Temporal evolution of dissolved gases in groundwater of Tenerife Island

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.