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Lanzarote (795 km2) is a volcanic island with 58 km long and 21 km wide located in the eastern part of the Canary Islands and it is approximately 130 km away from the African coast. The island has experienced the longest historical eruption occurred in the Canarian archipelago, Timanfaya eruption, from 1730 to 1736. It created more than 30 volcanic cinder cones along several NE – SW fissures and produced voluminous amounts of lava flows that covered an area of 200 km2, which corresponds to almost a quarter of the island. Nowadays, several thermal anomalies remain active in the Islote de Roque Hilario. The latest eruption recorded in the island was in 1824, known as Tinguatón eruption, along an ENE – WSW fracture.Since there are not visible gas emanations at the surface environment of Timanfaya Volcanic Field (TVF), diffuse degassing surveys became a useful tool to monitor the volcanic activity. Among degassing phenomena, soil CO2 efflux is important because of the characteristics of CO2: it is the major gas species after water vapor in both volcanic fluids and magmas and it is an effective tracer of subsurface magma degassing due to its low solubility in silicate melts at low to moderate pressures (Gerlach and Graeber, 1985). To do so, since 1999, diffuse CO2 emission surveys have been yearly undertaken in and around the TVF. Between September and October 2022, a new survey was performed with 410 sampling sites, covering an area of 252 km2 in order to have a homogeneous distribution. Soil CO2 efflux was measured following the accumulation chamber method. Soil temperature was determined by inserting a thermocouple at each sampling site at a depth of 40 cm. Soil CO2 efflux values ranged from non-detectable (

São Jorge is one of the nine islands of the Azores Archipelago. On March 19, 2022, a seismic crisis began in the island, with more than 32,300 earthquakes recorded until May 2022, of which nearly 300 were felt by the population. On April 5, 2022, an INVOLCAN team travelled to the Azores and, in collaboration with CIVISA/IVAR of the Universidade dos Açores, carried out a soil gas and CO2 diffuse efflux survey to monitor the activity in this volcanic island. The absence of visible volcanic gas emissions (fumaroles, hot springs, etc.) at the surface environment of Manadas volcanic system -the most recent volcanic system in São Jorge- makes this type of studies an essential tool for volcanic surveillance purposes.Soil CO2 efflux was measured in 400 observation points on the island following the accumulation chamber method using a non-dispersive infrared LICOR-830 CO2 analyzer. Soil gas samples were taken at 40 cm depth to study the chemical composition of diffuse emanations. The chemical composition of the gas samples (He, Ne, H2, CO2, CH4, N2 and O2) was analyzed daily on the island by means of a micro-gas chromatograph (micro-GC).The results reported herein are related to the soil He and H2 degassing. The former has exceptional characteristics as a geochemical tracer while the second is one of the most abundant trace species in volcanic systems and it is fundamental in many redox reactions occurring in the reservoir gas. Mapping He and H2 soil effluxes may be useful to detect hidden tectonic structures. In addition, detection of significant changes in the diffuse emission of both gases in volcanically active areas, as well as changes in their spatial distribution, is usually linked to movements of magma in the subsoil and/or changes in seismic-volcanic activity. These variations may be excellent early warning signals of changes in the activity of the system.Spatial distribution maps of the diffuse flow of He and H2 in the 237.59 km2 area of the island were constructed following the sequential Gaussian simulation (sGs) procedure to quantify the diffuse He and H2 emission from the studied area. He fluxes varied from 0 to 3.7 mg·m-2·d-1 (mean value, 0.4 mg·m-2·d-1), while H2 fluxes ranged between 0 and 4.7 mg·m-2·d-1 (mean value, 0.15 mg·m-2·d-1). He and H2 diffuse emission was estimated in 101 and 28 kg·d-1, respectively, with diffuse degassing values of 0.369 and 0.102 kg·km-2·d-1.The principal H2 flux anomaly is in the central part of the island and coincides with the ground deformation obtained by synthetic aperture radar data acquired by the ESA Sentinel-1 satellite at the beginning of the crisis. He highest fluxes are dispersed in different areas and most of the highest values are located in the Manadas volcanic system, where most of the seismicity focused. The spatial distribution maps of He and H2 fluxes seem to show the existence of areas that could be acting as preferential zones of vertical permeability allowing the migration of deep source gases, which is very important to follow any seismic-volcanic crisis.

Carbon dioxide (CO2) is a gas that is found in small amounts in Earth's atmosphere. It is released into the atmosphere by volcanoes, but it is typically diluted to low concentrations very quickly and is not harmful to people or animals. However, in certain stable atmospheric conditions, cold CO2 gas can flow into low-lying areas and reach higher concentrations. At these concentrations, CO2 can be dangerous. If the air contains more than 3% CO2, it can cause symptoms such as headaches, dizziness, increased heart rate, and difficulty breathing. At concentrations above 15%, CO2 can quichly lead to unconsciousness and death. The boundary between healthy air and lethal gas can be extremely sharp; even a single step upslope may be adequate to escape death.Since the Tajogaite eruption ended on December 13, 2021, high concentrations of CO2 up to 20% have been observed inside the buildings of La Bombilla and Puerto Naos (La Palma, Canary Islands). Anomalous concentrations of CO2 are detected mainly in the ground-floor and basement of the buildings in Puerto Naos, and their distribution is not homogeneous or uniform throughout the Puerto Naos area (Hernández P.A. et al, 2022). Because of the ultradilute nature of CO2 in the atmosphere, chemical sorbents with strong CO2-binding affinities are typically employed for CO2 capture. For this study, we set up a network of 45 alkaline traps stations located on the ground-floor (streel level) inside buildings of Puerto Naos. These chemical sorbents were made up KOH 1.2 M and replace weekly. The aim of this study was to distinguish between indoor areas that had high levels of CO2 in their alkaline traps and those with lower levels of CO2. This was done through weekly surveys conducted from May to October 2022 in order to identify and define areas with the greatest CO2 hazard. The statistical analysis of the results shows that 75% of the variance of the amounts of CO2 retained weekly in the alkaline traps seems to be related to an endogenous CO2 source while 25% of the variance observed is related to external variables. Hernández, P. A., Padrón, E., Melián, G. V., Pérez, N. M., Padilla, G., Asensio-Ramos, M., Di Nardo, D., Barrancos, J., Pacheco, J. M., and Smit, M.: Gas hazard assessment at Puerto Naos and La Bombilla inhabited areas, Cumbre Vieja volcano, La Palma, Canary Islands, EGU General Assembly 2022, Vienna, Austria, 23–27 May 2022, EGU22-7705, https://doi.org/10.5194/egusphere-egu22-7705, 2022.

Carbon dioxide (CO2) is a colourless and odourless gas. It is non-flammable, chemically non-reactive and 1.5 times as heavy as air; therefore, may accumulate at low elevations. CO2 is a toxic gas at high concentration, as well as an asphyxiant gas (due to reduction in oxygen). Irritation of the eyes, nose and throat occurs only at high concentrations. Since the Tajogaite eruption ended on December 13, 2021, high concentrations of CO2 up to 20% (200.000 ppmv) have been observed inside of buildings of the neighborhoods of La Bombilla and Puerto Naos (La Palma, Canary Islands), which are located about 5 km distance from the Tajogaite eruption vent. Anomalous concentrations of CO2 are manily detected in the ground-floor and basement of the buildings in Puerto Naos, and the distribution of relatively high CO2 concentrations is not homogeneous or uniform throughout the Puerto Naos area (Hernández P.A. et al, 2022).The purpose of this study was to use the Tunable Laser Diode (TDL) absorption spectroscopy method to monitor the indoor CO2 concentration of the ground-floor of one of the buildings of Puerto Naos. A CO2-TDL was installed on 9 January 2022 and continues measuring the CO2 concentration along an optical path of about 6 meters. During the period January-March 2022, daily averages of CO2 concentrations from fifteen-minute data ranged from 5000 to 25000 ppmv reaching values up to 40000 ppmv (4%). Over time, a clear decreasing trend of the indoor CO2 concentration has been observed at this observation site and the daily CO2 averages from fifteen-minute data during the last 3 months (October-December 2022) ranged from 1000 to 2500 ppmv. This clear decreasing trend over time has not been observed at other observation sites where the concentration of CO2 inside buildings is being monitored. This observation indicates the complexity of the problem and the need to install a dense network of sensors to monitor CO2 for civil protection purposes. Hernández, P. A., Padrón, E., Melián, G. V., Pérez, N. M., Padilla, G., Asensio-Ramos, M., Di Nardo, D., Barrancos, J., Pacheco, J. M., and Smit, M.: Gas hazard assessment at Puerto Naos and La Bombilla inhabited areas, Cumbre Vieja volcano, La Palma, Canary Islands, EGU General Assembly 2022, Vienna, Austria, 23–27 May 2022, EGU22-7705, https://doi.org/10.5194/egusphere-egu22-7705, 2022.

