5:15 PM - 6:45 PM
[AHW24-P05] Groundwater flow characteristics of Akaike Pond and its surrounding area: Insights from water quality, stable isotope ratios, and hydrogeological structure
Keywords:Temporary Pond, Water quality, Stable isotopes, Hydrogeology
Akaike is a small temporary pond located at the northern foot of Mt. Fuji, typically appearing after heavy rainfall events. Previous studies measured the water quality and environmental isotopes of Akaike Pond in 2020 and 2021, revealing that the water flowing into Akaike Pond primarily originated from the preceding rainfall events and appeared in a relatively short period after heavy rainfall events, without deep underground percolation. Additionally, despite experiencing approximately half the amount of rainfall in 2021 compared to 2020, the concentrations of major dissolved ions in the Akaike Pond in 2021 were much lower than those in 2020. This result suggested that different amounts of rainfall likely affected the flow rate and volume of groundwater reaching the Akaike Pond. However, the conditions and the mechanisms governing the formation of Akaike Pond remain largely unknown owing to a lack of hydrological observations.
In this study, we installed two observation wells (at depths of 1.45 m and 2.7 m) in the center of Akaike Pond in 2022 to monitor groundwater levels and analyze groundwater quality and stable isotope ratios of water to elucidate water quality characteristics. Additionally, a borehole survey was also conducted to investigate the relationship between the hydrogeological structure of Akaike and the groundwater quality characteristics.
The results of the groundwater survey conducted at the observation wells highlighted the variations in groundwater quality across different well depths, indicating the presence of two types of groundwater with distinct recharge sources. The Borehole survey revealed the deposition of gray-brown clay layer (0.85–1.26 m thick), black clay layer (0.45–0.65 m thick), white clay layer (0.8–1 m thick), and brown soil (about 0.5 m thick) over the Aokigahara lava and basement rock at 2.6–2.8 m below ground level. The clay layers contained a significant amount of diatom fossils, except for the black clay layer, indicating deposition in water. The 14C age of plant remains in the black clay layers is 925 ± 25 yr BP, suggesting temporary terrestrial deposition around this period. Considering the hydrogeological structures, the origin of the groundwater in the upper part of the black clay layer is lateral water flow from the permeable shallow part of the lava slope. Conversely, below the black clay layer, water isotopic ratios show monthly fluctuations, suggesting a more extensive groundwater flow through the lava body.
In this study, we installed two observation wells (at depths of 1.45 m and 2.7 m) in the center of Akaike Pond in 2022 to monitor groundwater levels and analyze groundwater quality and stable isotope ratios of water to elucidate water quality characteristics. Additionally, a borehole survey was also conducted to investigate the relationship between the hydrogeological structure of Akaike and the groundwater quality characteristics.
The results of the groundwater survey conducted at the observation wells highlighted the variations in groundwater quality across different well depths, indicating the presence of two types of groundwater with distinct recharge sources. The Borehole survey revealed the deposition of gray-brown clay layer (0.85–1.26 m thick), black clay layer (0.45–0.65 m thick), white clay layer (0.8–1 m thick), and brown soil (about 0.5 m thick) over the Aokigahara lava and basement rock at 2.6–2.8 m below ground level. The clay layers contained a significant amount of diatom fossils, except for the black clay layer, indicating deposition in water. The 14C age of plant remains in the black clay layers is 925 ± 25 yr BP, suggesting temporary terrestrial deposition around this period. Considering the hydrogeological structures, the origin of the groundwater in the upper part of the black clay layer is lateral water flow from the permeable shallow part of the lava slope. Conversely, below the black clay layer, water isotopic ratios show monthly fluctuations, suggesting a more extensive groundwater flow through the lava body.