In order to understand the hydrogeochemical processes of spring water affected by volcanic gases on northern and eastern flanks of Asama volcano, central Japan, sulfur isotopic ratio (δ³⁴S) were analyzed. The groundwater flow pattern in relation to the structure of a composite volcano with large-scale collapse during the eruptive process was also discussed based on the distribution pattern and chemical evolution of spring water.
According to previous studies using a major anion compositions and a carbon isotopic ratio, the spring waters affected by volcanic gases were classified into two types. One is neutral sulfate-bicarbonate or sulfate-chloride types with a large amount of chloride, in the northern and eastern flanks of Mt. Maekake (younger volcano), and the other is bicarbonate-rich and acid sulfate-rich types with a little amount of chloride, located on Jigokudani fumarole, in the southern slope. The former δ³⁴S values were higher (8.8∼15.9‰) and the latter one, acid sulfate-rich type, was lower (2.2‰). On the basis of the difference of both δ³⁴S values, the chemical evolutions of both types were proposed as follows:
The former is formed by mixing of shallow groundwater and deeply acid sulfatechloride thermal water with neutralization by rock dissolution. On the other hand, the latter is formed by contacting groundwater with low-temperature diffusive volcanic gases, as H₂S or CO₂ near the ground surface.
The fact that the influence of acid sulfate-chloride type water which may be formed beneath the crater of Mt. Maekake (active younger volcano) can be found in the spring waters distributed on northern and eastern flanks suggests that the groundwater preferentially flows north and⁄or east directions in the body of Mt. Maekake. Because Mt. Maekake has grown on a large scale collapse caldera of Mt. Kurofu (older volcano), groundwater flow in the mountain may be strongly restricted by the shape of the collapse caldera inclined down to northeast direction.

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The author has carried out the analysis of tritium and stable isotopes of surface water and groundwater to evaluate the groundwater flow system in the Yiluo River basin, China.
1) Tritium concentrations of the groundwater in the slope of the southern and northwestern mountain range were higher than 10 T.U. and suggested that the residence time of these groundwater was younger than 50 years. On the other hand, tritium concentrations of the deep groundwater along the Yiluo River and in the lower area of the basin were very low. Therefore, the residence time were estimated to be longer than 60 years.
2) The stable isotopic ratios of hydrogen and oxygen of the groundwater showed relatively low value in the southern peripheral part of the basin and showed relatively high value in the central part. In the eastern part, down-stream part, and the southeastern part of the basin, groundwater with low δD and δ¹⁸O flows toward the lower area of the basin.

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