The eruption of the Tajogaite volcano occurred between September and December 2021, being the longest one recorded at La Palma Island. During the course of the eruption, the emission of gas and pyroclasts was constant, forming a cone of approximately 200m. Once the eruption ended, the gas emission has gradually decreased with fumaroles of low and high temperatures along craters. In situ sampling techniques, close as possible to the gas emission sources, are the most suitable tools to investigate the chemical composition of primary emitted volcanic gases. For this reason, we applied the sampling technique proposed by Noguchi & Kamita (1963) to monitor C/S, S/Cl, S/F and F/Cl ratios in the air inside the crater of Tajogaite by using alkaline traps (named JB1, JB3 and JB6) spatially distributed in order to access to different volcanic gas emission spots. This method gives the advantage of an integral monitoring of the acid gases during a certain sampling interval. Initially, two redundant sampling sites were use at three selected locations in April 2022 to study whether there were significant differences in the data from each pair of traps. The results indicated no significant differences in the results of the redundant sampling sites. The alkaline solutions, consisting of 2N KOH solutions, are analyzed using both volumetric and chromatographic methods. The carbon is analyzed by volumetric titrations, whereas Cl, S and F are analyzed by Ion Chromatography (IC). Samples are collected weekly for subsequent analysis.Measured C/S ratios varied between 0.003-0.02, 0.001-002 and 0.002-4.1 for JB1, JB3 and JB6, respectively. S/Cl ratios varied between 11.5-62.3, 64.3-241.3 and 0.1-482.0 for JB1, JB3 and JB6, respectively. S/F ratios varied between 0.8-82.1, 6.3-78.9 and 0.01-74.4 for JB1, JB3 and JB6, respectively. F/Cl ratios varied between 0.4-17.8, 1.8-22.6 and 0.6-169.7 for JB1, JB3 and JB6, respectively. Results show a progress increase on the C/S ratio during all sampling period (April-November 2022), except for the JPB1 and JB3 during October and November 2022, which is explained in base of a observed slight decreasing trend in S content, which is to be expected since the post-eruptive activity should decrease as well as the decrease in the fumarolic output. However, the variations observed in the S/Cl, S/F and F/Cl ratios show a more chaotic behavior, depending on the alkaline trap and therefore its location. The fumaroles that come into contact with the alkaline traps are not the same, and the possible effects of external variables such as wind, can considerably reduce the transport of minor fumarolic gases from their source to the traps producing these variations. S/Cl ratio seems to show higher values during the first period of monitoring, that may be related to variations in the fumarolic activity whereas S/F and F/Cl do not show in general a clear temporal tendency. Direct sampling of plume/fumarolic gases by alkaline traps is a cheap and easy technique to monitor the relationship between various chemical species and volcanic activity.Noguchi & Kamiya (1963). Bull. Volcanol., 24, 367-378.

Tenerife (2,034 km2), the largest island of the Canarian archipelago, is characterized by three volcanic rifts NW-SE, NE-SW and N-S oriented, with a central volcanic structure, Las Cañadas caldera, hosting Teide-Pico Viejo volcanic complex. The North-West volcanic Rift Zone (NWRZ, 72 km2) is one of the youngest and most active volcanic systems of the island, where three historical eruptions (Boca Cangrejo in the 16th Century, Arenas Negras in 1706 and Chinyero in 1909) have occurred. The North-East volcanic Rift Zone (NERZ, 210 km2) has showed one historical eruptive activity during 1704 and 1705 across 13 km of fissuraleruption (Siete Fuentes-Arafo-Fasnia eruption). As part of the volcano monitoring program of NWRZ and NERZ, diffuse CO2 degassing surveys have been carried out yearly since 2000 at the NWRZ and since 2001 at the NERZ. In-situ measurements of CO2 efflux from the surface environment were performed according to the accumulation chamber method using a portable non-dispersive infrared (NDIR) sensor. Soil CO2 efflux contour maps were constructed to identify spatial-temporal anomalies and to quantify the total CO2 emission using the sequential Gaussian simulation (sGs) interpolation method. Previous studies in the NWRZ has shown a temporal correlation between long-term variations in the diffuse CO2 output with the occurrence of a seismic unrest in 2004 and the 2016 seismic swarm (Hernández et al., 2017). The last diffuse CO2 emission survey performed in the survey in the NWRZ in 2022 showed values between non-detectable ones and 32.1 g·m-2·d-1, with an average value of 5.6 g·m-2·d-1 for NWRZ. In the case of the NERZ, the last diffuse CO2 emission survey performed in 2022 showed values ranging between non-detectable values and 59.1 g·m2·d-1, with an average value of 6.7 g·m-2·d-1. Diffuse CO2 emission rate were estimated in 360 ± 15 t·d-1 for NWRZ and 1,444 ± 42 t·d-1 NERZ were obtained in the 2022 surveys. The normalized CO2 emission value by area was estimated in 5 t·d-1·km-2 for NWRZ and in 6.9 t·d-1·km-2 for NERZ. Temporal evolution of diffuse CO2 emission at both NWRZ and NERZ of Tenerife shows a clear relationship with the volcano seismic activity in and around Tenerife Island. These geochemical observations are clear evidence of changes of processes operating deep in the hydrothermal-magmatic system of Tenerife.Hernández et al. (2017). Bull Volcanol, 79:30, DOI 10.1007/s00445-017-1109-9.

The recent eruption of Tajogaite volcano occurred between September and December 2021 at La Palma Island is considered the most devastating of Europe since that of Vesuvio in 1944. The post-eruptive period is being characterized by the appearance of high outdoor and indoor CO2 concentrations at inhabited areas such as La Bombilla and Puerto Naos (Hernández et al., 2022), forcing the eviction of numerous homes. However, anomalous concentrations of CO2 have not only appeared in inhabited areas, but also in cultivated lands. In fact, the highest CO2 concentration values measured in the outdoor environment during the entire post-eruption period have been in a banana plantation of approximately 4,200 m2 that INVOLCAN has been monitoring since June 2022, named Las Hoyas.Since June 2022, 26 scientific observation surveys have been carried out at Las Hoyas consisting of the measurement at 39 homogeneously distributed sites of the CO2 and O2 concentration at two heights, 40 and 170 cm from the ground, as well as sampling of atmospheric air in 19 sites at 40 cm for carbon isotope ratio of air CO2. Air CO2 and O2 concentrations are measured with a hand portable Dräger X-am® 8000 meter and the carbon isotope ratio of air CO2 (expressed as δ13C-CO2 ‰ vs. VPDB) is analyzed at ITER/INVOLCAN lab by a Thermo Finnigan MAT 253 mass spectrometer. Spatial distribution maps have been constructed following the sequential Gaussian simulation (sGs) to evaluate the spatial distribution of the air CO2 concentration. Observed air CO2 concentration values ranged from air value (412ppm) up to 69%, the highest ever measured during the post-eruptive period, with an average value of 7.1%. Air O2 concentration values ranged between 7.9% to air value (20.9%), with an average value of 19.2%. δ13C-CO2 values ranged between –8.90 to -2.66‰, with an average value of -4.87‰, indicating a clear volcanic-hydrothermal origin for the anomalous CO2 emitted from Las Hoyas banana plantation and ruling out a single biogenic origin.In order to investigate the temporal evolution of the observed high CO2 concentrations in Las Hoyas, a Sinclair statistical graphic analysis was applied to the data from each survey. Time series of background and peak populations does not show a clear trend, with the occurrence of peaks and valleys throughout the entire series, and maintaining values much higher than those of the air. Likewise, the temporal evolution of the δ13C-CO2 values shows a trend towards heavier values, indicating that the volcanic-hydrothermal contribution increases with time. The spatial distribution of the air CO2 concentration measured at 40 cm shows that in most of the surveys, the anomalous values (>10%) are located mainly along the walls of Las Hoyas and in the NW sector, where in more than a year after the eruption, the banana plants are still withered and dead terrestrial and aerial fauna constantly appear due to poisoning and suffocation from CO2 inhalation. Hernández et al. EGU General Assembly 2022, Vienna, Austria, 23–27 May 2022, EGU22-7705, https://doi.org/10.5194/egusphere-egu22-7705, 2022.

Tenerife is the largest (2,034 km2) and highest (3,718 m) island of the Canarian archipelago and is the only one hosting an active stratovolcano (Teide-Pico Viejo volcanic system). Its structure is controlled by a volcanotectonic rift-system with NW, NE and NS directions, with the Teide-Pico Viejo volcanic system located in the intersection. The last eruption Teide-Pico Viejo volcanic system occurred in 1798 through an adventive cone. Although Teide volcano shows a weak fumarolic system, volcanic gas emissions observed in the summit cone consist mostly of diffuse CO2 degassing (Hernández et al., 1998; Mori et al., 2001).More than 200 diffuse CO2 efflux surveys have been performed in the summit crater of Teide Volcano during the period 1999-2023. During each survey, diffuse CO2 emission was estimated in 38 sampling sites, homogeneously distributed inside the crater, by means of a portable non dispersive infrared (NDIR) CO2 fluxmeter using the accumulation chamber method. CO2 emission rates was estimated after spatial distribution maps constructed by sequential Gaussian simulation (sGs) algorithm. During 23 years of the studied period, CO2 emissions ranged from 2.0 to 346 t/d. The most remarkable feature of the temporal evolution of diffuse CO2 emission rate was an important increase that began few weeks after the occurrence of a seismic swarm of long period events was recorded on Tenerife in 2 October 2, 2016, a followed by a general increase of the seismic activity in and around the island (D’Auria et al., 2019). Several geochemical parameters showed significant changes during ∼June–August of 2016 and 1–2 months before the occurrence of the October 2, 2016, long-period seismic swarm (Padrón et al., 2021). Since then, anomalously high diffuse CO2 emission rates has remained in the crater of Teide. This change might be explained as an input of magmatic fluids triggered by an injection of fresh magma and convective mixing after the 2 October 2016 seismic swarm (D'Auria et al., 2019; Padrón et al., 2021). This work reflects how useful are the diffuse studies to understand the behaviour of the volcanic system and to forecast future volcanic activity. Monitoring the diffuse degassing rates has demonstrated to be an essential tool for the prediction of future seismic–volcanic unrest and has become an important monitoring tool to reduce volcanic risk in Tenerife.D'Auria, L., et al. (2019). J. Geophys. Res.124,8739-8752Hernández, P., et al. (1998). Geophys. Res. Lett. 25, (17) 3311-3314Mori, T., et al. (2001). Chem. Geol. 177, 85–99Padrón, E., et al. (2021). J. Geophys. Res.126,e2020JB020318

