[U-05] Advanced understanding of Quaternary and Anthropocene hydroclimate changes in East Asia

[U05-P03] Long-Term monthly measurements of dissolved inorganic radiocarbon in water at Lake Motosu and Lake Kawaguchi to understand seasonal hydrological variations

*Ota Kosuke¹, Yusuke Yokoyama¹, Yosuke Miyairi¹, Shinya Yamamoto², Toshihiro Miyajima¹ (1.University of Tokyo, 2.Mount Fuji Research Institute, Yamanashi Prefectural Government)

Keywords:Radiocarbon, Oxygen isotope, Fuji Five Lakes, Lake water, groundwater

The process of water flowing into lakes through precipitation, groundwater, and rivers varies from region to region and between seasons. In order to understand the future hydrological state of lakes, it is important to clarify the details of the hydrological cycle in the long term. Lake Motosu is a volcanic dammed lake at the northwestern foot of Mount Fuji. The lake has only temporal inflow and outflow rivers, and lake water characteristics are generally defined by a combination of precipitation and groundwater inflow. Previous surveys on water quality in Lake Motosu, including vanadium concentration and water temperature in surface water during summer and winter, and the vertical distribution of water temperature and water quality throughout the year revealed that precipitation is the dominant factor in the water quality of the lake. It is, therefore, possible to quantitatively estimate the water transport pathways, including the carbon transport process in the hydrosphere.
In this study, we investigated the water pathways in Lake Motosu by measuring radiocarbon concentration (Δ¹⁴C), and oxygen and hydrogen (δ¹⁸O and δD) stable isotopes in precipitation, groundwater, and lake water from 2018 to 2021, as well as the spring water at the lake bottom. The results of groundwater measurements and water balance surveys were combined to obtain a comprehensive picture of water exchange. The mass balance of water was also calculated from the groundwater recharge altitude, lake observation data, and the Δ¹⁴C box model of groundwater and lake water.
The Δ¹⁴C, and the δ¹⁸O and dD of water were measured using a single-stage accelerator mass spectrometer and a wavelength scanning cavity ring-down spectrometer at the Atmosphere and Ocean Research Institute of the University of Tokyo. The Δ¹⁴C of surface water in Lake Motosu was found to increase during winter with the highest value in January. In contrast, the Δ¹⁴C values became lower in summer. The three years records of Δ¹⁴C, δ¹⁸O, and dD in Kawaguchi Lake will also be presented, and the water cycle of the Fuji Five Lakes will be discussed.