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

[AHW18-15] Boron isotope as a tracer for groundwater contamination in Quaternary limestone aquifer in the southern part of Okinawa Island, Japan

*KE-HAN SONG¹, Ryuichi Shinjo^1,2, Yuto Ito¹, Yumi Moromizato³, Jun Yasumoto⁴ (1.Graduate School of Engineering and Science, University of the Ryukyus, 1 Senbaru, Nishihara, Okinawa 903-0213, Japan, 2.Research Institute for Humanity and Nature, Motoyama 457-4, Kamigamo, Kita-ku, Kyoto 603-8047, Japan, 3.Department of Physics and Earth Sciences, University of the Ryukyus, 1 Senbaru, Nishihara, Okinawa 903-0213, Japan, 4.Department of Regional Agricultural Engineering, Faculty of Agriculture, University of the Ryukyus, Senbaru 1, Nishihara, Okinawa 903-0213, Japan)

Keywords:Boron, Tracer, Groundwater contamination

We present a practical example for quantitative analyses using boron isotope of hydrological samples as a tracer for environmental contamination in the southern part of Okinawa Island, Japan. The studied areas are characterized by Quaternary limestone (so-called Ryukyu Limestone) aquifers. Since intensified human activities have affected groundwater, monitoring groundwater quality is important in the region. We report boron concentrations and δ¹¹B values of groundwater, spring water, soil leachate, and leachates of possible contaminant sources. Boron concentrations of groundwater are positively correlated with nitrate concentrations, indicating that boron source is similar to nitrate sources. We assume four end-members, having different boron characteristics, as diagnostic of contamination processes with mixing lines. We estimate that the least contaminated groundwater is rainwater containing 0.6% seawater component, which is obtained from the best fit of water samples to two trends on the mixing lines of fertilizer materials (manure and chemical fertilizer). Based on the mixing lines, contribution rates of fertilizers are less than 1% and around 3% for groundwaters and soil leachates, respectively. The quantitative application with boron isotopes has potential for unraveling contamination processes in the Ryukyu Limestone aquifers.