1:45 PM - 3:15 PM
[SCG59-P02] Geochemical signals from spring water related to crustal activities in Southern Kyusyu
Keywords:Radon, Hutagawa fault, Hinagu fault, Hot spring
1.Introduction
There are a large number of hot springs in Japan. Groundwater which is the major source of these springs contains a wide variety of chemical components. Many studies have been discussed the status of subsurface conditions and/or groundwater provenance using groundwater chemistry. Studies on faulting and geology using radon concentration in the groundwater, for example, and studies on the origin of water using dissolved ions and isotope ratios are the key topics in this research area. The purpose of this study is to clarify the relationship between groundwater and geology or geological structures such as faults in southern Kyushu based on sampling of hot and cold spring water in Kagoshima Prefecture and Yatsushiro Sea area in Kumamoto Prefecture.
2.Sampling Procedures
Sampling of hot springs was conducted over a wide area around the Yatsushiro Sea in Kumamoto Prefecture and the whole Kagoshima Prefecture. Samples were taken avoiding atmospheric air contamination as much as possible, and were basically brought back to the laboratory. The radon concentration and ions were analyzed at our laboratory, and the stable isotopes of hydrogen and oxygen were analyzed by an external institution. Some samples were analyzed at AIST.
3.Result and discussing
Hydrogen and oxygen isotope ratios are plotted mostly along the Meteoric Water Line, indicating that the samples are basically meteoric in origin. However, in some samples located north of the Yatsushiro Sea area, oxygen isotope ratios show a large positive shift, suggesting the mixture of fluids originating from andesitic magma. Although the major ions are basically characterized by regional geology or fault. Some samples on the north of the Yatsushiro Sea area showed anomaly in their isotope ratios. For several samples, Li-/Cl- index indicates deep-originated fluid such as magmatic water (Kazahaya et al., 2014), suggesting that the fluid or gas components are upwelling through the Hinagu and Hutagawa fault zones. In terms of radon concentrations, many of the samples show a strong influence of geology, especially in the granite zone, where the concentrations are very high compared to the surrounding areas. This is in harmony with previous studies (e.g.; Toshihiro et al., 2019). It is also possible that the results imply the groundwater flow paths originating from those granites. On the other hand, in the area around the Yatsushiro sea, there were several samples with significantly high concentrations around the Hinaku fault zone. Considering the location being near the Yatsushiro sea section of the Hinagu fault and the results of other analyses, high radon concentration may be resulted from the fault fracture zones.
4.Reference
Kazahaya, K., Takahashi, M., Yasuhara, M., Nishio, Y., Inamura, A., Morikawa, N., Sato, T., Takahashi, H., Kitaoka, G., Osawa, S., Oyama, Y., Owada, M., Tsukamoto, H., Horiguchi, K., Tosaki, H. and Kirita, J. (2014): Distribution and characteristics of deep fluids of slab origin in southwest Japan, JJHM, 44, 3-16
Toshihiro K., Fujiwara M. and Hatanaka K. (1996) : Radon concentration in groundwater in Yamaguchi Prefecture, Yamaguchi Eisei Kenkyu Gyoho, 17, 22-25
There are a large number of hot springs in Japan. Groundwater which is the major source of these springs contains a wide variety of chemical components. Many studies have been discussed the status of subsurface conditions and/or groundwater provenance using groundwater chemistry. Studies on faulting and geology using radon concentration in the groundwater, for example, and studies on the origin of water using dissolved ions and isotope ratios are the key topics in this research area. The purpose of this study is to clarify the relationship between groundwater and geology or geological structures such as faults in southern Kyushu based on sampling of hot and cold spring water in Kagoshima Prefecture and Yatsushiro Sea area in Kumamoto Prefecture.
2.Sampling Procedures
Sampling of hot springs was conducted over a wide area around the Yatsushiro Sea in Kumamoto Prefecture and the whole Kagoshima Prefecture. Samples were taken avoiding atmospheric air contamination as much as possible, and were basically brought back to the laboratory. The radon concentration and ions were analyzed at our laboratory, and the stable isotopes of hydrogen and oxygen were analyzed by an external institution. Some samples were analyzed at AIST.
3.Result and discussing
Hydrogen and oxygen isotope ratios are plotted mostly along the Meteoric Water Line, indicating that the samples are basically meteoric in origin. However, in some samples located north of the Yatsushiro Sea area, oxygen isotope ratios show a large positive shift, suggesting the mixture of fluids originating from andesitic magma. Although the major ions are basically characterized by regional geology or fault. Some samples on the north of the Yatsushiro Sea area showed anomaly in their isotope ratios. For several samples, Li-/Cl- index indicates deep-originated fluid such as magmatic water (Kazahaya et al., 2014), suggesting that the fluid or gas components are upwelling through the Hinagu and Hutagawa fault zones. In terms of radon concentrations, many of the samples show a strong influence of geology, especially in the granite zone, where the concentrations are very high compared to the surrounding areas. This is in harmony with previous studies (e.g.; Toshihiro et al., 2019). It is also possible that the results imply the groundwater flow paths originating from those granites. On the other hand, in the area around the Yatsushiro sea, there were several samples with significantly high concentrations around the Hinaku fault zone. Considering the location being near the Yatsushiro sea section of the Hinagu fault and the results of other analyses, high radon concentration may be resulted from the fault fracture zones.
4.Reference
Kazahaya, K., Takahashi, M., Yasuhara, M., Nishio, Y., Inamura, A., Morikawa, N., Sato, T., Takahashi, H., Kitaoka, G., Osawa, S., Oyama, Y., Owada, M., Tsukamoto, H., Horiguchi, K., Tosaki, H. and Kirita, J. (2014): Distribution and characteristics of deep fluids of slab origin in southwest Japan, JJHM, 44, 3-16
Toshihiro K., Fujiwara M. and Hatanaka K. (1996) : Radon concentration in groundwater in Yamaguchi Prefecture, Yamaguchi Eisei Kenkyu Gyoho, 17, 22-25