Time Series Change in the Chemistry of Hot Pool Water Collected from Fumarole Areas around Ebinokogen Ioyama

After the eruption of Ebinokogen Ioyama in April 2018, the formation of hot water pools at some craters was observed. Time-series changes in the chemical composition of the hot pool waters are studied over the period of more than six years from July 2018, with a view to monitoring time variations of water chemistry in the waning stage of geothermal activities. According to the observed time-series changes in fumarole and pool water temperatures, geothermal activities in the Ioyama fumarole area are in a waning stage in Period I (July, 2018–May, 2019) and Period II (Aug., 2019–July, 2021), whereas they show an increasing trend in Period III (Aug., 2021–Oct., 2022) and Period IV (Nov., 2022–Dec., 2024). The water chemistry of hot pools Y2a and Y2b in this area shows time-series changes from a rather high pH (up to pH = 2.2) and a low Cl/SO4 molar ratio (< 0.02) in Period II to an extremely low pH (pH < 0) and a high Cl/SO4 molar ratio (> 10) in Period IV, which is in accordance with that of geothermal activities. On the other hand, geothermal activities in the Ioyama-west fumarole area show a peak in Period II, when the temperature of the fumarole beside the hot pool W4 in this area reached 125°C. At around the same time, the hot pool water chemistry showed fluctuations in pH (pH = 0.5-1.6) and in Cl/SO4 (Cl/SO4 = 1.0-2.7). After hot pool W4 disappeared at the end of Period II, the water of another hot pool M8 in this area showed a rather constant chemistry, pH = 1.1-1.7 and Cl/SO4 = 0.01-0.04. From observations in these two areas, time-series changes of pH in hot pool waters are likely to reflect a flux of the fumarole that supplies magmatic-derived Cl and SO4 into the hot pool. On the other hand, Cl/SO4 in hot pool waters could be affected by several factors. The observed low Cl/SO4 in hot pools Y2a and Y2b in Period II is attributed to a process in an underground hydrothermal reservoir where HCl is separated from the fumarole due to preferential partitioning into the liquid phase. Whereas, the observed high Cl/SO4 ratio in hot pools Y2a and Y2b in Period IV is likely to reflect the short residence time of magmatic gas in the hydrothermal reservoir due to an extremely high fumarole flux. It is difficult to find a good correlation between the observed time-series changes of hot water chemistry and magmatic activity of the Ebionokogen Ioyama represented by frequency of earthquakes. On the other hand, a sharp decrease of the Cl/SO4 ratio in some hot pool waters is observed just after heavy rainfall in this area. This result suggests that the ground-water systems beneath the fumarole area are substantially affected by a large amount of meteoric waters being supplied. The contribution of meteoric water could be a specific factor, but it is also one of the important factors for understanding the geochemical process in the hydrothermal system of Ebionokogen Ioyama located in the area rich in rainfall.