Anomalous CO2 degassing of volcanic origin was observed by the end of November 2021 in the neighborhoods of La Bombilla and Puerto Naos, located in the western flank of La Palma, about 5 km distance southwestern of the 2021 Tajogaite eruption vents (Hernández et al., 2021). In this study zone, continuous monitoring of CO2 concentration in the outdoors ambient air at 200 cm from the surface has reached a daily average of maximum and mean values about 28,000 and 10,000 ppm, respectively. We started recently to perform CO2 concentration and stable isotope surveys in the outdoors ambient air of Puerto Naos at 140 cm from the surface by means of a Delta Ray analyzer installed in an electrical car which was driving through the streets of Puerto Naos. This instrument is a high performance, mid-infrared laser-based, isotope ratio infrared spectrometer (IRIS) which offers the possibility of performing simultaneous determination of δ13C and δ18O in CO2 at ambient concentrations with a precision as low as 0.05‰. One major advantage of IRIS techniques with respect to more traditional ones (e.g., isotopic ratio mass spectrometry -IRMS-) is the possibility to perform (semi)continuous measurements at high temporal resolution. Since October 2022, seven surveys have been performed at Puerto Naos making up a total of about 600 measurements. The observed CO2 concentrations and the δ13C-CO2 values in the outdoors ambient air ranged from 420 to 3,500 ppm and from -9.0 to -3.2 ‰ vs. VPDB, respectively. Survey data analysis showed a good spatial correlation between relatively high CO2 concentrations with δ13C-CO2 values less 13C-depleted (i.e., volcanic CO2). These observations highlight that stable isotope surveys allow to evaluate the impact of volcanic degassing on the air CO2 concentration and provide valuable results to identify the volcanic CO2 gas hazard zones.Hernández, P. A., Padrón, E., Melián, G. V., Pérez, N. M., Padilla, G., Asensio-Ramos, M., Di Nardo, D., Barrancos, J., Pacheco, J. M., and Smit, M.: Gas hazard assessment at Puerto Naos and La Bombilla inhabited areas, Cumbre Vieja volcano, La Palma, Canary Islands, EGU General Assembly 2022, Vienna, Austria, 23–27 May 2022, EGU22-7705, https://doi.org/10.5194/egusphere-egu22-7705, 2022.

The most common types of survey in known geothermal areas are water and gas sampling of natural discharges. However, where wáter and gas discharge features are few and where the extent of the field is not known, soil and soil-gas surveys can prove useful. These surveys can identify areas in a field that are permeable and potential upflow or boiling zones. They can also outline the boundaries of a geothermal system, often in conjunction with geophysical surveys to provide more comprehensive understanding, especially when geophysical data is difficult to interpret. For all of the above, a soil and soil-gas survey for geothermal exploration had been carried out at Cumbre Vieja volcano that is characterized by neither having visible manifestations of volcanic gases nor zones of hydrothermal alteration on its surface. We have divided the soil and soil-gas survey for geotermal exploration into four categories based on the number of sampling sites per square kilometer: 1) Regional exploration survey (1 sampling site/km2), 2) Regular exploration survey (about 5 sample sites/km2), 3) Detailed I exploration survey (between 10 and 100 sampling sites/km2) and 4) Detailed II exploration survey (more than 500 sampling sites/km2). We have classified our soil and soil-gas survey for geothermal exploration at Cumbre Vieja volcano as a regular exploration survey due to the fact that 1200 observation or measurement points were selected for its 220 km2 area, resulting in approximately 5.5 sampling sites per km2. Similar soil gas surveys had been performed in other study áreas of the Canarian archipelago (Rodríguez et al. 2021). At every sampling site, we performed in-situ measurements of soil CO2 efflux and 222Rn activity in the soil gas atmosphere. In addition, soil gas samples were collected at a depth of 40 cm for further chemical and isotopic analysis in our geochem lab. Simultaneously with the soil gas survey, soil samples were collected from the base of the B Horizon-Subsoil (30–50 cm) which were subsequently dried and sieved (80 mesh) for subsequent leaching in order to analyze those volátiles (Hg0, As, B and NH3) of interest that were fixed in the organic-clay fraction of the soils. Some preliminary results of this soil and soil-gas survey for geothermal exploration at Cumbre Vieja are presented.Rodríguez F., Pérez N. M., Melián G. V., Padrón E., Hernández P. A., Asensio-Ramos M., Padilla G. D., Barrancos J. and D’Auria L. (2021). Exploration of deep-seated geothermal reservoirs in the Canary Islands by means of soil CO2 degassing surveys. Renewable Energy, https://doi.org/10.1016/j.renene.2020.09.065

The 2021 Tajogaite eruption in Cumbre Vieja volcano (La Palma, Canary Islands), which started on Sep. 19, 2021, and lasted 85 days, caused extensive damages because of the lava flows and ash fall. However, since the middle of Nov. 2021, some areas located about 5 km SW of the eruptive center started to be affected by intense diffuse CO2 emission. Among them are the urban centers of La Bombilla and Puerto Naos (Hernández et al., 2022). These emissions prevented the population of these two centers from returning to their houses because of high concentrations of CO2 in indoor and outdoor environments.In this work, we model the CO2 dispersion process in Puerto Naos to obtain hazard maps with the maximum CO2 concentrations which can be reached in the town in the outdoor environment. To achieve these results, we combined field observations with numerical modelling. Field surveys were realized in low wind conditions, measuring the CO2 concentration with portable sensors at 15 and 150 cm from the ground at measurement points spaced approximately 10 m from each other along the streets of Puerto Naos.We realized numerical modelling using the software TWODEE-2, a code for modeling the dispersion of heavy gases based on the solution of shallow water equations (Folch et al., 2009). For this purpose, we used a detailed digital topographic model, including the edifices of Puerto Naos. Using a trial-and-error approach, we determined the gas emission rates from a set of discrete source points in no-wind conditions. Subsequently, we repeated the numerical modelling, keeping the same sources and simulating all the realistic wind conditions in terms of direction and intensity. For each simulation, we determined the maximum CO2 concentration at different elevations from the ground. This allowed obtaining a hazard map with the maximum CO2 outdoor concentrations for each part of the townThe main results highlight that the outdoor environment is affected by a dense layer of CO2, whose flow is strongly conditioned by the urban infrastructures. Furthermore, we evidenced how even light winds can change the gas concentration pattern radically in a few minutes, evidencing the possibility of sudden changes in the CO2 concentration outdoors with no warning.Folch A., Costa A., Hankin R.K.S., 2009. TWODEE-2: A shallow layer model for dense gas dispersion on complex topography, Comput. Geosci., doi:10.1016/j.cageo.2007.12.017Hernández, P. A., Padrón, E., Melián, G. V., Pérez, N. M., Padilla, G., Asensio-Ramos, M., Di Nardo, D., Barrancos, J., Pacheco, J. M., and Smit, M.: Gas hazard assessment at Puerto Naos and La Bombilla inhabited areas, Cumbre Vieja volcano, La Palma, Canary Islands, EGU General Assembly 2022, Vienna, Austria, 23–27 May 2022, EGU22-7705, https://doi.org/10.5194/egusphere-egu22-7705, 2022.

The Cape Verde Islands are a group of 10 intraplate oceanic islands of which 3 show significant levels of recent volcanic activity: Fogo, Santo Antao and Brava. Of these, Fogo is the only historically active volcano with intense activity up to 1725 AD followed by less frequent, mainly effusive eruptions in the last 280 years. The last eruption began on 23 November 2014, and continuing until 8 February 2015. This eruption occurred 19 years after the previous eruptive event, but on the contrary of the 1995 eruption, when there was not monitoring program for the Cape Verde volcanic surveillance, a simple and multidisciplinary volcanic monitoring program, based on a collaborative research volcano monitoring program between Cape Verde and Spanish scientists, was operative to face volcanic unrest such as the 2014 eruption. As part of this volcano monitoring program, from May 2007 to January 2015, forty eight diffuse CO2 emission surveys were performed at the summit crater of Pico do Fogo, covering homogeneously an area of about 0.142 km2. Measurements of diffuse CO2 emission were performed at the surface environment following the accumulation chamber method. The emission rate was calculated after the construction of spatial distribution maps following the sequential Gaussian simulation (sGs) algorithm. The emission rate showed a first anomalous peak in the diffuse CO2 emission, suggesting the occurrence of a first magmatic reactivation likely due to a deep magma intrusion beneath Pico do Fogo volcano between November 2008 and February 2009. Diffuse CO2 emission data suggest the occurrence of a second magmatic intrusion that trigger the eruption process as can be observed by the significant increase of CO2 emission on March 2014. The increment of the diffuse CO2 emission rate shows a good temporal correlation with the satellite-based long-wavelength infrared data reported by Girona et al., (2021). The diffuse CO2 emission data and satellite-based thermal infrared radiance measurements prior the 2014-15 Fogo eruption demonstrate the importance of measuring both parameters to identify and evaluate changes in the volcanic activity of Pico do Fogo volcano. Girona, T. et al., (2021). Nature Geoscience 14, 238–241.

