Water quality and stable isotope characteristics of Akaike, a temporary pond at the northern foot of Mt. Fuji, August 2021

Akaike is a small temporary pond located at the northern foot of Mt. Fuji, which appears following heavy rainfall events. However, the paucity of water quality and isotope data prevents us from elucidating the details of the formation mechanism of Akaike Pond. In this study, we examined hydrogen (δD) and oxygen (δ18O) stable isotope ratios and dissolved major ion contents in water samples from Akaike Pond and the surrounding area in August 2021, and compared the results with those in 2020 to reveal the origin of Akaike Pond. The appearance of Akaike Pond in 2021 is closely related to the fluctuation of the water level difference between Lake Sai and Lake Shoji, suggesting that the appearance of Akaike is not solely attributed to the rise in the water level of Lake Shoji. The δD and δ18O of Akaike Pond were higher than those of the surface and groundwater samples in the surrounding area, showing values closer to those of the rainfall sample in the same period. In addition, the concentrations of Ca2+and HCO3- in Akaike Pond were lower than those in the water samples around the pond, suggesting that the water in Akaike Pond in 2021, as in 2020, was mainly from the preceding rainfall infiltrated and flowed in a relatively short period. Furthermore, even though the amount of rainfall in 2021 was about half of that in 2020, the concentrations of major dissolved ions in Akaike Pond in 2021 were much lower than those in 2020. Such a relationship cannot be explained by the dilution effect of rainfall, suggesting that different amounts of rainfall likely affected the flow rate and the volume of groundwater that flowed into Akaike Pond.