Distribution and origin of dissolved gases of groundwaters at Las Cañadas aquifer, Tenerife, Canary Islands

Volcanic aquifers commonly trap an important fraction of the main soluble components of fluids released from volcanic-hydrothermal system (CO2, SO2, H2S, HCl, HF, etc.). In particular, the interactions between volcanic gases and volcanic aquifers have been studied through hydrogeochemical parameters, as major and minor ions contents and dissolved gases in groundwaters. In the context of hydrogeochemical studies applied to active volcanic areas, studies of dissolved gases species in groundwater could be a useful tool to better understand the subsurface processes as gas-water-rock interaction or to strengthen the geochemical seismic-volcanic surveillance programs. In this work, we report the results of the geochemical characterization of dissolved gases (N2, O2, Ar, CO2, CH4, CO, H2, He, 222Rn, δ13CTDIC) in 96 groundwater samples from Las Cañadas aquifer (around Teide volcano) between May and October, 2006. The main aims of this work are: (1) to determine the background level of magmatic gas input in the aquifer during quiescent periods, (2) to better define the origin of dissolved gases in Las Cañadas aquifer, specially CO2, (3) to evaluate the gas-water-rock interaction processes occurring at depth, and (4) to delineate high permeable pathway of upward migration of volcanic-hydrothermal gases. In general, the dissolved gas phase in groundwaters of Las Cañadas aquifer is relatively enriched in endogenous gases (CO2, He and H2) while it is relatively depleted in atmospheric gases (N2, O2 and Ar). N2-O2-CO2 triangular diagram shows that dissolved gases in most of analyzed groundwater are variable mixtures of CO2-rich fluids from the volcanic-hydrothermal system (as represent the Teide fumaroles) with dissolved air. The relatively high N2/O2 ratio in some groundwater compared to the air saturated water suggests an O2 consumption during gas-water-rock interactions occurring at depth. Spatial distribution maps show anomalous concentration of 222Rn CH4, H2 and CO2 dissolved in groundwater at the westernmost area of Las Cañadas aquifer, which is in good spatial correlation with geophysical and geochemical anomalies related to 2004-2005 seismic-volcanic unrest at Tenerife Island. Determinations of δ13C values in the total dissolved inorganic carbon (TDIC) in groundwater in Las Cañadas aquifer ranging from -5 to +5 (‰ vs PDB). This result was explained by isotopic fractionation of either volcanic-hydrothermal CO2 partially dissolved in groundwater or due to precipitation of CaCO3 and CO2 degassing related to silicate hydrolysis dissolving Ca2+.