[A-HW23] 水循環・水環境

[AHW23-08] Coseismic changes of shallow groundwater traced from long term temperature and level dataset in paddy field well

★Invited Papers

*井手 淨¹、嶋田 純²、島 武男³ (1.芝浦工業大学、2.熊本大学、3.国立研究開発法人農業・食品産業技術総合研究機構)

The Kumamoto area, located in the central part of Kyushu, is one of the famous groundwater use area in Japan since the excellent groundwater aquifer system with high infiltration capacity and water-holding capacity. Furthermore, the active groundwater management as winter flooded rice paddy has conducted in groundwater recharge area, and was confirmed effectivity to increase of groundwater discharge in downstream-area such as Ezu-lake [1]. At the Maki district in Ozu town situated in a groundwater recharge area, continuous monitoring of shallow groundwater has been performed by water level and temperature logger to check the effectivity of the artificial recharge system from 2015. During the monitoring period, the large earthquake disaster (Kumamoto earthquake, maximum intensity: M_w7.0) occurred on April 14 and 16^th in 2016, which caused extensive human and property damage. These earthquakes caused many surface rupture zones over area between discharge and recharge area including the Maki area. Plural hydrological studies focusing to changes of Kumamoto groundwater flow system after these coseismic events, were discussing the release of additional mountain waters and transportation to downslope groundwater flow system, and water drawdown into newly seismic rupture near the epicenter [2]. These previous studies imply continuous monitoring records of groundwater temperature and level in recharge area have the potential of the tracing with the property of the infiltration due to artificial groundwater recharge, and the influence of the change in water circulation before and after the Kumamoto earthquake in 2016. Then we would like to introduce the discussions used these monitoring results. Three loggers are located within 500 m area in the Maki district. Land use pattern is rice paddy field, and the surface geology is alluvium (surrounded by Aso pyroclastic flow deposits). Installation depth of loggers from the upstream side are 20.5, 5.5, 12.5 (GL-m), respectively. Surface elevation of corresponding wells are 289, 264, 258 (m.a.s.l). Even in same land-use pattern and surface geology, time series patterns of temperature and level data are different in sites, especially for the downstream logger (GL-12.5 m) has a cyclic fluctuation characteristic despite its installation depth. It is difficult to explain this temperature pattern by the heat conduction of air temperature, and the fact that the suspend of periodical temperature change during the period between after irrigation to start artificial recharge suggests that the downward infiltration rate of water due to the irrigation is reasonably fast. Moreover, during the period when the cyclic fluctuation stopped, the water temperature is assumed to be in equilibrium with the groundwater temperature flowing down through the area. This temperature observation result is harmonized with previous studies discussions that releasing the mountain groundwater and flowing into the area from higher elevations than original source, after the 2016 Kumamoto earthquake. We would like to continue the monitoring observation, and hope to provide information that will lead to the study of long-term changes in the hydrological environment due to earthquakes, and to research activities on groundwater use in the event of a large-scale earthquake.

Acknowledgments
Part of this study was financially supported by the Japanese cabinet office, Government of Japan, Cross-ministerial Strategic Innovation Promotion Program (SIP),"Enhancement of National Resilience against Natural Disasters".

References
1. Kagabu, M., Matsunaga, M., Ide, K., Momoshima, N., Shimada, J., 2017. Groundwater age determination using ⁸⁵Kr and multiple age tracers (SF₆, CFCs, and ³H) to elucidate regional groundwater flow systems. J. Hydrol. Reg. Stud. 12, 165–180.
2. Hosono, T., Saltalippi, C., Jean, J.-S., 2020. Coseismic hydro-environmental changes: Insights from recent earthquakes. J. Hydrol. 585, 124799.