Chapter 20 Permanent monitoring of soil CO2 degassing at Furnas and Fogo volcanoes (São Miguel Island, Azores)

Eight years of permanent soil CO2 diffuse degassing monitoring at Furnas and Fogo volcanoes shows that several environmental variables may influence soil CO2 flux to a different extent depending on the location. Stepwise multivariate regression analysis applied to the data acquired by the permanent flux stations installed on Sao Miguel showed that the monitored environmental variables may influence the gas flux in a proportion between 18 and 51%. The external variables that most significantly correlate with the soil CO2 flux values for Fogo and Furnas volcanoes monitoring sites are the rainfall/soil water content with its effect on permeability; the barometric pressure pumping effect; the wind speed owing to the atmospheric air intrusion and the air temperature through the action of atmospheric tides. These variables are responsible not only for spike-like oscillations in CO2 emission, but also for the long-term oscillations with higher and lower values registered usually during winter and summer months, respectively. Residual time-series, calculated using regression models, are compared with other geophysical and geochemical monitoring data in order to recognize changes that may be correlated with the volcanic/hydrothermal systems.