Variations of the groundwater flow system due to heavy rainfall in Unzen Volcano, Japan

Miku Ishibashi, Koichi Sakakibara, Keisuke Suzuki, Makoto Kagabu
Received 6 December, 2023
Accepted 7 March, 2024
Published online 20 June, 2024

Miku Ishibashi1), Koichi Sakakibara2), Keisuke Suzuki2), Makoto Kagabu3)

1) Graduate School of Science and Technology, Shinshu University, Japan
2) Faculty of Science, Shinshu University, Japan
3) Faculty of Environmental Science, Nagasaki University, Japan

Volcanic regions have complicated groundwater flow systems because of their geological structures, and the effects of rainfall on their groundwater recharge/discharge are still largely unknown. A field survey and chemical analyses were conducted on springs near an andesitic volcano in Japan to investigate changes in groundwater flow processes associated with heavy rainfall. After heavy rainfall, some springs showed a decrease in tracer values while others at lava or pyroclastic flow termini were stable. This indicates that the rainfall component rapidly discharges or the bedrock groundwater component discharges from deeper subsurface areas depending on the structure, length, and permeability of the volcanic ejecta. Hence, the groundwater mixing process in areas with ejecta can be approximated by the binary mixing model. In contrast, tracer values increased after heavy rainfall in areas near agricultural land, which suggests that nitrate ions were loaded into groundwater. Thus, the changes in various groundwater flow processes in volcanic regions after rainfall can be attributed to not only geological/geographical differences but also the groundwater stored in mountain bodies.

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Copyright (c) 2024 The Author(s) CC-BY 4.0

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