[A-HW22] 流域生態系における物質輸送と循環：源流から沿岸まで

[AHW22-09] Nitrate contamination source identification by using multiple isotope ratios in Ryukyu limestone aquifer, southern Okinawa island, Japan

*Oktanius Richard Hermawan¹、Takahiro Hosono²、Jun Yasumoto³、Kazuko Sawada⁴、KE-HAN SONG⁵、Ryuichi Shinjo⁶ (1.Earth and Environmental Science, Graduate School of Science and Technology, Kumamoto University、2.Faculty of Advanced Science and Technology, Kumamoto University、3.Department of Regional Agriculture Engineering, University of the Ryukyus、4.Center for Strategic Research Projects, University of the Ryukyu、5.Physics and Earth Sciences Course, Graduate School of Engineering and Science, University of the Ryukyus、6.Department of Physics and Earth Sciences, Faculty of Science, University of the Ryukyus)

キーワード：nitrate isotopes, nitrate contamination, limestone aquifer, sulfate isotopes

Numerous studies have reported intense nitrate groundwater contamination in the Ryukyu limestone aquifer, southern part of Okinawa Island. The majority land use of the research area is agricultural fields, especially sugarcane farm. Due to the constant demand of groundwater extraction from that sustainable agriculture practices, Komesu and Giza subsurface dam were made in 2005. Nitrate concentration in the groundwater was relatively high, NO₃^--N concentration was more than 10 mg/L at 9 observation sites among 62, with the average concentration of 7.2 ± 3.9 mg/L and maximum value of 24.8 mg/L. Determining nitrate pollution source is critical to control the ongoing pollution for a better groundwater management since 30% groundwater in Giza area is used for drinking purpose. Previous researchers reported that the most dominant nitrate source came from abusing chemical fertilizer (ammonium sulfate) for the agricultural practices. N and O isotope ratios in nitrate was measured to fingerprint the dominant pollutant source. Sulfur isotope ratio in sulfate was also measured. Soil samples were used as the representative of the end members. This is because soil water acts as an ultimate source of all the pollutants before infiltrating into the groundwater. From the soils, 3 end members could be categorized based on their distinct isotopic compositions; soil water with high chemical fertilizer input (δ¹⁵N_NO3 = 3.1 to 4.7‰), soil water with high manure input (δ¹⁵N_NO3 = 7.8-9.0‰) and volatilized manure (δ¹⁵N_NO3 = 18.1 to 20.3‰). Giza’s groundwaters have δ¹⁵N_NO3 value between 4.0-9.0‰, on the other hand Komesu’s groundwater have higher ratio of δ¹⁵N_NO3 from 5.6-11.8‰. By using simple binary mixing model and comparing the ratio with the soil end members, it can be proven that there is another major contaminant in the research area which is manure input. Meanwhile, the higher δ¹⁵N_NO3 and δ¹⁸O_NO3 in Komesu area were due to inappropriate storage of manures. There was a lot of manure fertilizer piles stored in the field without any cover, causing those volatilized manure leached into the groundwater. In addition, nitrate versus sulfate mol ratio did not follow ammonium sulfate fertilizer trend (2:1 trend), thus chemical fertilizer only cannot explain the cause of elevated sulfate concentration in the research area (>100 mg/L). Groundwater δ³⁴S_SO4 for both Komesu and Giza area showed homogenous ratio between -2 to 5‰ and in the range of soil samples ratio (-5 to 5‰). The soil in the research area is a transported from the Shimajiri Formation. Shimajiri Formation consists of fine-grained sedimentary rocks which may contain sufficient reduced sulfur as the form of pyrite. The range of sulfate stable isotope ratios also suggested the sulfate contamination was enhanced by manure as the form of organic fertilizer (δ³⁴S_SO4 = -2 to 6‰ and δ¹⁸O_SO4 = 4 to 6‰), especially in Komesu area. At the end, the difference in hydrogeological setting and subsurface dam setting also control the contamination tendency between Komesu and Giza areas. The mixing process between contamination of manure and chemical fertilizer seems to proceed well in Giza area due to the active groundwater flow from recharge area to discharge area, meanwhile it is more site specific and heterogenous in Komesu area in response of sluggish flow areas generated by the subsurface dam.