Temporal evolution of dissolved gases in groundwater of Tenerife Island.

This work presents the temporal evolution (April 2016 to June 2020) of the chemical and isotopic content of dissolved gas species (CO 2 , He, O 2 , N 2 and CH 4) present in groundwater at the Fuente del Valle (FV) and San Fernando (SF) galleries (Tenerife, Spain), and their relationship with the seismic activity observed on the island. CO 2 is the major dissolved gas species in the groundwater at both galleries, and the δ13C-CO 2 data show a clear endogenous origin as a result of interaction with deep fluids. A bubbling gas sample from Fuente del Valle revealed CO 2 rich gas with a considerable He component. The isotopic data of both components (δ13C-CO 2 and 3He/4He) in the bubbling gas support the sources inferred from the dissolved gas compositions, with the endogenous component affected by changes in volcanic-hydrothermal system activity. On October 2, 2016, a seismic swarm occurred, followed by an increase in seismic activity throughout Tenerife. After the event, geochemical variations were registered in the dissolved gas species, e.g., an increase in dissolved CO 2 and He content and CO 2 /O 2 , He/CO 2 , He/N 2 and CH 4 /CO 2 ratios. These changes suggest an injection of fluids into the Tenerife hydrothermal system during October 2016, confirming a connection between groundwater and the hydrothermal system. This investigation highlights the relevance of dissolved gases in groundwater as a tool for volcanic surveillance. • First extensive study of dissolved gases in groundwaters from Tenerife • Dissolved gas concentrations show changes coupled with a seismic swarm on Tenerife • Groundwaters have considerable dissolved He and CO 2 • Isotopic data provide important information about the endogenous origin of dissolved gases [ABSTRACT FROM AUTHOR]