During and after the end of the 2021 Tajogaite eruption (La Palma, Canary Islands), anomalous CO2 degassing has been detected in the neighborhoods of La Bombilla and Puerto Naos, located around 5 km distance southwestern of the 2021 Tajogaite eruption vents. The aim of this study is to determine the indoor air quality of the houses of the aforementioned neighborhoods. For that purpose, from August 11 to October 24, 2022, air samples were taken, for further analysis, from indoors of 10 locations in Puerto Naos, on a weekly basis. In addition, on September 22, 2022, a discrete survey of the indoor ambient air was carried out in 10 houses of La Bombilla, consisting on in-situ measurements and gas sampling for further analysis.Gas samples were taken for a complete geochemical characterization (i.e., He, Ar, Ne, H2, N2, O2, CH4, CO contents) by micro-gas chromatography (micro-GC) and quadrupole mass spectrometry (QMS) and, as well as for carbon isotopic analysis of the CO2 (δ13C-CO2) by isotopic ratio mass spectrometry (IRMS). In-situ measurements of CO2, O2, 222Rn, 220Rn, H2S and Hg0 were conducted in La Bombilla with and without natural ventilation.National Health Systems in the European Union reflect that the upper limit of the acceptable CO2 concentration range for long-term exposure in the indoor ambient air of buildings for residential use should be of the order of 1,000-1,200 ppm to guarantee people health. The concentrations of CO2 registered in the indoor ambient air of the 10 houses of La Bombilla determined by the in-situ measurements showed relatively high values -above 5,000 ppm- even reaching a maximum of 183,900 ppm in conditions without natural ventilation. In these conditions of absence of ventilation, a certain displacement of O2 was observed, which dropped to 18.7% in the worst case. Under conditions with natural ventilation for a period of 2 hours, the range of CO2 concentration fell to a range between 1,050 and 14,200 ppm and the O2 concentration registered was 20.9%. These results reflect that natural ventilation, and even more forced ventilation, would contribute to reduce CO2 concentration in the ambient air inside buildings. Regarding the results of the indoor gas samples analysis from Puerto Naos, the CO2 concentration and the δ13C-CO2 mean values ranged from 1,190 to 230,952 ppm and -7.9 to -4.8‰ vs. VPDB, respectively. These results of the chemical and isotopic composition of the indoor ambient air of Puerto Naos and La Bombilla demonstrate the importance of these studies to monitor and manage these silent hazards that pose a threat to the population and restrict access to their houses.

Monitoring the volcanic activity of Teide, the only active stratovolcano in Tenerife, the largest island of the Canary Islands, is extremely important for the prevention and reduction of the volcanic disasters of the island. As part of the geochemical monitoring of the Teide volcanic activity, during the last three decades the volcano has been the subject of a geochemical monitoring of the fumarole discharges, characterized by low flux emission of fluids with temperatures of ∼83°C, located at the Teide summit crater (Pérez et al., 1992 and 1996; Melián et al., 2012). Teide fumaroles show chemical compositions typical of hydrothermal fluids, i.e., meteoric steam dominates the gas composition, followed by CO2, N2, H2, H2S, HCl, Ar, CH4, He, and CO (Pérez et al., 1992). The temporal variations in fumarole gas chemistry at Teide volcano was useful to detect significant changes in the chemical composition of the Teide fumarole, including the appearance of SO2, and increases in the HCl and CO concentrations, one year before a seismic crisis that occurred in Tenerife Island between April and June 2004, what suggested that the associated temporal changes in seismic activity and magmatic degassing indicate that geophysical and fluid geochemistry signals in this system are unequivocally related (Melián et al., 2012). The average of the air-corrected 3He/4He ratio during the period 1991-2022 was 6.80 RA (being RA the atmospheric ratio), with the maximum value of the time series (7.57 RA) measured in August 2016, when an input of magmatic fluids triggered by an injection of fresh magma and convective mixing took place beneath Teide volcano (Padrón et al., 2021). After such input of magmatic fluids, increases in the CO2/H2O, C/S and He/CO2 ratios, a decrease in the CO/CO2 ratio were observed together with a significant increase in the seismic activity recorded in the island of Tenerife. This work highlights the important role of volcanic gases in the monitoring of volcanic activity, paying attention to different chemical and isotopic species in the fumarolic discharges. Melián G.V. et al., (2012), Bull. VolcanoL. 74, 1465–1483.Padrón E. et al., (2021), J. Geophys. Res. 126, e2020JB020318.Pérez N.M. et al., (1992), Actas de las sesiones científicas. III Congreso Geológico de España, 1, 463–467.Pérez N.M. et al., (1996), Geophys. Res. Lett. 23(24), 3531–3534.

After the Tajogaite eruption at Cumbre Vieja volcano (La Palma, Canary Islands), volcanic gas hazard continues to affect the inhabited coastal areas of Puerto Naos and La Bombilla, as well as the nearby agricultural fields, which are located about 6 km distance from the eruptive vents. This gas hazard is primarily due to CO2 (Hernández et al. 2022) and persist during the post-eruptive phase of the eruption. According to long-term geochemical studies conducted by INVOLCAN, the high levels of CO2 emissions in these coastal areas were first recorded approximately three weeks before the end of the Tajogaite eruption on December 13, 2021.To monitor this anomalous diffuse CO2 degassing at La Bombilla, 46 surveys consisting of approximately 84 sampling observation sites have been regularly conducted since December 2021, covering an area of 0.033 square kilometers. In-situ measurements of soil CO2 efflux and ground temperature as well as collection of samples of the soil gas atmosphere at a depth of 40cm for chemical and isotopic analysis were carried out at each sampling site. Soil CO2 efflux measurements have been performed following the accumulation chamber method. Soil gas chemical analysis were carried out by means of a microGC and the carbon isotope ratio of soil gas CO2 (expressed as δ13C-CO2 ‰ vs. VPDB) was analyzed also in our geochem lab by a Thermo Finnigan MAT 253 mass spectrometer. Spatial distribution maps have been constructed following the sequential Gaussian simulation (sGs) to evaluate the spatial distribution of the soil CO2 efflux measurements and quantify the diffuse CO2 emission from the studied area. Observed soil CO2 efflux values ranged from

El Hierro (278 km2) is one of the eight islands of the Canary Islands archipelago. It is an oceanic island formed ~1.2 Ma ago and one of the most active from a volcanic point of view of all the Canary Islands. Its last historical eruption was submarine eruption that began on 12 October, 2011 until 5 March, 2012, off its southern coast, which was the first to be monitored in the Canary Islands. Since 1998, periodic diffuse CO2 surveys have been carried out (except for the 2011-2012 seismic-volcanic unrest and eruptive periods, when the surveys frequency increases, Melián et al., 2014) by means of the accumulated chamber method measuring the diffuse CO2 emission at 601 points distributed homogeneously throughout the emerged 278 km2 of El Hierro. CO2 emission rate measured in the entire island has been varying over time, registering highest values in the pre-eruptive and eruptive periods where a significant increase of this gas emission rate was registered. Soil CO2 efflux values for the 2022 survey ranged between non-detectable (

La Palma Island (708 km2) is located at the north-west and is one of the youngest (~2.0My) of the Canarian Archipelago. The current stage of shield-building is manifest by the construction of Cumbre Vieja volcano, at the southern part of the island, where volcanic activity has taken place exclusively in the last 123 ka. On September 19, 2021, a new volcanic eruption occurred at Cumbre Vieja volcanic system (Tajogaite eruption). The erupting fissure (~1.0 km-length) is characterized by lava effusion, strombolian activity, lava fountaining, ash venting and gas jetting. After 85 days, the eruption finished on December 13, 2021. The 2021 Tajogaite eruption, with a magnitude VEI=3 (Bonadonna et al., 2022), resulted in the longest volcanic event on the island during the last 600 years and the most important eruption of Europe during the last 75 years.At the time of this study, the volcanic gas emissions observed at Tajogaite volcanic cone consisted mostly in diffuse CO2 degassing and residual fumarolic activity. Here we report the first diffuse CO2 and H2S emission surveys that have been carried out in Tajogaite volcanic cone.The measurements of soil CO2 efflux have been performed following the accumulation chamber method in 94 sites and the spatial distribution maps have been constructed following the sequential Gaussian simulation (sGs) procedure to show the location of CO2 and H2S diffuse degassing structures (DDS) and to quantify the diffuse CO2 and H2S emission from the studied area. The diffuse CO2 emission released to the atmosphere from Tajogaite volcanic cone ranged between 0 to 11.4 kgm-2·d-1 with an average of 0.90 kgm-2·d-1. The main DDS was located in the easternmost area of ​​the cone. Regarding the diffuse H2S emission, the data ranged between 0 to 44. 7 kgm-2·d-1 with an average value of 3.0 kgm-2·d-1. Two main DDS were identified: one coinciding with the CO2 DDS, in the easternmost zone, and other in the northern area of the cone. This study represents a starting point to study the degassing of the residual magma bodies beneath Tajogaite volcanic cone.Bonadonna, C. et al. (2022), EGU22-11927, https://doi.org/10.5194/egusphere-egu22-11927.

Active/recent volcanism indicates the presence of high-enthalpy resources at depth, but sometimes there is not any evidence of endogenous fluids manifestations at surface, that confirms the presence of an active geothermal system. In this study we present an extensive study of published and new diffuse 4He and 3He emission and thermal energy released data from fifteen volcanic systems, confirming a direct relationship between diffuse helium emissions and thermal energy released associated with the rise of mantle fluids. The results are useful to infer the existence of deep and/or hidden geothermal resources and to estimate roughly its potential at those areas without obvious geothermal manifestations at the surface, but with measureable anomalous emissions of 4He and 3He. Tracing diffuse He emission anomalies at the surface environment of volcanic systems provides very important information for the exploration and discovery of geothermal resources at those areas where the resources are either hidden or lie at great depth.

