5:15 PM - 6:45 PM
[SCG47-P06] Continuous Monitoring of Dissolved Gases in Thermal Water at Aso-Kumamoto Region
Keywords:Hydrothermal Water, Continuous Groundwater Monitoring, Deep-Origin Gas
Deep-origin fluids supplied from conductive zones C1 and C2, which are located under the area indicated by Aizawa+(2021), are thought to reach the ground surface through the Futagawa Fault, and the conduit and faults of Mt. Aso. Therefore, we observed dissolved gases in hydrothermal water at the KUM and ASO stations to examine how the supply state of deep-seated gas changes with seismic and volcanic activities.
An automatic sampling extractor was developed to collect of dissolved gas in thermal water without exposing it to the atmosphere. The extractor, which has one peristaltic pump and two electromagnetic valves, can easily introduce hydrothermal water into an airtight container, extract dissolved gas, and drain only remaining hydrothermal water. Composition of extracted gas was analyzed by a modified residual gas analyzer for field observation. The measured mass spectra were corrected by molecular ionization cross sections and cracking patterns as well as detector sensitivity to determine the concentration of the gas species. The signal height of oxygen in a mass spectrum was considered to be contribution of atmospheric source gases in the dissolved gas. The amount of deep source gas in the dissolved gas was calculated by subtracting the amount of the standard atmospheric which are evaluated by the oxygen signal intensity (Giggenbach (1992)).
The dissolved gas composition in the KUM began to change slowly at the end of December 2021 and returned to its original level in April 2022. The ratio of atmospheric-origin to deep-origin gas in the hydrothermal water began to increase gradually in December 2021, peaked at the end of February 2022, and recovered to the original level in April 2022. The composition of dissolved gas in ASO became unstable around October 2021 and changed irreversibly in December 2021. The ratio of atmospheric-origin to deep-origin gas in the hydrothermal water showed an irreversible change in December 2021. On the other hand, the fraction of andesitic and basaltic magma in the deep-origin gas was generally constant at both KUM and ASO. Since no disturbance to hydrothermal wells due to human manipulation of the source or piping was recorded during the observation period at both stations, we believe these changes can be considered to be a compositional change in the dissolved gas.
Despite that the KUM and ASO are located approximately 40 km apart, the timing of the observed changes in dissolved gas is in close proximity. Although there were six earthquakes of magnitude 3.5 or greater in the area during the observation period, they did not relate to the origin time of these earthquakes, so the changes were not caused by seismic activity. Possible causes of the observed changes include crustal movement around the area and volcanic activity of Mt. Aso. We will discuss the results of continuous observations of hydrothermal dissolved gases at the two stations and whether they are related to crustal deformation and/or volcanic activity.
An automatic sampling extractor was developed to collect of dissolved gas in thermal water without exposing it to the atmosphere. The extractor, which has one peristaltic pump and two electromagnetic valves, can easily introduce hydrothermal water into an airtight container, extract dissolved gas, and drain only remaining hydrothermal water. Composition of extracted gas was analyzed by a modified residual gas analyzer for field observation. The measured mass spectra were corrected by molecular ionization cross sections and cracking patterns as well as detector sensitivity to determine the concentration of the gas species. The signal height of oxygen in a mass spectrum was considered to be contribution of atmospheric source gases in the dissolved gas. The amount of deep source gas in the dissolved gas was calculated by subtracting the amount of the standard atmospheric which are evaluated by the oxygen signal intensity (Giggenbach (1992)).
The dissolved gas composition in the KUM began to change slowly at the end of December 2021 and returned to its original level in April 2022. The ratio of atmospheric-origin to deep-origin gas in the hydrothermal water began to increase gradually in December 2021, peaked at the end of February 2022, and recovered to the original level in April 2022. The composition of dissolved gas in ASO became unstable around October 2021 and changed irreversibly in December 2021. The ratio of atmospheric-origin to deep-origin gas in the hydrothermal water showed an irreversible change in December 2021. On the other hand, the fraction of andesitic and basaltic magma in the deep-origin gas was generally constant at both KUM and ASO. Since no disturbance to hydrothermal wells due to human manipulation of the source or piping was recorded during the observation period at both stations, we believe these changes can be considered to be a compositional change in the dissolved gas.
Despite that the KUM and ASO are located approximately 40 km apart, the timing of the observed changes in dissolved gas is in close proximity. Although there were six earthquakes of magnitude 3.5 or greater in the area during the observation period, they did not relate to the origin time of these earthquakes, so the changes were not caused by seismic activity. Possible causes of the observed changes include crustal movement around the area and volcanic activity of Mt. Aso. We will discuss the results of continuous observations of hydrothermal dissolved gases at the two stations and whether they are related to crustal deformation and/or volcanic activity.