Monitoring of fumarole discharge and CO2 soil degassing in the Azores: contribution to volcanic surveillance and public health risk assessment

Fluid geochemistry monitoring in the Azores involves the regular sampling and analysis of gas discharges from
fumaroles and measurements of CO2 diffuse soil gas emissions. Main degassing areas under monitoring are associated
with hydrothermal systems of active central volcanoes in S. Miguel, Terceira and Graciosa islands. Fumarole
discharge analysis since 1991 show that apart from steam these gas emissions are CO2 dominated with
H2S, H2, CH4 and N2 in minor amounts. Mapping of CO2 diffuse soil emissions in S. Miguel Island lead to the
conclusion that some inhabited areas are located within hazard-zones. At Furnas village, inside Furnas volcano
caldera, about 62% of the 896 houses are within the CO2 anomaly, 5% being in areas of moderate to high risk.
At Ribeira Seca, on the north flank of Fogo volcano, few family houses were evacuated when CO2 concentrations
in the air reached 8 mol%. To assess and analyse the CO2 soil flux emissions, continuous monitoring stations
were installed in S. Miguel (2), Terceira and Graciosa islands. The statistical analysis of the data showed
that some meteorological parameters influence the CO2 flux. The average of CO2 flux in S. Miguel stations
ranges from 250 g/m2/d at Furnas volcano to 530 g/m2/d at Fogo volcano. At Terceira Island it is about 330
g/m2/d and at Graciosa 4400 g/m2/d.