Groundwater and soil CO2 efflux weekly monitoring network for the volcanic surveillance of Tenerife, Canary Islands

Tenerife, with 2034 km², isthe largest active volcanic island of the Canarian archipelago and boasts over 1,000 galleries used for groundwater explotation, enabling access to the aquifer at varying depths and elevations. From mid-2016 to present, we've diligently sampled two important galleries - Fuente del Valle and San Fernando - on a weekly basis for volcanic monitoring. On-site measurements of water's physicochemical parameters such as temperature (ºC), pH, and electrical conductivity (E.C., µS·cm^-1) were conducted at each sampling point. Subsequently, the water's chemical/isotopic composition and dissolved gases were analyzed in the laboratory. Noteworthy trends in certain parameters, including increased conductivity, sulfate (SO₄^2-) concentration, chloride, bicarbonate, and the SO₄^2-/Cl^- molar ratio, suggest an infiltration of deep-seated gases into the groundwater. Isotopic data further revealed a robust interaction with endogenous gases like CO₂, H₂S, H₂, etc. Additionally, correlations were discerned between specific hydrogeochemical parameters in the gallery groundwaters, correlating with observed seismic activity changes. This study underscores the sensitivity of monitoring the chemical and isotopic composition of groundwater in Fuente del Valle and San Fernando galleries to fluctuations in volcanic activity on Tenerife. Exploring groundwater associated with a volcanic system offers insights into magmatic gas input into the aquifer, models groundwater flow within the edifice, and provides vital geochemical information potentially indicating an imminent eruption.

Concurrently, a cost-effective method to gauge CO₂ fluxes using alkaline traps has significantly contributed to Tenerife's volcanic surveillance. In the summer of 2016, a network of 31 closed alkaline traps was strategically placed across Tenerife's three volcanic rifts (NE, NW, and NS) and at Cañadas Caldera, persisting until the present. The weekly replacement of alkaline solutions facilitated subsequent laboratory titration analysis of the trapped CO₂, expressed as weekly integrated CO₂ efflux. Across the study period, the average CO₂ efflux stood at 6.41 g·m^-2·d^-1, with variations across regions: 8.41 g·m^-2·d^-1 for the NE rift-zone, 5.11 g·m^-2·d^-1 for Cañadas Caldera, 6.36 g·m^-2·d^-1 for NW rift-zone, and 6.35 g·m^-2·d^-1 for NS rift-zone. Notably elevated CO₂ effluxes were observed in the NE rift-zone, exhibiting maximum values. While the temporal evolution of CO₂ efflux estimated by closed alkaline traps exhibited minimal variation during the study, seasonal fluctuations were noted. The systematic use of closed static chamber alkaline traps proves to be a straightforward and economical method aiding volcanic surveillance, especially in areas lacking visible volcanic gas manifestations.

This comprehensive approach using chemical analysis of groundwater and CO₂ flux monitoring through alkaline traps showcases their combined efficacy in advancing Tenerife's volcanic surveillance, potentially serving as a crucial precursor to future volcanic activity.