ABSTRACT

Chemistry of surface waters from the Tateyama caldera known as a site of frequent landslides in northern central Japan was investigated to obtain the relationship between their geochemical characteristics and the landslide density in the watershed. Most waters in the caldera have water chemistry of a Ca–SO₄ type with high total ionic concentrations. Stream waters originating from the landslide areas have notably high sulfate concentration. Dissolution of rock-forming minerals such as plagioclase by sulfuric acid produced by oxidation of hydrothermal pyrite that was formed during alteration of volcanic rocks is responsible for the high calcium and sulfate concentration. The δ³⁴S values of dissolved sulfate in the surface waters support this view. Intense rock weathering at the head areas of sulfate-rich streams is likely to have caused the 1964 large-scale landslide there. Positive correlation between SO₄²⁻ concentration in the streams and a ratio of landslide area to the catchment area indicates a high probability of landslide in the area where SO₄²⁻ concentration in water is high. Hydrogeochemical information at landslide areas has a potential of identifying the area of future landslide.