Hot Spring Waters Alert Volcanic Eruptions

Volcanic eruptions can cause not only pyroclastic flows and the ejection of
volcanic rocks, but also landslides and tsunamis. Worldwide, more than 1,500
active volcanoes have the potential to produce such hazards, and about one billion
people live in at-risk areas.
To help reduce the impact of volcanic disasters, we attempted to predict eruptions
at Sakurajima Volcano. Before an eruption, changes in volcanic activity can cause
gases released from magma to dissolve into nearby groundwater and hot spring
water. We therefore investigated whether hot spring water chemistry could be used
as an indicator of impending eruptions.
We analyzed chemical components related to volcanic gases—CO₂ and SO₂,
which can increase when magma intrudes beneath Sakurajima, and HCl and HF,
which are expected to be released from the magma system shortly before an
eruption. We detected compositional changes that preceded the continuous
eruption sequence in May 2025. To make monitoring feasible in the field, we
developed and improved (i) a simple, low-cost, rapid method to measure total
carbonate concentration without expensive or time-consuming instruments, and
(ii) a hot spring water analysis method using a homemade spectrophotometer. Our
results suggest a two-step chemical warning system: an increase in the molar
[Cl]/[Na] ratio about 3–4 months in advance as a first warning sign, followed by an
increase in F⁻ concentration about one month in advance as a second warning sign.
These indicators may help support future eruption forecasting and disaster
preparedness