On January 12, 2020, Taal volcano in Philippines erupted, 43 years after its previous eruption in 1977. This eruption was preceded by diffuse CO

The Canary Islands are the only Spanish territory that holds potential for high enthalpy geothermal resources, although there is no evidence in the islands of hydrothermal fluid discharges in the surficial environment except for Teide volcano fumaroles in Tenerife. We herein report the results of several diffuse carbon dioxide (CO2) degassing surveys carried out at six mining licenses across Tenerife (Berolo, Guayafanta, Abeque, Garehagua and Garehagua II) and Gran Canaria (Atidama), covering a total of 640 km2 and more than 3230 sampling sites. Selected geochemical molar ratios, spatial distribution of CO2 efflux, statistical-graphical analysis of CO2 efflux, isotopic composition of CO2 (δ13C–CO2), the calculated volcano-hydrothermal contribution of CO2 and the deep-seated CO2 efflux were studied. These data were combined and used for geothermal exploration, demonstrating an effective reconnaissance strategy that reduces risk and cost. Garehagua II, Garehagua (both located in the north-south volcanic rift of Tenerife) and Abeque (located in the northwest volcanic rift of Tenerife) are, in this order, the mining licenses studied that showed the highest potential to contain geothermal resources, based on the geochemical indicators ranking proposed.

OBJECTIVES This article presents new radiocarbon and isotopic data to provide further information about the diet of the indigenous population of La Gomera and its possible changes across time. MATERIALS AND METHODS δ13 C and δ15 N of 53 human and 19 faunal samples from different sites on the island have been obtained and analyzed. Of these, 52 have been radiocarbon dated to provide insight on chronological changes. RESULTS Human dates range from the 3rd to 15th centuries AD, while faunal dates range from the 1st to 17th centuries AD. Stable carbon and nitrogen values are significantly different between the human and goat samples and have also a trophic increase of 3.4‰. Although male and female δ15 N data are not significantly different. Both δ13 C and δ15 N values of both human and animal samples tend to discretely decrease over time. DISCUSSION Radiocarbon dates from humans correlate with other dates obtained in the rest of the archipelago. Animal radiocarbon dates generally coincide except for one date, which requires further study. Isotopic δ15 N data suggest a mix of marine and terrestrial protein consumption in humans, the latter being more abundant given the seasonality of the first. δ13 C data also suggest a possible mixed diet in humans, with a predominance of C3 plants, like Hordeum vulgare, the only grain found in archaeological sites so far. Variations of both δ13 C and δ15 N over time suggest a slight modification on the diet, which could be related to environmental changes.

The recent volcanic eruption of Cumbre Vieja, on the island of La Palma, has beenconsidered by many to be the most important and devastating urban eruption of the last 100 years in Europe. After its completion on December 13, 2021, some urban areas not directed damaged by lava flows are affected by strong carbon dioxide (CO2) emissions from the soil. CO2 is a toxic gas at high concentration, as well as an asphyxiant gas and may be lethal when present in concentrations higher than 15 V%. The base of the small cliff where the La Bombilla neighborhood is located as well as the basements and garages of numerous buildings in the town of Puerto Naos, seem to represent leaking pathways along which CO2 related to the volcanic-hydrothermal activity rises to the surface. In order to assess the hazard represented by the endogenous gas emissions, a scientific observational study was undertaken by means of diffuse CO2 and H2S efflux measurements as well as gas sampling from the soil atmosphere at 40cm depth and the measurement of the soil temperature at 15cm and 40cm in 97 points homogeneous distributed at La Bombilla and Puerto Naos, in order to delimit anomalous gas emission zones and to know the emission rates of the measured gases. Also we carried out the installation of a Tunable Diode Laser system to measure continuously the CO2 air concentrations in the basement of a building at Puerto Naos and three permanent CO2 monitoring stations. Diffuse CO2 efflux values measured in the Puerto Naos area were relatively low (between not detected and 24 g m-2 d-1). However, in numerous points of the built-up area of Puerto Naos, air CO2 concentration values measured both in the street at a height of about 40 cm and in the lower part of several garage doors were generally over 1-2%V, with some sites with values higher than 20%V. The area with the highest CO2 diffuse efflux values is located in the La Bombilla neighborhood, reaching values higher than 7 kg m-2 d-1. δ13C-CO2 values of soil gases ranged from -19.2 to -1.7‰ vs. VPDB, confirming a volcanic-hydrothermal origin for those samples exhibiting high CO2 effluxes and concentration. No H2S effluxes as well as air concentrations were registered. During the survey, many animals were found dead due to high concentrations of CO2 and low levels of O2 in the air .All these anomalous CO2 emissions are not associated to thermal anomalies. Results of this study show that in many sites at La Bombilla and Puerto Naos areas there is a dangerous CO2 air concentration that exceeds the hazardous thresholds. These zones should be continuously monitored for gas hazard and the multi-measurement approach adopted in the present study is of paramount importance for decision-making of people's return to their homes.

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 CO2 emission surveys have been yearly performed in summer periods to minimize the influence of meteorological variations. Measurements of soil CO2 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 CO2 emission from the studied area. In the period 2001-2016, the diffuse CO2 output released to the atmosphere from Cumbre Vieja volcano ranged between 320 to 1,544 t·d-1. During pre-eruptive period (2016-2021), time series of the diffuse CO2 emission showed a change with an increasing trend from 788 t·d-1 up to 1,870 t·d-1, coinciding with the beginning of the seismic swarms. This increase of diffuse CO2 emission is interpreted as a geochemical precursory signal of volcanic eruption of Cumbre Vieja, on September 19, 2021. The observed increase on the diffuse CO2 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 CO2 emission showed strong temporal variations with a minimum value of the diffuse CO2 emission in October 21, followed by an increase trend of up to 4,435 t·d-1 on December 14, the highest of time series and coinciding with the end of the eruption. During the post-eruptive period, the diffuse CO2 emission has shown a descending trend. Our results demonstrate that periodic surveys of diffuse CO2 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.

Cumbre Vieja is the most active volcano of the Canary Islands since it has been the scenario of 8 of 17 historical eruptions in this archipelago. A recent magmatic reactivation started at Cumbre Vieja volcano on October 2017, and 9 additional seismic swarms occurred until the recent eruption which started on September 19, 2021, and ended on December 13, 2022 after 85 days of eruption. Since the first day of the eruption, extending to current days, INVOLCAN performed the monitoring of SO2 realesed by this eruption using a miniDOAS on terrestrial (car), sea (ship) and air (helicopter) mobile position. More than 360 measurements of SO2 emission rates were carried out daily. The standard deviation of the estimated values obtained daily was ~ 20%. During the first days of the eruption, estimated SO2 emission rates reached more than 30,000 tons/day, and maintaining weekly average values above 10,000 tons/day until the end of the eruption. After a final paroxysmal phase with an eruptive column of 8,500 m altitude, decreased significantly to averages values of 250 tons/day. Estimated SO2 emission rates from the 2021 Cumbre Vieja eruption became a powerful tool to contribute to the understanding of eruptive dynamics.

Cumbre Vieja (220 km2) is the most active volcano in the Canary Islands. It has been the location of 8 of the 17 historical eruptions in the archipelago during the last 600 years. The establishment of a geochemical monitoring program by our research group for the volcanic surveillance of Cumbre Vieja started in 1997. This program was mainly focused on diffuse degassing monitoring because of the absence of visible volcanic degassing manifestations (fumaroles, plumes, etc.) as well as other obvious geothermal features at Cumbre Vieja up to the 2021 eruption which started on September 19, ended on December 13 and lasted 85 days. The INVOLCAN’s soil degassing monitoring at Cumbre Vieja is carried out by means of a geochemical instrumental permanent network (soil CO2 efflux, soil gas 222Rn and soil C isotope ratio) and regular geochemical surveys covering the entire area of Cumbre Vieja (diffuse CO2, He and H2 emissions). Several soil degassing anomalies have been observed and some of them years before the 2021 eruption, which illustrates the importance of diffuse degassing monitoring for volcanic surveillance. The single visible manifestation of volcanic degassing at La Palma is a cold CO2-rich site at Taburiente volcano. Regular helium-3 emission monitoring of this observation site has been carried out since 1991 in collaboration with Tokyo Univ., and provided a clear early warning signal of the 2021 Cumbre Vieja eruption. Because of the registration of seismic swarms, and to strengthen the INVOLCAN geochemical monitoring program of Cumbre Vieja volcano, regular sampling of groundwater for chemical and isotopic analysis started in October 2017. The results of this hydrogeochemical monitoring also showed significant changes related to the recent volcanic unrest of Cumbre Vieja. Since the 2021 eruption onset, INVOLCAN performed daily observations of SO2 emissions using a miniDOAS in traverse mode, on terrestrial (car), sea (ship) and air (helicopter) mobile position recording relatively high SO2 emissions (> 50.000 t/d). Static scanners and satellite instruments were used also to monitoring the SO2 emission released by this eruption; a task lead by the volcano research group of Manchester University. Additional plume geochemical monitoring was carried out using OP-FTIR spectrometers and UAV, helicopter and ground-base MultiGas units to characterize the chemical composition of the plume degassing in collaboration with scientists from Manchester Univ., Palermo Univ., UCL, INGV, IPGP and Azores Univ. Carbon isotope analysis of the CO2 gas plume was also undertaken in collaboration with New Mexico Univ. Analysis of pristine ash leachates has been also performed in collaboration with Durham Univ. and Tokyo Institute of Technology since it is often used to estimate the composition of the gas phase during volcanic eruptions and provides important information on the eruption processes was also performed. The results of these geochemical observations during the inter-eruptive, pre-eruptive, eruptive and post-erupive phases have been tremendously useful to understand the recent magmatic reactivation of Cumbre Vieja volcano.

