Soil CO2 efflux and hydrogeochemical monitoring for volcanic surveillance of Tenerife, Canary Islands, Spain

Tenerife island, with 2034 km², is the largest active volcanic island of the Canarian archipelago and has remained in a state of volcanic calm since the Chinyero eruption in 1909. Apart from the fumarolic activity present at the summit crater of Teide volcano, nowadays not any visible evidence of gas emission is found at Tenerife. The main geophysical event recorded in Tenerife in the study period is a significant seismic swarm of long-period events recorded on 2 October 2016, with a subsequent significant increase in seismic activity recorded in and around the island observed to date. As part of the INVOLCAN Volcano monitoring program for the reduction of volcanic risk in Tenerife and with the aim of strengthening the geochemical monitoring program, we have conducted on a weekly basis, two distinct studies: (1) physical-chemical and chemical/isotopic composition studies in the groundwater of two galleries (Fuente del Valle and San Fernando) and its associated dissolved gases from mid-2016 to the present; and (2) a weekly study (since March 2024) of diffuse CO₂ emissions at 31 selected points along the three volcanic-ridges of the island (NE, NW, and NS)  and Las Cañadas caldera (central volcanic complex). 

The most relevant results obtained in the hydrogeological study, were significant changes observed during and after the 2 October 2016 seismic swarm measured in the dissolved gases (CO₂ and He), likely produced by the dissolution of volcanic hydrothermal gases released after the input of magmatic fluids in the groundwater system. 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.  

The diffuse CO₂ emissions values measured at the 31 selected points ranged between non-detected values (<0.5 g·m^-2d^-1) and 26.2 g·m^-2d^-1, with maximum values measured in Las Cañadas caldera and the north-east volcanic ridge. Although the values ​​recorded have been low, with values ​​typical of biogenic degassing, the time series that began in March 2024 serves as a baseline in periods of volcanic calm to assess possible future increases related to changes in the activity activity of Tenerife.