Changes in crater lake chemistry after the 2021 eruption at Aso Volcano, Japan: insights from UAV-based hot water sampling

• Development and application of a novel UAV-based hot lake water sampling technique. • High-temperature crater lake water chemistry at Aso Volcano soon after the 2021 eruption. • Anion and isotope variations indicate rapid transitions in the hydrothermal system. Several volcanoes in Japan have summit craters filled with water, including Zao, Kusatsu-Shirane, Kirishima, and Aso volcanoes. Aso Volcano, located in central Kyushu, is an active caldera volcano with a crater lake whose water volume varies from nearly zero to full capacity depending on volcanic activity. Following the latest eruption of Aso Volcano in October 2021, we conducted UAV-based hot water sampling at the Nakadake First Crater (NFC) in 2022, 2023, and 2024. These efforts enabled the development of sampling method using a weight, rope, and sterilized sampling bottle with a long fluorinated ethylene propylene (FEP) sleeve, allowing hot water collection at temperatures higher than previously possible. We found that crater lake chemistry changed drastically within one year, with sulfate concentrations decreasing rapidly from 211,900 ppm in 2022 to 19,940 ppm in 2023 (approximately one-tenth), while temperature changes were smaller (about 5 °C). Compared with intermittent records from 1993 to 2009, these sulfate values were up to twice as high as previously reported maxima. Such deviations suggest increased inputs of magma-derived fluids, possibly associated with the 2021 eruption. Based on compiled data, we propose that extremely high chlorine concentrations in lake water can serve as significant indicators for predicting forthcoming eruptions, regardless of temperature changes. Our findings indicate that short-interval monitoring of the NFC is necessary to understand the transition between open and closed hydrothermal systems, especially after eruptions. Combined with UAV-based methods capable of sampling during eruptions, this approach will enhance understanding of crater lake hydrothermal dynamics and eruption forecasting.