[A-HW22] Material transportation and cycling in watershed ecosystems; from headwaters to coastal areas

[AHW22-16] Groundwater flow system inferred by estimates of hydrological, thermal and chemical budgets in Okama Crater Lake, Zao Volcano, Japan

*Kazuhisa A. Chikita¹, Akio Goto², Jun Okada³, Satoshi Miura⁴, Mare Yamamoto⁴ (1.Arctic Research Center, Hokkaido University, 2.Center for Northeast Asian Studies, Tohoku University, 3.Volcanology Research Department, Meteorological Research Institute, 4.Research Center for Prediction of Earthquakes and Volcanic Eruptions, Faculty of Science, Tohoku University)

Keywords:Okama Crater Lake, hydrological budget, chemical budget, thermal budget, geothermal flux

In order to clarify a groundwater flow system below Zao Caldera, hydrologcal, thermal and chamical budgets of a crater lake, Okama, were estimated. In the summer of 2020, water and sediment samplings and profilings on a boat and hydro-meteorological observations at lake shore were carried out (see Fig. 1). This is a report about the evaluation of groundwater input and output in Okama by estimating hydrological and chemical budgets. Okama is at ca. 1550 m asl with ca. 25 m at maximum depth and pH=3.2-3.7. The Okama water probably leakes into the headwater region of the Nigori River at 1540 m asl, which could affect the ecosystem in the downstream region. The estimate of hydrological and chemical budgets allowed us to numerically obtain groundwater input of 0.007 - 0.019 m³/s and groundwater output of 0.035 - 0.047 m³/s. Meanwhile, the lake water leakage into the Nogiri River was estimated at ca. 0.01 m³/s. Thus, the lake water could be leaked at 0.025 - 0.037 m³/s into the deeper zone. In the completely ice-covered period of 5 December 2019 - 5 May 5 2020, bottom water temperature in Okama gradually increased in spite of the lake-ice growth (then, the lake is cooled). This suggests that a certain significant geothermal input occurs at the bottom as one of volcanic activities.