In order to determine the source and formation process of dissolved inorganic carbon (DIC) in spring water and to evaluate quantitatively the contribution of volcanic gas to water chemistry of springs distributed on and around Asama volcano, the carbon isotopic ratio of DIC (δ
13C
DIC) with major dissolved solids has been measured. The measurements of carbon isotopic ratios of volcanic and soil CO
2, which are the source materials of DIC, were also carried out in Jigokudani fumarole and in the forest soil of several points of volcano flank, respectively.
The spring waters in Asama volcano have been classified into nine groups (A∼I) based on the physicochemical characteristics, such as water temperature, electrical conductivity and chemical compositions. As δ
13C
DIC increase with increasing DIC content, the origin of DIC in spring water from Asama volcano was can be assessed by mixing process between isotopically light soil CO
2 (organic origin) and
13C-enriched volcanic CO
2 (deep origin with mantle component), except for the springs of group B. On the basis of two components mixing, the contribution rate of volcanic CO
2 to DIC in spring water was computed by using the carbon isotopic ratio of CO
2 equilibrated with DIC (δ
13C
CO2) as an indicator. Consequently, the contribution rates of volcanic CO
2 were ranged from 40 to 60% in the groups C, F and H located on the flank of the mountain. In particular, the strong contribution of more than 90% was confirmed in the group I located on the higher part of the mountain, that is near the crater. These groups were correspondent with those in which influence of volcanic gases was assumed from the geochemical characteristics of spring water. By contrast, influence of volcanic CO
2 was almost not found in other groups A, D, E and G.
The spring waters of group B which are not plotted on the two components mixing line and located at the terminal of Onioshidashi lava flow have highest δ
13C
DIC in spite of low DIC content. These
13C-enriched spring waters are probably derived from dissolved CO
2 degassing of thegroundwater affected by volcanic CO
2 during the discharge process. Since the groundwater moves in the clinker, which is fractured zone developed in lower and upper part of the lava flow and is extremely porous, as not laminar flow but turbulent flow, the CO
2 degassing would be effectively caused.
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