Japan Geoscience Union Meeting 2019

Presentation information

[J] Poster

A (Atmospheric and Hydrospheric Sciences ) » A-HW Hydrology & Water Environment

[A-HW24] Isotope Hydrology 2019

Mon. May 27, 2019 5:15 PM - 6:30 PM Poster Hall (International Exhibition Hall8, Makuhari Messe)

convener:Masaya Yasuhara(Rissho Univ.), Kohei Kazahaya(Geological Survey of Japan, AIST), Shinji Ohsawa(Institute for Geothermal Sciences, Graduate School of Science, Kyoto University), Kazuyoshi Asai(Geo Science Laboratory)

[AHW24-P08] Analysis of spring water formation mechanism used by 222Rn concentration at Masutomi-Onsen located in Yamanashi, Japan

*Eiki Mochizuki1, Takuya Hayakawa1, Hiroshi Kobayashi1 (1.Yamanashi Institute of Public Health and Environment)

Keywords:hot spring, Masutomi-Onsen, Yamanashi prefecture, Radon-222, formation mechanism

The 222Rn content of Masutomi-Onsen in the northern part of Yamanashi Prefecture is one of the best in the country and many users are visiting as spa resort areas. In order to use this hot spring for a long term, planned use, it is important to clarify the current formation mechanism of spring water.
In Masutomi-Onsen, it is reported that the sedimentary rock near the surface of the earth is a source of 222Rn. Therefore, the behaviors of the 222Rn concentration and other component concentrations except 222Rn are considered to be different. Therefore, we attempted to estimate the formation mechanism of spring water based on 222Rn concentration and other component concentration.
In this study, we regularly monitored Nyuzawa spring in the Masutomi-Onsen area for 2 years from April, 2016 to March, 2018, about discharge amount , 222Rn concentration , other component concentrations, and water temperature.
As a result, periodic fluctuations were observed in 222Rn concentration range from 3,700 to 7,800 Bq / kg. However, except for Unusual decrease in concentration at October 2017 and March 2018, other component concentrations, discharge amount, and water temperature were constant.
In October 2017 and March 2018 in which Unusual decrease in other component concentration was observed, both were consistent with the time when the periodic fluctuation of 222Rn concentration showed minimum. lot of rain was observed immediately before monitoring, and it seemed that this was related to 222Rn and other component concentration reduction. From this, we analyzed the variation of 222Rn concentration , Unusual decrease of other component concentrations and the relevance of precipitation.
When water was supplied to the surface by rainfall, 222Rn concentration decreased, and Then it took 4 months to recover. other component concentrations were constant regardless of the amount of precipitation except Unusual decrease.
I set the following hypothesis to explain this. 222Rn arising from sedimentary rocks becomes 222Rn gas in the soil, and this 222Rn gas is one of 222Rn sources in spring water. Normally, shallow groundwater does not intersect with spring water, but shallow groundwater deprives 222Rn gas according to its amount, so 222Rn concentration in spring water fluctuates.
In order to verify this hypothesis, we confirmed the relationship between shallow groundwater level estimated from precipitation and 222Rn concentration, and as a result, a significant negative correlation was observed, and a result supporting the hypothesis was obtained.
From these facts, it was possible to explain formation mechanism of spring water based on 222Rn concentration, other component concentrations and precipitation in Nyuzawa spring.