[A-HW18] 流域圏生態系における物質輸送と循環：源流から沿岸海域まで

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

*SONG KE-HAN¹、新城 竜一^1,2、伊藤 湧人¹、諸見里 ゆみ³、安元 純⁴ (1.琉球大学理工学研究科、2.総合地球環境学研究所、3.琉球大学理学部物質地球科学科、4.琉球大学農学部農学部地域農業工学科)

キーワード：ホウ素、トレーサー、地下水汚染

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.