A volcanic eruption began at Cumbre Vieja volcano (La Palma, Canary Islands, Spain) on September 19, 2021, and resulted in the longest volcanic event since data are available on the island: it finished after 85 days and 8 hours of duration and 1,219 hectares of lava flows. This volcanic eruption is part of the volcanic evolution of La Palma Island, the fifth in extension (706 km2) and the second in elevation (2,423 m a.s.l.) of the Canarian archipelago. Cumbre Vieja volcano, where the volcanic activity has taken place exclusively in the last 123 ka, forms the southern part of the island. The first geophysical precursory signals of the last eruptive process, started on October 7th and 13rd, 2017, when two remarkable seismic swarms interrupted a seismic silence of 46 years in Cumbre Vieja volcano with earthquakes located beneath Cumbre Vieja volcano at depths ranging between 14 and 28 km with a maximum magnitude of 2.7. Five additional seismic swarms were registered in 2020 and four in 2021, the last being the one that preceded the eruption, beginning a week before it. 3He/4He ratio has been monitored at Dos Aguas cold mineral spring in La Palma Island since 1991 to date as an important volcano monitoring tool able to provide early warning signal of future volcanic unrest episodes, as magmatic helium emission studies have demonstrated to be sensitive and excellent precursors of magmatic processes occurring at depth. A significant increase was observed from 2011 to 2012, when the 3He/4He ratio reached the highest value of the period 1991-2019: 10.24 RA (being RA the ratio in atmospheric helium) in September 2012. At that time, this was the highest 3He/4He ratio reported from the Canarian archipelago measured either in the lavas or terrestrial fluids (Padrón et al., 2015). We suggest the occurrence of aseismic magma rising episodes beneath La Palma within the upper mantle towards an ephemeral magma reservoir in the period 2011-2012. Later, in the period 2017-2020, magma rising continued and produced seismic swarms that were accompanied also by the highest 3He/4He ratio measured at Dos Aguas (September, 2020). In 2021, 3He/4He ratio decreased ~0.32 RAsince the beginning of the year, including the eruptive period. 3He/4He ratio values suggest that upward magma migration that caused the 2021 Cumbre Vieja eruption likely started in or before 2012.Padrón et al., (2015). Bull Volcanol 77:28. DOI 10.1007/s00445-015-0914-2

In October 2017, two remarkable seismic swarms interrupted a 46-year seismic silence in Cumbre Vieja volcanic system, La Palma, Canary Islands, Spain. As a response to this seismic unrest episode, INVOLCAN strengthened the volcano monitoring in the island with the installation of a new automatic geochemical station in the municipality of Fuencaliente (LPG08) in the southern part of the island, which included a Delta RayTM Isotope Ratio Infrared Spectrometer (Thermo Fisher Scientific), to measure the content and isotopic composition (δ13C-CO2) of the soil gas CO2 using a PVC trap buried in the soil at 40 cm depth and transporting the gas through a polyamide pipe. After different seismic swarms occurred in the following years, a volcanic eruption started in Cumbre Vieja on September 19, 2021, lasting 85 days and 8 hours, the longest historical eruption in the island. On September 22, 2021, INVOLCAN installed an additional automatic geochemical station in the municipality of Los Llanos de Aridane (LPG10, around 5 km far from the eruption site) in the western part of the island, including another DeltaRayTM analyzer. In this work, we show the results from August 2020 to December 2021 measured at LPG08, and from September 2021 to January 2021 measured at LPG10. LPG08 data showed a range of δ13C-CO2 from -24.3 to -17.9‰ vs. VPDB (this last value just before the eruption started), with an average value of -20.9‰, during the study period. A clearly increasing trend to less negative values of δ13C-CO2 was detected from the beginning of 2021 to the moment when the eruption started, showing an increasing magmatic component in the soil CO2 measured, which was corroborated by plotting δ13C-CO2 vs. 1/[CO2] mean monthly values. During and after the eruptive period, the values showed a decreasing trend. Regarding LPG10, the values ranged from -18.8 to -7.3‰ vs. VPDB, with a mean value of -13.4‰. In this case, a general decrease trend of the δ13C-CO2 values to more negative values was observed after the eruption finished, while mean monthly values in the δ13C-CO2 vs. 1/[CO2] plot showed a shift from values ​​with a higher contribution of deep-seated CO2 at the beginning of the eruption to values ​​with a lower contribution at its end. This data demonstrates that the continuous measuring of carbon isotopic composition in soil gases before, during and after a volcanic eruption constitutes a powerful new tool for volcano monitoring.

The oceanic active volcanic island of Tenerife (2,034 km2) is the largest of the Canarian archipelago. There are more than 1,000 galleries (horizontal drillings) in the island, which are used for groundwater exploitation and allow reaching the aquifer at different depths and elevations. This work presents the first extensive study on the temporal variation of dissolved gases in groundwaters from Fuente del Valle and San Fernando galleries (Tenerife, Spain) since April 2016 to June 2020. This investigation is focused on the chemical and isotopic content of several dissolved gas species (CO2, He, O2, N2 and CH4) present in the groundwaters and its relationship with the seismic activity registered in the island. The results show CO2 as the major dissolved gas specie in the groundwater from both galleries presenting a mean value of 260 cm3STP·L-1 and 69 cm3STP·L-1 for Fuente del Valle and San Fernando, respectively. The average δ13C-CO2 data (-3.9‰ for Fuente del Valle and -6.4‰ for San Fernando) suggest a clear endogenous origin as result of interaction of them with deep-origin fluid. A bubbling gas sample from Fuente del Valle gallery was analysed, obtaining a CO2 rich gas (87 Vol.%) with a considerable He enrichment (7.3 ppm). The isotopic data of both components in the bubbling gas support the results obtained in the dissolved gases, showing an endogenous component that could be affected by the different activity of the hydrothermal system. During the study period, an important seismic swarm occurred on October 2, 2016, followed by an increase of the seismic activity in and around Tenerife. After this event, important geochemical variations were registered in the dissolved gas species, such as dissolved CO2 and He content and the CO2/O2, He/CO2, He/N2 and CH4/CO2 ratios. These findings suggest an injection of fluids into the hydrothermal system during October 2016, a fact that evidences the connection between the groundwaters and the hydrothermal system. The present work demonstrates the importance of dissolved gases studies in groundwater for volcanic surveillance.

On September 19, 2021, a new eruption began at the west flank of Cumbre Vieja volcano (La Palma, Canary Islands), after an inter-eruptive period of 50 years from the previous eruption (Teneguía, October 1971). The 2021 event was a fissure and powerful strombolian eruption with a magnitude VEI=3 and it has been considered as the most important eruption of Europe during the last 75 years in terms of the significant amount of SO2 released and the serious damage caused by the lava flows.In this work we report the leachate analyses of volcanic ash from the beginning of the eruption, focused on determining the relationship between chemical composition of water‐soluble components adhering to volcanic ash and the volcano’s activity episodes. A total of 5 main control sites or ash-collecting stations were established and ash was picked up in a daily basis. These were located around the main eruptive vents at different distances. Water-extractable concentrations of the samples leached at 1:25 for 2 hours were analyzed by ion chromatography (Cl-, SO42-, F-, Br-, NO3-, Na+, Ca2+, Mg2+, K+) and ICP-MS (Li, B, Al, Si, Sr, Ba, Fe, Ti, Cu, Sb, Rb, Ni, Co, Cd, V). The most abundant components in the leachates were SO42- for the anions and Na+ for the cations, with mean concentrations of 854 and 455 mg/kg, respectively. The results showed the following trend, in decreasing order of abundance: SO4-2>Na+>Cl->F->Ca2+>Al3+>K+>Mg2+. Fluoride, an element of primary concern for human and animal health, showed a range of 16 and 733 mg/kg and an average of 239 mg/kg, which is relatively higher than global median value (129 mg/kg). The S/Cl molar ratio in the ash leachate presented a ranged of values from 0.16 to 5.9 and the observed values > 3.0 seems to be related to ash-rich phases of the eruption. Preliminary results show significant temporal variations in ash leachate compositions, revealing changes in the eruption dynamics.

Tenerife Island (2,034 km2) is the largest island of the Canarian archipelago. Its structure is controlled by a volcano-tectonic rift-system with NW, NE and NS directions, with the Teide-Pico Viejo volcanic system located in the intersection. Teide is 3,718 m.a.s.l. high and its last eruption occurred in 1798 through an adventive cone of Teide-Pico Viejo volcanic complex. Although Teide volcano shows a weak fumarolic system, volcanic gas emissions observed in the summit cone consist mostly of diffuse CO2 degassing. In this study we investigate the Teide-Pico Viejo volcanic system evolution using a comprehensive diffuse degassing geochemical dataset 216 geochemical surveys have been performed during the period 1999-2021 at the summit crater of Teide Volcano covering an area of 6,972 m2. Diffuse CO2 emission was estimated in 38 sampling sites, homogeneously distributed inside the crater, by means of a portable non dispersive infrared (NDIR) CO2 fluxmeter using the accumulation chamber method. Additionally, soil gases were sampled at 40 cm depth using a metallic probe with a 60 cc hypodermic syringe and stored in 10 cc glass vials and send to the laboratory to analyse the He and H2 content by means of quadrupole mass spectrometry and micro-gas chromatography, respectively. To estimate the He and H2 emission rates at each sampling point, the diffusive component was estimated following the Fick’s law and the convective emission component model was estimated following the Darcy’s law. In all cases, spatial distribution maps were constructed averaging the results of 100 simulations following the sequential Gaussian simulation (sGs) algorithm, in order to estimate CO2, He and H2 emission rates. During 22 years of the studied period, CO2 emissions ranged from 2.0 to 345.9 t/d, He emissions between 0.013 and 4.5 kg/d and H2 between 1.3 and 64.4 kg/d. On October 2, 2016, a seismic swarm of long-period events was recorded on Tenerife followed by an increase of the seismic activity in and around the island (D’Auria et al., 2019; Padrón et al., 2021). Several geochemical parameters showed significant changes during ∼June–August of 2016 and 1–2 months before the occurrence of the October 2, 2016, long-period seismic swarm (Padrón et al., 2021). Diffuse degassing studies as useful to conclude that the origin of the 2 October 2016 seismic swarm an input of magmatic fluids triggered by an injection of fresh magma and convective mixing. Thenceforth, relatively high values have been obtained in the three soil gases species studied at the crater of Teide, with the maximum emission rates values registered during 2021. This increase reflects a process of pressurization of the volcanic-hydrothermal system. This increment in CO2, He and H2 emissions indicate changes in the activity of the system and can be useful to understand the behaviour of the volcanic system and to forecast future volcanic activity. Monitoring the diffuse degassing rates has demonstrated to be an essential tool for the prediction of future seismic–volcanic unrest, and has become important to reduce volcanic risk in Tenerife.D'Auria, L., et al. (2019). J. Geophys. Res.124,8739-8752Padrón, E., et al., (2021). J. Geophys. Res.126,e2020JB020318

