[A-HW30] Isotope Hydrology 2025

[AHW30-08] Seasonal variation analysis of end-members composing Aso Caldera spring water using multiple tracers

*MORI Ueyama¹, MAKOTO Kagabu¹ (1.Faculty of Environmental Science, Nagasaki University)

Keywords:spring water, end-member analysis, seasonal variation, major ions, stable isotopes, eDNA

The Aso Caldera, located in central Kyushu, is home to numerous springs, which have been extensively studied. This study focuses on the Takezaki Spring, situated in the central Nangoudani Valley in the southern Aso Caldera, which exhibits one of the highest discharge volumes among the caldera's springs. The characteristics of spring water, including water quality and stable isotope ratios, vary significantly depending on the discharge location within the caldera. These variations have led to the broad classification of two primary groundwater flow systems: the "central cones system" and the "caldera rim mountain system".Located at the boundary of these two groundwater flow systems, the Takezaki Spring exhibits unique environmental tracer characteristics. Water quality, oxygen and hydrogen stable isotope ratios fluctuate seasonally, reflecting the influence of both flow systems and their distinct end-members. This study hypothesizes that the end-members of the two major flow systems contributing to the Takezaki Spring's discharge exhibit seasonal variations in their mixing ratios. To investigate this hypothesis, multiple tracers, including isotope ratios, dissolved ions, dissolved gases, and environmental DNA, will be employed. Oxygen and hydrogen stable isotope ratios are influenced by altitude. Groundwater originating from the "central cones system," characterized by a higher recharge elevation, exhibits lower isotope ratios. Consequently, the contribution of each flow system can be assessed by analyzing fluctuations in these ratios. This study specifically examines sulfate and bicarbonate ions as dissolved component tracers. Previous research has demonstrated that springs associated with the "central cones system" exhibit higher sulfate ion concentrations, suggesting that sulfate ions can serve as an indicator of the "central cones system" contribution. Conversely, bicarbonate ions, which are characteristically high in the "caldera rim mountain system," are considered a parameter for evaluating the contribution of this system. Dissolved gases like CFCs and SF₆, employed for groundwater age estimation, suggest that the "central cones system," with its higher recharge elevation, has a larger flow system compared to the "caldera rim mountain system," resulting in a longer residence time. While the application of environmental DNA to spring water is limited, the Takezaki Spring presents a unique opportunity. The groundwater of the "central cones system," influenced by volcanic activity, may contain unique DNA, potentially serving as a novel tracer for capturing seasonal fluctuations in end-members.
Three surveys conducted to date (October 2024, December 2024, and January 2025) have revealed seasonal fluctuations in dissolved components and oxygen and hydrogen stable isotope ratios. Analysis of dissolved components indicated an increase in sulfate ion concentration from October to January, accompanied by a decrease in bicarbonate ion concentration. Furthermore, oxygen and hydrogen stable isotope ratios exhibited a gradual decrease from October to January. The observed seasonal fluctuations in these tracers are consistent with the hypothesis that the contribution of the "central cones system" increases from autumn to winter, a relatively dry period. This phenomenon is likely attributed to the increased abundance of "central cones system" groundwater, which, due to its larger flow volume, is less susceptible to seasonal precipitation variations compared to the "caldera rim mountain system".