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

[AHW24-P02] Identification of the occurrence of denitrification in the Ryukyu limestone aquifer in southern Okinawa

*Oktanius Richard Hermawan¹, Takahiro Hosono², Jun Yasumoto³, KE-HAN SONG⁴, Ryuichi Shinjo⁵, Takada Ryogo⁶ (1.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.Graduate School of Engineering and Science, University of the Ryukyus, 5.Research Institute for Humanity and Nature, 6.Center for Strategic Research Projects, University of the Ryukyus)

Keywords:subsurface dam, limestone aquifer, denitrification, stable isotopes, nitrate contamination, Ryukyu limestone

The nitrate contamination issue in the limestone aquifer has not been observed in detail, although there are a lot of contaminated soil-mantled limestone areas because of human activities and extensive agricultural practices. Hence, nitrate attenuation becomes a principal process to consider as natural remediation for better groundwater management. There are two possible nitrate attenuation processes: dilution is the mixing of waters of high concentrations with low concentration water, and denitrification reaction enhanced by microbial activities. The complex, random and fast-flowing hydrogeological system makes it challenging to discuss the nitrogen cycle in the limestone aquifer. Albeit it seems unlikely, denitrification also happened in the Ryukyu limestone, the southern part of Okinawa Island. Additionally, it is interesting that two subsurface dams were established in our research area (the Komesu and Giza subsurface dams). Those subsurface dams were constructed to meet agricultural and domestic use demands in 2005. In southern Okinawa, 9 of 48 observation wells showed relatively high NO₃-N concentration of groundwater samples more than 10 mg l^-1. Anyhow, in the preliminary survey, we detected some depletion anomalies in their concentrations (NO₃-N < 2 mg l^-1 and DO < 1 mg l^-1) for some particular wells. N and O stable isotope ratios of nitrate ion identified contamination sources that were caused mainly by manure and animal waste. The stable isotopes value of δ¹⁵N_NO3 and δ¹⁸O_NO3 mostly around 6 to 14‰ and -1 to 9‰, respectively. In contrast, the samples from the low concentration anomaly typically show higher δ¹⁵N_NO3 exceeding up to 35‰, implying the occurrence of denitrification. We further searched seasonal changes for those particular wells through monthly monitoring across 13 months and collected 96 groundwater samples to get details of the denitrification process. As the result, we observed systematic seasonal patterns of decreasing concentration of NO₃-N with increasing δ¹⁵N_NO3 up to 50‰. We also confirmed the occurrence of denitrification was systematically controlled by the seasonal hydrologic cycle. Although the occurrence of denitrification reaction has rarely been identified in the limestone aquifers with relatively fast groundwater flow regimes, three factors are considered to generate possible denitrification settings in the study area. First, denitrification reaction seems preferentially proceeding in the lithological gap generated due to the major fault system; second, sluggish groundwater area around groundwater divide; and third, the existence of the subsurface dam itself. The most intriguing finding is the role of the Komesu subsurface dam that can enhance the denitrification process by holding the groundwater during low water season and creating stagnant conditions. In contrast, the more dynamic and active groundwater system in the Giza area prevents the denitrification process. This is because the Giza subsurface dam always outflows the groundwater outside the intake wall and cannot create a sluggish condition in the whole season.