[A-HW23] 水循環・水環境

[AHW23-02] Elucidation of the mechanism of groundwater spring discharge around the Lake Ezu, Kumamoto City

*Irfan Tsany Rahmawan¹、Haruchika Hamatake²、Kimpei Ichiyanagi³、Jun Shimada³ (1.Graduate School of Science and Technology, Kumamoto University、2.Faculty of Science, Kumamoto University、3.Faculty of Advanced Science and Technology, Kumamoto University)

キーワード：Lake Ezu, First and second aquifers, Spring waters, Water chemistry, Stable water isotopes

Kumamoto area has abundances of groundwater which feeds 100% of Kumamoto City's tap water. There are two abundant groundwater aquifers, the first unconfined aquifer is composed of the Aso pyroclastic-flow deposit (Aso-4), and the second confined aquifer is composed of the Aso-1~Aso-3 and Togawa lava. Since Lake Ezu, which is the research area, is formed mainly by groundwater spring discharge, much research has been done. However, it is not clear how much discharge ratios of the first and second aquifers into the Lake Ezu. From the previous studies, the first aquifer is characterized by a high NO₃^- concentration due to excessive fertilization and livestock production on the ground surface, and the second aquifer is characterized by a high concentration of sulfate ions due to river water intrusion from the mid-stream area of the Shira River to the Lake Ezu. Previous studies also mentioned that the stable isotopes of groundwater in the first aquifer are relatively higher than in the second aquifer. Therefore, the purpose of this study is to estimate the contribution rate of the first and second aquifers based on the water quality and stable isotope ratios of spring and lake waters collected around Lake Ezu. The six river waters and five spring waters were collected monthly from February to October 2021. As a result, the NO₃^- in spring water in the upstream (the Kami-Ezu lake) area and downstream (the Shimo-Ezu lake) areas were 11~14 mg/L and 18~21 mg/L, respectively. The SO₄^2- in spring water in the Kami-Ezu lake and Shimo-Ezu lake areas were around 20~23 mg/L and 28~32 mg/L, respectively. Also, the δ¹⁸O in spring water in the Kami-Ezu lake and Shimo-Ezu lake areas were around -7.2 ‰ and -7.4 ‰, respectively. These results indicate that the groundwater in the Kami-Ezu lake area mainly comes from the first aquifer, while that in the Shimo-Ezu lake area mainly comes from the second aquifer. Based on SO₄^2- concentrations in springs and groundwaters, the mixing ratio of first and second aquifers are estimated at around 80% and 20% in the Kami-Ezu lake area, while those are around 30% and 70% in the Shimo-Ezu lake area. These results could become a helpful input for groundwater preservation in Kumamoto.