Hydrogeochemical characteristics and the monitoring from 2020 to 2023 of hot spring waters in Tatun Volcanic Group, Taiwan

The Tatun Volcanic Group (TVG) is considered potentially active as indicated by the chemistry of fumarole gases and seismological data. The presence of magma volatiles in thermal water serves as a key signal of volcanic activity, making their monitoring crucial. However, other factors such as water–rock interactions can markedly obscure the water chemistry. The hydrogeochemical understanding of each hot spring site is essential in clarifying the source of components in hot springs. Based on the knowledge, the changes sourced from magma volatiles can be identified and help efficient monitoring of volcanic activities. In this study, nine spatially distributed hot spring sites were observed and monitored for 3 years in TVG. The monitored items include pH and major ions SO₄^2-, Cl^-, concentrations of total B, Na, Mg, Ca, K, Si, Al, Fe, Sr, Ba, ΣRFE, along with their ratios (B/Cl^-, Cl^-/SO₄^2-). Additionally, strontium isotope ratios (⁸⁷Sr/⁸⁶Sr), hydrogen isotopes (δ²H), and oxygen isotopes (δ¹⁸O) were also measured, albeit at a lower frequency. Based on the field observation and the comprehensive geochemical observation, we fully discussed the hydrogeochemical characteristics of each hot spring site. Recommendations are provided along with a database of three years’ worth of monitoring results, offering valuable information for future monitoring efforts.