Study on groundwater flow and chemical quality change during water circulation in hard rock body such as the Nohi rhyolitic and the Abukuma granodioritic rock body by water quality analysis, stable isotope and tritium analysis and gas analysis.
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Concentrations of major-elements, stable isotopes, tritium and bubble gas components including carbon and hydrogen isotope of methane were determined on water samples collected from rivers, springs, hot or mineral springs and drilled wells throughout the distribution areas of acidic rocks such as the Nohi-rhyolitic rocks, central Japan, and the Abukuma granodioritic rocks, northeast Japan. As a studied area, several types of aquifer lithologies were selected including the Nohi-rhyolitic welded tuff, Mino-sedimentary rocks and granitic rocks in the Nohi district, and including granodiorite, Takanuki-biotite schist and Yamizo sandstone in the Abukuma district.<BR>Groundwater type was divided into ten groups. The change mechanisms in characteristics of water quality were identified in each hydrological groundwater flow systems, such as local flow system and regional flow system, and were clarified by the analysis of dissolved constituents and tritium concentration.<BR>Stable isotope analysis indicates that both local and regional groundwaters of the two districts are originated in meteoric water. On the other hand, these have different residence time. Tritium concentration of the regional groundwaters are relatively low, which indicates that the residence time of those springs is longer than that of the water of local flow system. Regional groundwaters of the Abukuma district show lower tritium concentration, which was caused by the nature of fracture. In the Nohi rhyolitic area regional groundwater is easy to mix with local water by the occurence of cracks.<BR>From water-mineral equilibrium consideration, it is concluded that the groundwater of relatively low salinity, high pH with sodium bicarbonate type (NR and AR groups) was formed by dissolution of surrounding acidic rocks with consuming carbon dioxide contained in soil, flowing through fracture zone.<BR>Gero hot spring was formed by mixture of the Nohi regional groundwater with NaCl hydrothermal water related to the Yugamine rhyolitic rocks. Asahi village Agricultural Administration Bureau's well water which is NaCl type with high salinity reveals an original water and gas composition, which indicates the characteristics of water included in the Mino sedimentary rocks.<BR>Inert gas composition indicates mixing of air saturated water with groundwaters included in the Mino sedimentary rocks. Yuya, Shitajima and Nigorigo waters have much contribution of air saturated water. Methane isotope ratios indicate that methanes in bubble gas were originated by mixture of thermogenic methane formed from kerogens with sedimentary methane in the Mino sedimentary rocks which is isotopically lighter. Bubble gas methane of Nigorigo has much contribution from volcanic or geothermal origin.<BR>Gas analysis indicates that the depth of fracture system related to water with coexisting free gas extends to the Mino sedimentary rocks beneath the Nohi rhyolitic rock body. NR group groundwater without carbon dioxide bubble gas flow within the Nohi rhyolitic rock. AR group groundwater distributed in the Abukuma granodioritic rock body recharged in mountainous area about 10 km apart, flow through permeable fault, interrupted by impermeable fault such as Yujimata fault and flow upward to surface.
- Japanese Association of Groundwater Hydrologyの論文
Japanese Association of Groundwater Hydrology | 論文
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