiency by performing a straightforward nitrogen balance calculation that incorporated statistical data on the local water balance and nitrogen loads, alongside measurement results of nitrogen concentrations in the groundwater. The results indicated that most of the groundwater inside and outside the freshwater lens showed no prominent signs of denitrification.

Conversely, in certain areas, denitrification became pronounced as depth increased, predominantly prevailing in brackish and sea water zones. It was revealed that nitrate infiltrating into the freshwater zone is reduced and converted into nitrogen during the process of being transported toward the freshwater-saltwater interface at the bottom and peripheral margins of the freshwater lens. Additionally, it was estimated that approximately one-third of the infiltrating nitrogen is removed via denitrification at this freshwater-saltwater interface located at the bottom and periphery of the lens.

Translating this result into annual values, out of 103.7 tons of nitrogen that infiltrates into the groundwater, 34.6 tons are removed through underground denitrification, while 69.1 tons flow out into the coastal area via groundwater.

Hosono stated: "We have succeeded in quantitatively clarifying for the first time the underground natural purification function of nitrogen components loaded onto the ground surface, making this an achievement of both academic and social significance. We expect the outcomes of this study to be utilized for local environmental conservation and appropriate resource management. Currently, in addition to Tarama Island, we are expanding our target region to the entire Sekisei Lagoon, conducting surveys on the actual state of eutrophication in coastal areas and its relationship with coral cover. We hope to develop this research further and widely disseminate our findings."

Journal Information
Publication: Water Research
Title: Quantitative assessment of denitrification rates at the freshwater-saltwater interface of a limestone island based on isotopic tracers and mass balance calculation
DOI: 10.1016/j.watres.2026.125742