[A-HW23] Isotope Hydrology 2023

[AHW23-P06] The formation mechanism of Akaike Pond at the northern foot of Mt. Fuji in 2022 revealed by groundwater level observations and water quality and stable isotope analyses

*Shinya Yamamoto¹, Takashi Nakamura², Seongwon Lee³, Yuki Itoh³, Masaya Yasuhara³ (1.Mount Fuji Research Institute, Yamanashi Prefectural Government, 2.International Research Center for River Basin Environment, University of Yamanashi, 3.Faculty of Geo-Environmental Science, Rissho University)

Keywords:Mt. Fuji, temporary pond, water quality, stable isotopes

Several temporary water bodies are known to appear under specific conditions in Japan (e.g., Ikenodaira (Shizuoka Prefecture), Chinoike Pond (Gunma Prefecture), and Akaike Pond (Yamanashi Prefecture)). These water bodies are difficult to observe because they appear irregularly and disappear in a short period time, and therefore the detailed mechanisms of their occurrence are still unknown. In previous studies, we measured the hydrogen and oxygen stable isotope ratios of water from Akaike Pond, which appeared in 2020 and 2021, and suggested that the water flowing into Akaike Pond was mainly derived from the immediately preceding rainfall. We also measured major dissolved ion compositions, and the nitrogen and oxygen stable isotope ratios of nitrate ions, and suggested that water in Akaike Pond appeared in a relatively short period after the heavy rainfall events. However, these investigations only focused on the surface water and the detailed formation mechanism of Akaike Pond remains unclear. In this study, we attempted to elucidate the formation mechanism of Akaike Pond by observing the groundwater level and measuring the groundwater quality and stable isotope ratio of the groundwater. Water level observations were conducted from May 2022 using compact absolute pressure water level gauges (S&DL mini, Oyo) that are installed in observation wells at depths ranging from 80 cm to 2.7 m at the center of Akaike Pond and on the north-south slope of the pond. Groundwater samples were collected from the observation wells at the center of Akaike Pond approximately once a month from June to November 2022 using a baler-type water sampler. In this study, we will discuss the formation mechanism of Akaike Pond based on groundwater level fluctuations and changes in water quality and stable isotope ratios of water from mid to late September 2022, when the occurrence of Akaike Pond was confirmed due to heavy rainfalls caused by Typhoons No. 14 and 15.