[A-HW25_2AM2] Isotope Hydrology 2014

[AHW25-09] Contribution of volcanic gas to spring waters in the Mt. Yotei.

*Yukiko KUSANO¹, Masaya YASUHARA², Kazuyoshi ASAI³, Akihiko INAMURA², A., hiroshi TAKAHASHI², Noritoshi MORIKAWA² (1.The University of Tokyo, 2.Geological Survey of Japan, AIST, 3.Geo science Laboratory Co. Ltd.)

The Mt. Yotei is a stratovolcano in southwest Hokkaido with an altitude of 1893 m. There are a lot of springs in the foot of the mountain and most of these are found from the altitude of 200 to 260 m. This elevation is consistent with that of boundary between volcanic rocks of the volcano and basement consists of tuff and pumice. Northern, eastern, and southern slopes of the mountain are covered by pyroclastic fall and reworked deposits of that, while the western slope is covered by lava erupted from summit and parasitic volcanos in western slope (Katsui, 1956). It is revealed that springs in eastern and southern slopes tend to show lager discharge and lower dissolved components, while those in the western slope tend to show smaller discharge and higher dissolved components (Yamaguchi and Sato, 1971; Yamaguchi, 1972; Tsurumaki, 1989). Because of higher HCO₃^- and free CO₂ gas concentrations and higher temperature of springs in the western slope, addition of volcanic gas to those is estimated (Yamaguchi and Sato, 1971; Tsurumaki, 1989). However, contribution of volcanic gas to groundwater has not been evident. In this study, contribution of volcanic gas to spring waters will be discussed based on dissolved components and isotopic compositions.Spring water samples were collected on August 2013, and were analyzed for major dissolved components, isotopic compositions (δD, δ¹⁸O, δ¹³C, ³He/⁴He), and groundwater-age indices (CFCs and SF₆). Dissolved components of spring waters are Ca-HCO₃ type or Na-HCO₃ type. Spring waters in the western slope tend to show higher HCO₃^- and free CO₂ gas concentrations. These results are consistent with those of previous studies. δ¹³C of dissolved inorganic carbon (DIC) in spring waters in eastern and southern slopes are -21.7 to -17.7 ‰, while those of the western slope are -18.1 to -3.0 ‰. This result suggests that spring waters in western slope contain DIC of which origin is different from that contained in springs in eastern and southern slopes. Dissolution of marine carbonates is unlikely because groundwater flow in volcanic rocks. Relationship between δ¹³C and inverses of concentrations of total carbon are plotted in mixing zone of volcanic CO₂ and soil CO₂, and spring water showing higher δ¹³C and total carbon concentration were correspond to higher contribution of volcanic CO₂. These results suggest that spring waters in western slope showed higher δ¹³C and total carbon concentrations because of addition of volcanic CO₂. δD and δ¹⁸O of spring waters are plotted along the meteoric water line (GSJ, undisclosed data), suggesting contribution of magmatic water is quite small. Effect of parasitic volcanos in the wastern slopes is suggested by Yamaguchi and Sato (1971) as a possible factor for higher contribution of volcanic gas to springs in the western slope. Further discussions based on dissolved components, isotopic compositions, and groundwater-age indices will be performed to reveal relationship between contribution of volcanic gas and groundwater flow system.ReferencesKatsui, Y., 1956. Explanatory text of geological map of Japan, scale 1: 50,000, Rusutsu, appendix, Geological Curvey of Hokkaido, p14.Tsurumaki, M., Journal of Groundwater Hydrology, 31, 3, 165-173.Yamashita, K. and Sato, I., 1971. Report of of spring water survey in the Mt. Yotei, Geological Survey of Hokkaido, p27.Yamashita, K., 1972. Journal of Geography, 81, 5, 4-20.