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This study presents the first hydrogeochemical model of the hydrothermal systems of Turrialba and Irazú volcanoes in central Costa Rica, manifested as thermal springs, summit crater lakes, and fumarolic degassing at both volcanoes. Our period of observations (2007–2012) coincides with the pre- and early syn-phreatic eruption stages of Turrialba volcano that resumed volcanic unrest since 2004, after almost 140 years of quiescence. Peculiarly, the generally stable Irazú crater lake dropped its level during this reawakening of Turrialba. The isotopic composition of all the discharged fluids reveals their Caribbean meteoric origin. Four groups of thermal springs drain the northern flanks of Turrialba and Irazú volcanoes into two main rivers. Río Sucio (i.e. “dirty river”) is a major rock remover on the North flank of Irazú, mainly fed by the San Cayetano spring group. Instead, one group of thermal springs discharges towards the south of Irazú. All thermal spring waters are of SO4-type (i.e. steam-heated waters), none of the springs has, however, a common hydrothermal end-member. A water mass budget for thermal springs results in an estimated total output flux of 187 ± 37 L/s, with 100 ± 20 L/s accounted for by the San Cayetano springs. Thermal energy release is estimated at 110 ± 22 MW (83.9 ± 16.8 MW by San Cayetano), whereas the total rock mass removal rate by chemical leaching is ~ 3000 m3/year (~ 2400 m3/year by San Cayetano-Río Sucio). Despite Irazú being the currently less active volcano, it is a highly efficient rock remover, which, on the long term can have effects on the stability of the volcanic edifice with potentially hazardous consequences (e.g. flank collapse, landslides, phreatic eruptions). Moreover, the vapor output flux from the Turrialba fumaroles after the onset of phreatic eruptions on 5 January 2010 showed an increase of at least ~ 260 L/s above pre-eruptive background fumarolic vapor fluxes. This extra vapor loss implies that the drying of the summit hydrothermal system of Turrialba could tap deeper than previously thought, and could explain the coincidental disappearance of Irazú’s crater lake in April 2010.

<p>On September 19, 2021, a volcanic eruption began at the west flank of Cumbre Vieja, La Palma, the most northwestern of the Canary Islands. The lava flows caused the evacuation of thousands of residents living in the vicinity of the volcano, and 1,219 hectares were covered by lava flows. After 85 days of activity, the eruption ended on December 13, 2021. Since visible volcanic gas emissions (fumaroles, hot springs, etc.) do not occur at the surface environment of Cumbre Vieja, the geochemical program for the volcanic surveillance has been focused mainly on diffuse (non-visible) degassing studies. Since 2001, diffuse CO<sub>2</sub> emission surveys have been yearly performed in summer periods to minimize the influence of meteorological variations. Measurements of soil CO<sub>2</sub> efflux have been performed following the accumulation chamber method in about 600 sites and spatial distribution maps have been constructed following the sequential Gaussian simulation (sGs) procedure to quantify the diffuse CO<sub>2</sub> emission from the studied area. In the period 2001-2016, the diffuse CO<sub>2</sub> output released to the atmosphere from Cumbre Vieja volcano ranged between 320 to 1,544 t·d<sup>-1</sup>. During pre-eruptive period (2016-2021), time series of the diffuse CO<sub>2</sub> emission showed a change with an increasing trend from 788 t·d<sup>-1 </sup>up to 1,870 t·d<sup>-1</sup>, coinciding with the beginning of the seismic swarms. This increase of diffuse CO<sub>2</sub> emission is interpreted as a geochemical precursory signal of volcanic eruption of Cumbre Vieja, on September 19, 2021. The observed increase on the diffuse CO<sub>2</sub> emission during this time window suggests that in October 2017 a process of magma ascent began from the upper mantle to depths between 35-25 km, at which the seismic swarms were recorded for four years. During eruption period, diffuse CO<sub>2</sub> emission showed strong temporal variations with a minimum value of the diffuse CO<sub>2</sub> emission in October 21, followed by an increase trend of up to 4,435 t·d<sup>-1 </sup>on December 14, the highest of time series and coinciding with the end of the eruption. During the post-eruptive period, the diffuse CO<sub>2</sub> emission has shown a descending trend. Our results demonstrate that periodic surveys of diffuse CO<sub>2</sub> emission are extremely important in the volcanic surveillance tools of Cumbre Vieja to improve the detection of early warning signals of future volcanic unrest episodes.</p><p> </p><p> </p><p> </p>

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