4:30 PM - 4:45 PM
[SCG59-05] Time series variation in radon concentration in hot spring water at the rim of Aira Caldera
★Invited Papers
Keywords:222Rn, groundwater, crustal fluid
The aim of our research is to understand various phenomena associated with crustal deformation from the geochemistry of fluids. We have been regularly sampling hot spring waters around the Aira caldera in Kagoshima Prefecture, and have found that the concentration of radon (222Rn) in each hot spring water has a unique variation depending on the region. The concentration of radon in water is known to vary due to Ra concentration and specific surface area, i.e., rock type and rock fracturing. In this presentation, we show the results of radon concentration measurements in hot springs around the Aira Caldera and present the development of our automatic continuous radon concentration measuring device.
Aira Caldera is a huge caldera that forms Kagoshima Bay. The active volcano "Sakurajima" and the underwater volcano "Wakamiko" are very active. Under the influence of the volcano, there are many hot springs inside and outside the Aira caldera under the influence of the volcano. Over the past six years, we have regularly (once a month) sampled hot spring waters around the Aira caldera to investigate the relationship between crustal deformation and changes in fluid constituents. We have focused on the occasional spike-like fluctuations in radon concentrations in hot spring waters in the eastern part of the Aira caldera (Tarumizu City area). These variations are thought to be caused by changes in the stress state of the crust itself (increased fracturing and porosity) or by mixing of groundwater of different origins due to changes in stress state. Detailed clarification of the nature of the crustal deformation to which these changes in radon concentration respond is expected to lead to prediction of crustal deformation using fluids in the future. However, the current observation interval is monthly, and although it is possible to read annual cyclic variations such as seasonal changes, it is difficult to correlate these with large crustal deformations that occur unexpectedly. To solve this problem, it is necessary to decrease the data acquisition interval. However, the remote location of each hot spring makes it almost impossible to take more timely measurements by hand. Therefore, we have tried to install an automatic radon measurement device at each hot spring. The data will be continuously monitored.
In order to develop an automatic continuous radon measurement device, short-term observations were first carried out. It was known from the previous monthly sampling, that the radon concentration in the hot spring water in Tarumizu City changes in a spike-like trend. The measurement interval for automatic monitoring could not be determined without knowing the smallest period over which such changes occur. Therefore, we measured radon in the hot spring every two hours for 42 hours in the field. It was found that the radon concentration changed more than fourfold within a short period of two hours. Such rapid changes in concentration may be related to crustal deformation.
We have developed a radon gas measurement device . A separator was made to separate radon gas in water into gas phase, and a gas line was constructed to carry the gas from the separator through a drying device and desiccant to the radon meter. Three three-way valves were used to purge all gas lines in stages between measurements. The valves, radon meter, pumps, and data acquisition are controlled from a PC using the NI module. The presentation will include details of the radon time-series changes and the development of the instrument.
Aira Caldera is a huge caldera that forms Kagoshima Bay. The active volcano "Sakurajima" and the underwater volcano "Wakamiko" are very active. Under the influence of the volcano, there are many hot springs inside and outside the Aira caldera under the influence of the volcano. Over the past six years, we have regularly (once a month) sampled hot spring waters around the Aira caldera to investigate the relationship between crustal deformation and changes in fluid constituents. We have focused on the occasional spike-like fluctuations in radon concentrations in hot spring waters in the eastern part of the Aira caldera (Tarumizu City area). These variations are thought to be caused by changes in the stress state of the crust itself (increased fracturing and porosity) or by mixing of groundwater of different origins due to changes in stress state. Detailed clarification of the nature of the crustal deformation to which these changes in radon concentration respond is expected to lead to prediction of crustal deformation using fluids in the future. However, the current observation interval is monthly, and although it is possible to read annual cyclic variations such as seasonal changes, it is difficult to correlate these with large crustal deformations that occur unexpectedly. To solve this problem, it is necessary to decrease the data acquisition interval. However, the remote location of each hot spring makes it almost impossible to take more timely measurements by hand. Therefore, we have tried to install an automatic radon measurement device at each hot spring. The data will be continuously monitored.
In order to develop an automatic continuous radon measurement device, short-term observations were first carried out. It was known from the previous monthly sampling, that the radon concentration in the hot spring water in Tarumizu City changes in a spike-like trend. The measurement interval for automatic monitoring could not be determined without knowing the smallest period over which such changes occur. Therefore, we measured radon in the hot spring every two hours for 42 hours in the field. It was found that the radon concentration changed more than fourfold within a short period of two hours. Such rapid changes in concentration may be related to crustal deformation.
We have developed a radon gas measurement device . A separator was made to separate radon gas in water into gas phase, and a gas line was constructed to carry the gas from the separator through a drying device and desiccant to the radon meter. Three three-way valves were used to purge all gas lines in stages between measurements. The valves, radon meter, pumps, and data acquisition are controlled from a PC using the NI module. The presentation will include details of the radon time-series changes and the development of the instrument.