A recent volcanic eruption has occurred in Cumbre Vieja volcano located in the west of the La Palma Island (at the northwest of the Canary Islands) from September 19 to December 13, 2021. A total of 85 days of eruption makes it the longest volcanic event since historical data have been recorder on La Palma. This volcanic eruption is part of the last stage in the geological evolution of La Palma Island, the fifth in extension (706 km2) and the second in elevation (2,423 m a.s.l.) of the Canarian archipelago. Cumbre Vieja volcano, where the volcanic activity has taken place exclusively in the last 123 ka, forms the southern part of the island. As a response to the occurrence of several seismic swarms and to strengthen the volcanic monitoring of Cumbre Vieja, a regular sampling of groundwater started in October 2017. Three sampling points have been selected, Las Salinas well and two horizontal galleries: Peña Horeb and Trasvase Oeste. Temperature (ºC), pH and electrical conductivity (EC, µS·cm 1) were measured in situ. Water samples were taken to measure the chemical and isotopic composition of the groundwaters in the laboratory. The temperature values showed mean values of 22.1 ºC, 23.7 ºC and 19.6 ºC for Las Salinas, Peña Horeb and Trasvase Oeste, respectively. The mean pH values were 6.50 for Las Salinas, 7.33 for Peña Horeb and 6.81 for Trasvase Oeste, while the mean E.C. values were 41,566 µS·cm-1, 1,684 µS·cm-1 and 426 µS·cm-1 for Las Salinas, Peña Horeb and Trasvase Oeste, respectively. The total alkalinity mean value of groundwater from Las Salinas well was 8.75 mEq·L-1 HCO3-, while that from Peña Horeb was 18.8 mEq·L-1 HCO3- and 4.4 mEq·L-1 HCO3- for Trasvase Oeste. The isotopic composition of O and H (δ18O and δD) showed a meteoric origin, with mean values of 0.8‰ and 7‰ vs. VSMOW for Las Salinas, -4.0‰ and -15‰ vs. VSMOW for Peña Horeb and -4.5‰ and -17 ‰ vs. VSMOW for Trasvase Oeste. Significant changes were observed during the eruptive period, likely related to interaction with endogenous gases such as CO2, H2S and H2. Regarding the isotopic composition of total dissolved carbon, expressed as δ13C-CO2, the average values were -4.6‰, -8.1‰ and -10.8‰ for Las Salinas, Peña Horeb and Trasvase Oeste, respectively, what suggests an endogenous origin for the CO2. Such deep-seated interaction seems stronger at Las Salinas. At this point, the isotopic composition of total dissolved carbon became more magmatic along the studied period, changing from an average of -3.79 in 2017-2018 to -5.17 from 2019 to 2021.Temporal variations were observed in the total alkalinity, δ13C-CO2 andδ18O and δD during the eruption period on La Palma Island. These changes observed in both chemical and isotopic composition were related to interaction between deep volcanic fluids and the groundwaters. Groundwaters studies associated to volcanic aquifers can provide important information about the magmatic gas input in the aquifer, to model groundwater circulations edifice and to strengthen the volcano monitoring.

Cumbre Vieja volcano is the last stage in the geological evolution of La Palma Island (Canarian Archipelago, Spain). The volcanic activity of La Palma has taken place exclusively in Cumbre Vieja in the last 123 ka, and has remained in volcanic quiescence in the last 50 years. After the occurrence of several seismic seismic swarms since 2017, a volcanic eruption began at Cumbre Vieja volcano on September 19, 2021, and resulted in the longest volcanic event since data are available on the island. The eruption lasted for 85 days and 8 hours and lava flows covered 1,219 hectares. As part of the volcano monitoring program of Cumbre Vieja, diffuse degassing of CO2 has been continuously monitored since 2005 at the southernmost part of Cumbre Vieja according to the accumulation chamber method. The monitoring site (LPA04) was selected because it shows anomalous diffuse CO2 degassing emission values with respect to the background values that had been measured in different surveys (Padrón et al., 2015). Meteorological and soil physical variables are also measured in an hourly basis and transmitted to ITER facilities about 150 Km far away. Since its installation, CO2 emissions ranged from non-detectable (-2d-1) to 1,464.0 gm-2d-1. The time series was characterized by a strong variability in the measured values that are modulated mainly by the atmospheric and soil parameters. Soil moisture is the monitored parameter that explains the highest variability of the data, being the dry season (spring y summer) the period with the highest observed diffuse emission values. This behavior in the time series changed after 2017 as an increasing trend was observed in a good temporal agreement with the increase of seismic activity recorded. Diffuse CO2 emission values showed a sustained increase reaching maximum values (up to 890 gm-2d-1) before de eruption onset. The observed diffuse CO2 emissions trend in the LPA04 geochemical station was useful to record the arrival of magmatic CO2 due to the occurrence of an upward magma migration beneath La Palma Island that caused the 2021 eruptive event.Padrón et al., (2015). Bull Volcanol 77:28. DOI 10.1007/s00445-015-0914-2

During July 2016, the first integrated heat flow, CO2, and 3He emission survey was conducted across 0.5 km2 of the summit cone and crater of Teide volcano, Tenerife, Canary Islands, Spain. The thermal energy released from Teide summit cone by diffuse degassing was 2.2 MW, and the heat flux calculated through Dawson’s method was 8.1 MW, difference due to the comparison of purely convective areas as the crater with diffusive areas as the flanks of the volcano. Diffuse CO2 output was 211±20 t d-1, and 3He emission was estimated to be within a range between 0.35 and 0.89 mol y-1. The obtained values of diffuse degassing and heat fluxes are close to others obtained for similar volcanic areas. The calculation of 3He/heat ratio for the first time in this volcanic system supports the presence of an important mantle source for the degassing of Teide volcano.

The South Rif Thrust (Northern Morocco) is characterized by several thermo-mineral water sources with outlet temperatures ranging from 24 °C to 54 °C. The anomalous temperatures of these resurgences are linked to deep circulation in areas where the geothermal gradients are abnormally high (>35 °C km−1). Nonetheless, the factors that govern the geological environment, heat flux and chemical characteristics of these thermo-mineral waters are still poorly defined. In order to provide a more comprehensive knowledge of their potential as geothermal resources, a multidisciplinary geological, hydrogeochemical, and isotopic study was carried out in 2014. The results show a hydrochemical heterogeneity displaying a large range of chemical facies, salinity and outlet temperatures, attributed not only to water-rock interactions but as well to different degrees of mixing processes between water types (regional deep diluted waters, shallow cold immature waters and deep thermal waters). The isotopic signature of thermo-mineral waters sampled (δ18O, δ2H and 3H) suggests a similar meteoric origin, recharged before the 1950’s, but from two different recharge areas: (1) the pre-Rif, the Western Meseta and/or the South Rif Thrust itself and (2) the Meseta-Atlas domain. Theoretical equilibrium temperatures at depth, ranging from 50 to 150 °C, confirm their potential as geothermal resources and the computed reservoir depths (between 0.7 and 3.0 km) which also suggest a deeper groundwater circulation than those proposed by previous studies which considered the Mesozoic aquifer (∼1.5 km depth) as the main source. Finally, the spatial correlation between thermo-mineral springs resurgences at the intersection between E-W Rif thrusts and NE-SW strike-slip fault systems, highlights the importance of those geological structures as preferential paths for groundwater circulation, privileging those intersections as priority sites to conduct further geothermal exploration.

