Tamagawa hyper-acidic hot spring and phreatic eruptions at Mt. Akita-Yakeyama Volcano: Part 2. Secular variations of SO₄/Cl ratios and their relationship to the phreatic eruptions

More than 70 years observation of the SO₄²⁻/Cl⁻ molar ratios of water from the Obuki hot spring in the vicinity of Mt. Akita-Yakeyama volcano indicated that the ratio started increasing rapidly from 1973, peaked in 1980 (SO₄²⁻/Cl⁻ = 0.34), remained at high values for about 18 years, then gradually decreased and returned to a normal baseline value (0.14) in 1991. The volcano erupted near the summit in 1948–1957 and again in 1997. The 1980 peak of SO₄²⁻/Cl⁻ ratios may reflect the 1948 phreatic eruption of the volcano. Similarly, high SO₄²⁻/Cl⁻ ratios were also observed in 2013 lasting for two years. The SO₄²⁻/Cl⁻ peak in 2013 appeared 16 years after the 1997 phreatic explosion. The delayed appearance of the SO₄²⁻/Cl⁻ peaks may represent slow upward movement of reservoir water to the surface after input of magmatic SO₂ into the fluid reservoir which produced HSO₄⁻ through disproportionation reaction. The slow movement of water is supported by a long subsurface residence time (13–25 years) of reservoir fluid estimated from tritium measurement of the Obuki hot spring water. One-dimensional channel model was applied to analyze the effect of volcanic activity on SO₄²⁻/Cl⁻ ratios of the Obuki hot spring water. The model supported the long travel time of the fluid implying that the effects of the 1997 eruption may appear in the spring water chemistry after about 29 years later, i.e., around 2026.