Origin of a Temporary Pond, Akaike, at the Northern Foot of Mount Fuji, Japan, July 2020

Abstract

Akaike is a small, temporary pond located about 1 km east of Lake Shoji at the northern foot of Mt. Fuji, which appears following heavy rainfall events. It is widely accepted that the pond is connected to Lake Shoji by groundwater. However, this connection has not yet been evaluated because of a lack of scientific data including water quality and environmental isotopes from both Akaike Pond and Lake Shoji. Major dissolved ion and trace element contents, hydrogen (δD) and oxygen (δ 18 O) stable isotope ratios of water, and nitrogen (δ 15 N-NO 3 ) and oxygen (δ 18 O-NO 3 ) stable isotope ratios of nitrate ions were measured in water samples from Akaike Pond, which appeared between July 9th and August 20th in 2020. These data are compared with surface water and rainfall samples collected from the surrounding area to reveal the origin of Akaike Pond. It is found that the δD and δ 18 O values of Akaike Pond significantly increased after the late July (21st28th) rainfall events. Mass balance analysis based on the δD and δ 18 O before and after the rainfall indicates that water flowing into Akaike Pond during this period was mainly derived from the immediately preceding rainfall. Major dissolved ion compositions in Akaike Pond show lower Ca 2+ and HCO 3 concentrations than water samples around the pond. This suggests that water in Akaike Pond appeared in a relatively short period (within a few days) after the heavy rainfall events, without deep underground percolation, which is further supported by δ 18 O-NO 3 and trace element composition.