MONITORING OF THE HYPERACID LAKE AT POÁS VOLCANO (COSTA RICA) USING DRONES: GEOCHEMICAL VARIATIONS FROM NOVEMBER 2023 TO MARCH 2025
The AVERT project (Anticipating Volcanic Eruptions in Real-Time) has been studying the active crater lake of Poás Volcano in Costa Rica, which is one of the most acidic natural water bodies on Earth, with extreme chemical conditions that challenge conventional sampling methods. Between November 2023 and March 2025, access to the crater was restricted due to volcanic unrest, explosions and the accumulation of toxic gases. To maintain the geochemical monitoring series, water samples were collected using a UAV (uncrewed aerial vehicle) system conditioned with a 3D printed device to carry a bayler bottler of 250 mL. Laboratory results showed a progressive acidification of the lake, a diminution of the water lake level, with pH values dropping from 0.13 in November 2023 to –1.04 in April 2024 and -1 in March 2025. Sulfate concentrations exceeded 130,000 ppm in some samples (May 2024 and March 2025), with high levels of fluoride (> 4000 ppm), magnesium (> 5000 ppm), and lithium (< 3.6 ppm). Increases in sodium and potassium were also observed during the most acidic months. The SO₄/Cl and Mg/Cl molar ratios revealed temporal variations in hydrothermal input, fluid-rock interaction, and phase separation processes within the volcanic system. This work demonstrates the effectiveness of drone-assisted sampling in active volcanoes with extreme volcanic environments and emphasizes the importance of continuous lake monitoring as a tool for understanding shallow hydrothermal dynamics and magmatic activity.