11:25 AM - 11:45 AM
[AHW26-03] Water budget of Lake Ashinoko and the Hydrothermal system in Hakone volcano
★Invited Papers
Keywords:Lake Ashinoko, Water budget, Hakone volcano
Lake Ashinoko, which is situated in the southwestern part of the Hakone caldera in Kanagawa prefecture, was formed about 3,000 years ago when the Hayakawa River was dammed by a debris flow generated by magma intrusion at Mt. Kamiyama. The area and the average depth of the lake are 6.9 km3 and 25m, respectively, and the reserved water amounts180 million m3. The catchment area of the lake is some 27.4km2, corresponding to 4 times of the lake’s area.
Itadera and Yoshida (2020) investigated monthly water budget of Lake Ashinoko based on the correlation between monthly potential inflow of water to the lake and monthly outflow of water from the lake, where potential inflow consists of rainfall on the lake surface and catchment area, and out flow is the change in water of the lake plus evaporation and intake from Fukara gate for irrigation that was constructed in Edo era. The investigation revealed that only about a quarter of the rainfall during a prior few months flows into the lake monthly and the average annual amount of precipitation that infiltrates under the ground of the catchment area was estimated at about 33 million m3. This is nearly two-fold larger than the value estimated by the Water Pollution Research Group of Hakone (1975) and as large as twelve times of the estimation by Matsuo et al. (1979).
Based on the results of the analysis on the water budget of Lake Ashinoko, Itadera and Yoshida (2020) proposed the idea that most of the rainfall on the central cones that occupy nearly half of the entire catchment area does not flow into Lake Ashinoko, but instead infiltrates under the ground and the large amount of water which is stocked in the volcanic edifice and supplied with heat and various chemical components by a reservoir of magna is the major source of thermal water that feeds the spas in the eastern flank of the central cones, There are not a few observational facts and analytical results that support this idea, i.e., many NW–SE trending fissures were observed on the surface of the central cones (Kobayashi, 2008), many cracks were estimated to exist in the edifice (Honda et al., 2014), and existence of abundant fluids at a depth of several km beneath the central cones was suggested by the analysis of seismic velocity structure (Yukutake et al. (2015).
At a volcanic unrest of Hakone volcano in 2015, news craters and fumaroles were formed in the Owakudani valley, from which emission of steam and volcanic gases have been continuing still now. In addition, in the valley, there have existed spring wells, surface water flows and artificial pits to gather volcanic gases that are used to make thermal waters since the time before the 2015 unrest. Some waters taken from these craters, fumaroles, spring wells, surface flows ad artificial pits show high concentration of chloride ions and large values of oxygen and hydrogen isotopic ratios. On the other hand, such waters that are very similar to rain are found. We consider following three processes are concerned in making these various kinds of waters, that is, 1) injection of thermal waters that are originated from magna reservoir, 2) enrichment of chloride ion and isotopes by evaporation of infiltrated water during the re-upwelling in hydrothermal system which occurs in the volcanic edifice, and 3) mixing of steam that is evaporated from water of high temperature with ground water in the shallow zone. We believe that the large amount of rainfall infiltrated into the central cones take an essential role in producing these diverse kinds of thermal waters.
References
Honda, R., Y. Yukutake, A. Yoshida, M. Harada, K. Miyaoka, and M. Satomura, Stress-induced spatiotemporal variations in anisotropic structures beneath Hakone volcano, Japan, detected by S-wave splitting: A tool for volcanic activity monitoring, J. Geophys. Res. Solid Earth, 119, 7043–7057, doi:10.1002/2014JB010978, 2014.
Itadera K, Yoshida A (2020) Water budget of Lake Ashinoko, the origin of Hakone thermal waters. Journal of Hydrology; Regional Studies, 28, doi: 10.1016/j.ejrh.2020.100682
Matsuo, S., M. Kusakabe, N. Niwano, T. Hirano, Y. Oki, Water budget in the Hakone caldera using hydrogen and oxygen isotope ratios, Isotopes in lake studies, 131-144. IAEA, Vienna, 1979.
Water Pollution Research Group of Hakone, 1975, Water pollution of thermal waters wasted from baths, Hakone, Bulletin of the Hot Springs Research Institute of Kanagawa Pref., 6(2), 87-116.
