ABSTRACT

A change in the chemical composition of Obuki Spring, a typical chloride-sulfate acid hot spring in Japan, has been revealed by continuous observation for more than 40 years. The concentration of SO₄^2- increased markedly from 1972 (∼1 g/l) to 1978 (∼3 g/l), then decreased gradually from 1978 to 1990 (∼1 g/l). Variations in the Cl^- concentration were apparently within a smaller range than those of the SO₄^2- concentration. The concentrations of numerous cations increased during 1972–1990, as did the SO₄^2- concentrations. This variation is thought to be the result of a mechanism involving a second source, namely, volcanic emanations of a sulfur-rich composition, which were temporarily added to emanations from the first source; the first source has continuously supplied volcanic emanations to the original acid hydrothermal solution at Obuki Spring, and the second source is responsible for the increase in SO₄^2-, Cl^-, and acidity. Variations in the concentrations of most other components can be explained as the result of changes in the interaction between the acid hydrothermal solution and the surrounding rocks; the latter changes were themselves the consequence of increases in acidity.