Environmental influences on soil CO2 degassing at Furnas and Fogo volcanoes (São Miguel Island, Azores archipelago)

Since October 2001, four soil CO2 flux stations were installed in the island of São Miguel (Azores archipelago), at Fogo and Furnas quiescent central volcanoes. These stations perform measurements by the accumulation chamber method and, as the gas flux may be influenced by external variables, the stations are equipped with several meteorological sensors. Multivariate regression analysis applied to the large datasets obtained allowed observing that the meteorological variables may influence the soil CO2 flux oscillations from 18% to 50.5% at the different monitoring sites. Additionally, it was observed that meteorological variables (mainly soil water content, barometric pressure, wind speed and rainfall) play a different role in the control of the gas flux, depending on the selected monitoring site and may cause significant short-term (spike-like) fluctuations. These divergences may be potentially explained by the porosity and hydraulic conductivity of the soils, topographic effects, drainage area and different exposure of the monitoring sites to the weather conditions. Seasonal effects are responsible for long-term oscillations on the gas flux.Before a reliable application of soil CO2 flux to seismic and/or volcanic monitoring, it is important to recognize those environmental influences on the gas flux. In addition, understanding the external meteorological influences on the gas flux may be important for the public health risk assessment, since meteorological parameters may cause also significant indoor CO2 increases. In a house at Furnas Village (in Furnas volcano caldera), the values detected reached percentages as high as 20.8% due to significant decreases in the barometric pressure.