[A-HW24] Material transportation and cycling at the land-sea interface: from headwaters to the ocean

[AHW24-P05] Evaluation Of Sources and Fate of Nitrate in the Agricultural Island of Osakishimojima (Seto Inland Sea, Japan) Using Nitrogen Isotope

*Sharon Bih Kimbi¹, Shin-ichi Onodera², Mitsuyo Saito³, Takuya Ishida², Makoto _Tamura¹, Yusuke Tomozawa², Nagasaka Itaru² (1.Graduate School of Integrate Arts and Sciences, Hiroshima University, 2.Graduate School of Advanced Science and Engineering, Hiroshima University, 3.Graduate school of Environmental and Life Science, Okayama University)

Keywords:Nitrogen stable isotopes, Nitrate sources, Groundwater pollution, Seto Inland Sea

Nitrate pollution is a common environmental problem worldwide and is one of the major causes of both surface and subsurface water quality degradation. Thus, determining these diffuse sources of nitrate pollution in the various water resources is of utmost importance for the proper management of water quality. The isotopic composition of nitrate (δ15NNO3 and δ18ONO3) was used to evaluate nitrate contamination and geochemical processes in the groundwaters of Kubi and Ocho villages in Osakishimojima Island (Seto Inland Sea). The study area is essentially an agricultural zone comprising of citrus plantations where the use of organic manure and synthetic nitrogen fertilizer is common practice in the villages of Kubi and Ocho respectively. A groundwater sampling campaign was conducted on 22 samples in Kubi village (19 wells,1 borehole,1 spring and 2 rivers) and 33 samples in Ocho village (30 wells,1 river and 2 springs) in the month of October 2021. Isotopic data of nitrate indicates that nitrate contamination in Kubi village mainly originates from Synthetic fertilizers and organic sources meanwhile in Ocho village, nitrate sources mainly originate from manure and septic systems. These results are contrary to expectations as observed in the village of Kubi whereby farmers mainly use manure on their agricultural fields.