Seasonal variation in end-members and discharge mechanisms of a caldera spring with multiple groundwater flow systems: insights from multi-tracer analysis in the Aso Caldera, Japan

A hydrogeochemical investigation was conducted at Takezaki Spring, located in the Nango Valley within the Aso Caldera – one of the largest caldera volcanoes in the world – where the spring is thought to be formed by the mixing of multiple groundwater flow systems. The study was conducted from October 2024 (wet season) to June 2025 (dry season). Seasonal variations in dissolved ion concentrations and stable isotope ratios of oxygen and hydrogen revealed that, toward the dry season, the contribution of groundwater from the central cone flow system – characterized by a larger and more stable flow – became increasingly dominant in the spring discharge. In addition, a marked rise in the oxygen isotope ratio and an increase in discharge was observed after mid-April, when paddy field irrigation commenced upstream. These observations suggest that irrigation water may have significantly contributed to spring discharge even during the dry season. Analysis using multiple hydrological tracers clarified the seasonal and temporal variability of end-members from the wet to the dry season.