Hydrogeochemical monitoring of groundwater as a tool for volcanic surveillance in Tenerife

The chemical and isotopic composition of groundwater in active volcanic oceanic islands is highly sensitive to the input of magmatic gases. On Tenerife (2,034 km2), the absence of visible peripheral gas emissions, aside from the Teide summit fumaroles, makes the island’s extensive network of water galleries a “window” into the volcanic aquifer. Since 2016, following a significant long-period (LP) seismic swarm on October 2 (D'Auria et al., 2019), a multidisciplinary geochemical monitoring program has been maintained across ten representative galleries (horizontal drillings) to detect deep-seated magmatic signals.Current results reveal a relatively homogeneous hydrochemical facies, primarily bicarbonate-sodium-calcium type, consistent with CO2-driven water-rock interaction. However, long-term time series analysis (2016–2025) across several galleries, including Fuente del Valle, San Fernando, Barranco de Vergara, and Buen Viaje, demonstrates significant temporal fluctuations in key volcanic tracers. High-frequency sampling during the study period identified distinct peaks in total alkalinity (HCO3−), pCO2, and Na+/Cl− ratios that correlate with episodes of increased seismicity, including volcano-tectonic (VT) and hybrid swarms.Notably, Fuente del Valle and San Fernando galleries exhibited sustained increasing trends in HCO3− and SO42−/Cl− molar ratios (Amonte et al., 2021), particularly surrounding the hybrid seismic swarms of 2019, 2022, and late 2024. Furthermore, sharp increases in pCO2 and dissolved fluoride (F−) concentrations in galleries such as El Almagre and Barranco de Vergara coincide with periods of renewed seismic unrest, suggesting the pulsative injection of magmatic CO2 and acidic volatiles into the hydrothermal-volcanic aquifer.These hydrogeochemical variations provide evidence of a dynamic hydrochemical connection between the Teide-Pico Viejo volcanic system and the underlying aquifer. By establishing robust baseline datasets and identifying pre-seismic geochemical anomalies, this monitoring approach serves as a critical early-warning tool. The integration of these hydrochemical "fingerprints" into the INVOLCAN volcano surveillance program enhances the ability to forecast changes in the volcanic system, ultimately contributing to volcanic risk reduction on Tenerife.ReferencesAmonte, C., Asensio-Ramos, M. et al. (2021) Hydrogeochemical temporal variations related to changes of seismic activity at Tenerife, Canary Islands. Bulletin of Volcanology, 83:24. https://doi.org/10.1007/s00445-021-01445-4D'Auria, L., Barrancos, J. et al. (2019). The 2016 Tenerife (Canary Islands) long‐period seismic swarm. Journal of Geophysical Research: Solid Earth, 124, 8739–8752. https://doi.org/10.1029/2019JB01787