Yukutake Y., R. Honda, M. Harada, R. Arai, M. Matsubara, A magma-hydrothermal system beneath Hakone Volcano, central Japan, revealed by highly resolved velocity structures. J. Geophys. Res. Solid Earth 120:3293–3308. https://doi.org/10.1002/2014JB011856, 2015.
Itadera and Yoshida (2020) investigated monthly water budget of Lake Ashinoko based on the correlation between monthly potential inflow of water to the lake and monthly outflow of water from the lake, where potential inflow consists of rainfall on the lake surface and catchment area, and out flow is the change in water of the lake plus evaporation and intake from Fukara gate for irrigation that was constructed in Edo era. The investigation revealed that only about a quarter of the rainfall during a prior few months flows into the lake monthly and the average annual amount of precipitation that infiltrates under the ground of the catchment area was estimated at about 33 million m3. This is nearly two-fold larger than the value estimated by the Water Pollution Research Group of Hakone (1975) and as large as twelve times of the estimation by Matsuo et al. (1979).
Based on the results of the analysis on the water budget of Lake Ashinoko, Itadera and Yoshida (2020) proposed the idea that most of the rainfall on the central cones that occupy nearly half of the entire catchment area does not flow into Lake Ashinoko, but instead infiltrates under the ground and the large amount of water which is stocked in the volcanic edifice and supplied with heat and various chemical components by a reservoir of magna is the major source of thermal water that feeds the spas in the eastern flank of the central cones, There are not a few observational facts and analytical results that support this idea, i.e., many NW–SE trending fissures were observed on the surface of the central cones (Kobayashi, 2008), many cracks were estimated to exist in the edifice (Honda et al., 2014), and existence of abundant fluids at a depth of several km beneath the central cones was suggested by the analysis of seismic velocity structure (Yukutake et al. (2015).
At a volcanic unrest of Hakone volcano in 2015, news craters and fumaroles were formed in the Owakudani valley, from which emission of steam and volcanic gases have been continuing still now. In addition, in the valley, there have existed spring wells, surface water flows and artificial pits to gather volcanic gases that are used to make thermal waters since the time before the 2015 unrest. Some waters taken from these craters, fumaroles, spring wells, surface flows ad artificial pits show high concentration of chloride ions and large values of oxygen and hydrogen isotopic ratios. On the other hand, such waters that are very similar to rain are found. We consider following three processes are concerned in making these various kinds of waters, that is, 1) injection of thermal waters that are originated from magna reservoir, 2) enrichment of chloride ion and isotopes by evaporation of infiltrated water during the re-upwelling in hydrothermal system which occurs in the volcanic edifice, and 3) mixing of steam that is evaporated from water of high temperature with ground water in the shallow zone. We believe that the large amount of rainfall infiltrated into the central cones take an essential role in producing these diverse kinds of thermal waters.
References
Honda, R., Y. Yukutake, A. Yoshida, M. Harada, K. Miyaoka, and M. Satomura, Stress-induced spatiotemporal variations in anisotropic structures beneath Hakone volcano, Japan, detected by S-wave splitting: A tool for volcanic activity monitoring, J. Geophys. Res. Solid Earth, 119, 7043–7057, doi:10.1002/2014JB010978, 2014.
Itadera K, Yoshida A (2020) Water budget of Lake Ashinoko, the origin of Hakone thermal waters. Journal of Hydrology; Regional Studies, 28, doi: 10.1016/j.ejrh.2020.100682
Matsuo, S., M. Kusakabe, N. Niwano, T. Hirano, Y. Oki, Water budget in the Hakone caldera using hydrogen and oxygen isotope ratios, Isotopes in lake studies, 131-144. IAEA, Vienna, 1979.
Water Pollution Research Group of Hakone, 1975, Water pollution of thermal waters wasted from baths, Hakone, Bulletin of the Hot Springs Research Institute of Kanagawa Pref., 6(2), 87-116.
Yukutake Y., R. Honda, M. Harada, R. Arai, M. Matsubara, A magma-hydrothermal system beneath Hakone Volcano, central Japan, revealed by highly resolved velocity structures. J. Geophys. Res. Solid Earth 120:3293–3308. https://doi.org/10.1002/2014JB011856, 2015.