The oceanic active volcanic island of Tenerife (2034 km2) is the largest of the Canarian archipelago. There are more than 1,000 galleries (horizontal drillings) in the island, which are used for groundwater exploitation and allow reaching the aquifer at different depths and elevations. During a two-year period (July 2016 to July 2018), a hydrogeochemical study was carried out in two galleries on Tenerife (Fuente del Valle and San Fernando) for volcanic monitoring purposes with weekly sampling. Physicochemical parameter of water, such us temperature (ºC), pH and electrical conductivity (E.C., µS·cm-1), were measured in-situ at each sampling point and chemical/isotopic composition of the water determined later in the laboratory.Temperature values showed mean values of 28.1 ºC and 19.0 ºC for Fuente del Valle and San Fernando galleries, respectively. According to the average pH values, which were 6.30 for Fuente del Valle and 7.13 for San Fernando, and based on the chemical composition, both galleries are sodium-bicarbonate (Na-HCO3) type. E.C. values in both galleries presented high ranges, with mean values of 975 and 1648 µS·cm-1 for Fuente del Valle and San Fernando, respectively. The total alkalinity mean value of groundwater from Fuente del Valle gallery was 11.3 mEq·L-1 HCO3-, while that from San Fernando was 17.3 mEq·L-1 HCO3-. The SO42-/Cl molar ratio was 0.59 and 3.4 for the samples from Fuente del Valle and San Fernando galleries, respectively.The δ18O and δD isotopic analyses showed a meteoric origin of groundwaters, with mean values of -6.2‰ and -26‰ vs. VSMOW for Fuente del Valle and -6.2‰ and -21 ‰ vs. VSMOW for San Fernando. The isotopic data showed a strong interaction with endogenous gases such as CO2, H2S, H2, etc. Regarding the isotopic composition of total dissolved carbon species, expressed as δ13CTDIC, average values of -0.17‰ and 0.26‰ were obtained for Fuente del Valle and San Fernando galleries, respectively. These results show an endogenous origin CO2 signature, heavier for Fuente del Valle gallery groundwater compared to that of San Fernando.Groundwater physicochemical parameters exhibited stable values throughout the study period, while significant temporal variations were observed in the total alkalinity, SO42-/Cl- molar ratio, δ18O and δD. Changes in isotopic ratios coincided with variations observed in the alkalinity and the SO42-/Cl- molar ratio. On October 2, 2016, a seismic swarm of long-period events was recorded on Tenerife followed by a general increase of the seismic activity in and around the island. A correlation was observed between some hydrogeochemical parameters in the groundwaters of the galleries, related to observed changes of the seismic activity. This study demonstrates the suitability of monitoring the chemical and isotopic composition of groundwater from Fuente del Valle and San Fernando galleries, as they are sensitive to changes in volcanic activity on Tenerife island. The study of groundwaters associated to a volcanic system can provide information about the magmatic gas input in the aquifer, modelling how the groundwaters flow through the edifice, and offer important geochemical information that could herald a future eruption.

Recent volcanic activity of La Palma island, the fifth in extension (706 km2) and the second in elevation (2,423 m a.s.l.) of the Canarian archipelago, has taken place exclusively in the last 123 ka at the southern part of the island, where Cumbre Vieja volcano, the most active basaltic volcano in the Canaries, has been constructed. A total of seven volcanic eruptions have been reported along the main north-south rift zone of Cumbre Vieja in the last 500 years. On October 7th and 13rd, 2017, two remarkable seismic swarms interrupted a seismic silence of 46 years in Cumbre Vieja volcano with earthquakes located beneath Cumbre Vieja volcano at depths ranging between 14 and 28 km with a maximum magnitude of 2.7. Five more seismic swarms were registered in 2020.3He/4He ratio has been monitored at Dos Aguas cold mineral spring in La Palma Island since 1991 to date as an important volcano monitoring tool able to provide early warning signal of future volcanic unrest episodes. Magmatic helium emission studies have demonstrated to be sensitive and excellent precursors of magmatic processes occurring at depth. The highest 3He/4He ratio reported to date from the Canarian archipelago has been measured at Dos Aguas: 10.24 RA (being RA the ratio in atmospheric helium) (Padrón et al., 2015). This value is higher than any value found either in the lavas or terrestrial fluid in the Canary Islands, and indicates an important mantle contribution. According to the temporal evolution of the magmatic component of helium at Dos Aguas, we suggest the occurrence of aseismic magma rising episodes beneath La Palma within the upper mantle towards an ephemeral magma reservoir in the period 2007-2017. However, in the period 2017-2020, magma rising have produced seismic swarms that were accompanied also by the highest 3He/4He ratio measured at Dos Aguas (10.42 RA); both geochemical and geophysical signals confirm an upward magma migration towards a subcrustal magma reservoir beneath La Palma island. Padrón et al., (2015). Bull Volcanol 77:28. DOI 10.1007/s00445-015-0914-2

The island of Tenerife (Canary Islands, Spain) relies on basalt-hosted aquifers to provide 90% of water for agriculture and human consumption. The island is characterised by a low-permeability core, overlain by permeable materials which are cut by impermeable dykes. The effect is a compartmentalised aquifer, which is exploited sequentially as each “pocket” of water is exhausted. The island is home to ~1 million people (with an additional 5 million visiting tourists per year), and although rain/snowfall can be heavy in winter storms, it is unpredictable from year to year, and rapid surface water run off occurs due to the steep geography. While net recharge into the upper zones of the Tenerife aquifer have been quantified (around 2 months between intense rainfall and water table fluctuations), water must then follow a tortuous path to recharge lower zones and aquifer “pockets”. Water recharge to the coastal aquifers is also interrupted and extracted during its journey. Human and agricultural pressure is highest near the coast, and has led to intensive exploitation of existing wells and horizontal galleries. In response to the intensification of water extraction and slow recharge rates, marine intrusions into the coastal aquifers of Tenerife have occurred, traditionally recorded by rising chloride levels and resulting in well/gallery closures as well as increased pressure on other extraction sites. However, in a volcanic ocean island setting, natural processes can mimic the appearance of salinisation in a coastal aquifer. Management of aquifer resources require careful consideration of seawater incursions vs. volcanic degassing contributions vs. ocean island rainfall. Full hydrochemical breakdown of 43 coastal aquifer extraction sites reveal seawater intrusion is affecting the western coastal aquifer, with the agreement of multiple parameters. The strontium isotopic signature of well samples was also measured, because it is not subject to the biological or physical fractionation processes of other isotopic systems, thereby forming distinct reservoirs for groundwater (87Sr/86Sr of host rock), and seawater. 87Sr/86Sr signatures suggest the northern coastal aquifers are also subject to seawater incursions. This parameter may be a more sensitive indicator than chlorides and conductivity markers for salinisation, especially in an ocean island environment where coastal aquifers are subject to intensive land use practices, seawater spray, and affected by diffuse volcanic degassing.

Cape Verde archipelago is a cluster of several volcanic islands arranged in a westward opening horseshoe shape located in the Atlantic Ocean, between 550 and 800 km-west of the coast of Senegal (Africa). Fogo Island is located in the southwest of the archipelago, and as main feature is a 9-km-north to south wide collapse caldera opened toward the east, within Pico do Fogo volcano rises 2,829 m.a.s.l. Pico do Fogo crater has an area of 0.142 km2 and its characterized by a fumarolic field composed by low and moderate temperature fumaroles, with temperatures around 95ºC and reaching 400ºC respectively. The last eruption of Fogo volcanic system took place between November 2014 and February 2015, when four new eruptive vents were formed, and destroyed partially the villages of Portela and Bangaeira (Silva et. al., 2015) forcing the evacuation of 1,300 inhabitants. In this work we present the temporal evolution of 3He/4He isotopic ratio, 3He and 4He emission and thermal energy released data measured from March 2007 to November 2018 in the crater of Pico do Fogo. In all the studied temporal evolutions, we can observe two main increases in the above parameters, the first in early 2010, suggesting a magmatic intrusion, and the second several months before the eruption onset. We have also observed that changes in the 3He emission might be accompanied by a significant increase in thermal output if the system is in an eruptive cycle. Our results confirm 3He emission studies are highly reliable indicator of imminent volcanic eruption and constitute a powerful tool to monitor the activity of volcanic areas around the world.Silva et al., (2015), Geophysical Research Abstracts Vol. 17, EGU2015-13378, EGU General Assembly.

Hydrogen (H2) is one of the most abundant trace species in volcano-hydrothermal systems and is a key participant in many redox reactions occurring in the hydrothermal reservoir gas. Although H2 can be produced in soils by N2-fixing and fertilizing bacteria, soils are considered nowadays as sinks of molecular hydrogen (Smith-Downey et al. 2006). Because of its chemical and physical characteristics, H2 generated within the crust moves rapidly and escapes to the atmosphere. These characteristics make H2 one of the best geochemical indicators of magmatic and geothermal activity at depth. Cumbre Vieja volcano (La Palma, Canary Islands) is the most active basaltic volcano in the Canaries with seven historical eruptions being Teneguía eruption (1971) the most recent one. Cumbre Vieja volcano is characterized by a main north–south rift zone 20 km long, up to 1950 m in elevation and covering an area of 220 km2 with vents located at the northwest and northeast. Cumbre Vieja does not show any visible degassing (fumaroles, etc.). For that reason, the geochemical volcano monitoring program at Cumbre Vieja volcano has been focused on soil degassing surveys. Here we show the results of soil H2 emission surveys that have been carried out regularly since 2001. Soil gas samples were collected in about 600 sampling sites selected to obtain a homogeneous distribution at about 40 cm depth using a metallic probe and 60 cc hypodermic syringes and stored in 10 cc glass vials. H2 content was analysed later by a VARIAN CP4900 micro-GC. A simple diffusive emission mechanism was applied to compute the emission rate of H2 at each survey. Diffuse H2 emission values were used to construct spatial distribution maps by using sequential Gaussian simulation (sGs) algorithm, allowing the estimation of the emission rate from the volcano. Between 2001-2003, the average diffuse H2 emission rate was ∼2.5 kg·d−1 and an increase of this value was observed between 2013-2017 (∼16.6 kg·d−1), reaching a value of 36 kg·d−1 on June 2017, 4 month before the first recent seismic swarm in October, 2017 at Cumbre Vieja volcano. Six additional seismic swarms had occurred at Cumbre Vieja volcano (February 2018, July-August 2020; October 8-10, 2020; October 17-19, 2020, November 21, 2020 and December 23-26, 2020) and changes of diffuse H2 emission related to this unrest had been observed reaching values up to ∼70 kg·d−1. Diffuse H2 emission surveys have demonstrated to be sensitive and excellent precursors of magmatic processes occurring at depth in Cumbre Vieja. Periodic diffuse H2 emission surveys provide valuable information to improve and optimize the detection of early warning signals of volcanic unrest at Cumbre Vieja volcano.