[A-HW18] Material transportation and cycling at the land-sea interface: from headwaters to the ocean

[AHW18-P15] Origin and behavior of nitrate nitrogen in the freshwater lens deduced by multiple stable isotope ratios in Tarama Island, Okinawa, Japan

*Ikehara Takatomo¹, Takahiro Hosono¹, Oktanius Richard Hermawan¹, Jun Yasumoto², Ko Yasumoto³, Maruyama Rio³, Mariko Iijima⁴, KE-HAN SONG², Ryuichi Shinjo⁵ (1.Kumamoto University, 2.University of Ryukyu, 3.Kitasato University, 4.National Institiute of Advanced Industrial Science and Technology, 5.Research Institute of Humanity and Nature)

Keywords:groundwater, freshwater lens, nitrogen pollution, denitrification

Tarama Island in Okinawa Prefecture is located in the Ryukyu Islands, and the freshwater lens exist in the aquifer. The freshwater lens is a valuable water resource for the islanders and is mainly used for domestic purposes. On the other hand, the freshwater lens is vulnerable to pollution by nitrate-nitrogen and other pollutants caused by human activities on the ground surface.
In this study, to clarify the origin and dynamics of nitrate-nitrogen in Tarama Island, Okinawa Prefecture, water samples were collected at various locations in the island and measured the concentration of nitrate-nitrogen dissolved in groundwater, as well as the stable isotope ratios of nitrogen and oxygen in nitrate ions and carbon in dissolved inorganic carbon. To understand the source of nitrate-nitrogen in the groundwater, compost and chemical fertilizers mainly used in the Tarama Island were sampled and their stable isotope ratios were measured.
The nitrate nitrogen concentrations were below the water environmental standard. The nitrogen and oxygen stable isotope ratios of nitrate ions suggested that the nitrate nitrogen may have originated both from livestock wastes and chemical fertilizers. These results are consistent with the origin-specific ratios of nitrogen loadings considering the nitrogen leaching rate. Carbon stable isotope ratios of dissolved inorganic carbon suggest that dissolved inorganic carbon in groundwater may be contributed by Ryukyu limestone and sugarcane. However, a large decrease in its values was observed with increasing water sampling depth in some boreholes in the central part of the island. The sites where these changes were observed had elevated nitrate nitrogen stable isotope ratios and were reducing environments, suggesting that heterotrophic denitrification was occurring. The results of the water and nitrogen budgets of Tarama Island suggest that the average residence time in the freshwater lens may be about 4 months, and it is further estimated that about 47% of the nitrogen load is purified by denitrification in the freshwater lens of Tarama Island.
It is advantageous to manage groundwater resources considering the denitrification characteristics of freshwater lens in Tarama Island.