High CO2Levels in Boreholes at El Teide Volcano Complex (Tenerife, Canary Islands): Implications for Volcanic Activity Monitoring

Emissions of CO2 have been known for more than a hundred years as fumarolic activity at the terminal crater of El Teide volcano and as diffuse emissions at numerous water prospection drillings in the volcanic island of Tenerife. Large concentrations of CO2 (>10% in volume) have been found inside galleries, long horizontal tunnels excavated for water mining. However, CO2 concentrations of only 2900 ppm have been observed at the surface of the central region of the island (Las Can˜ adas del Teide caldera). In this work we analysed CO2 concentrations in the subsurface of Las Can˜ adas caldera, in an attempt to study the vertical distribution of carbon dioxide and, in particular, the low emissions at the surface. This has been done through a series of 17 vertical profiles in two deep boreholes excavated in the Caldera. We found high levels of CO2, varying in time from 13 vol% up to 40 vol% in different profiles directly above the water table, while no significant concentrations were detected above the thermal inversion that takes places in both boreholes at approximately 100 m from the water table. Water analyses also showed high dissolved CO2 levels in equilibrium with the air, and an average d13C value in DIC of +4.7& (PDB), apparently induced by fast CO2 degassing in the bicarbonated water.