VIVEIROS, F., CARDELLINI, C., FERREIRA, T., CHIODINI, G., SILVA, C. (2008) - Soil CO₂ flux mapping at Furnas Volcano caldera (S. Miguel Island, Azores archipelago) – a tool for seismo-volcanic monitoring and for public health risk assessment. X Workshop IAVCEI de Gases Vulcânicos, México, 10 - 20 de Novembro (Poster).

Furnas Volcano is a quiescent central volcano located in the eastern part of the island of S. Miguel (Azores archipelago). Present day activity is marked by several visible degassing areas, mainly four low temperature fumarolic fields, thermal and CO₂ cold springs.

 

A total of 1244 soil CO₂ flux measurements were performed in an area of about 4.16 km², that connects the two main fumarolic fields of the volcano, in order to evaluate the extent of the soil diffuse degassing area. The measurements were performed based on the accumulation chamber method and due to the influences of meteorological parameters variations on soil gas flux the field surveys were carried out during summer months with more stable weather conditions. Soil CO₂ flux values varied from 0 to 28900 g/m²d, being the average value 227g/m²d.

 

Soil CO₂ degassing map was produced by means of Sequential Gaussian Simulation (sGs) algorithm as post-processing of 100 realizations of soil CO₂ flux distribution. For the first time the total CO₂ released by soil diffuse degassing was calculated at Furnas Volcano, which varied from 5508 t/d to 900 t/d with an average value of 700 t/d. The final maps show a low dispersion with a coefficient of variation of 8.9%.

 

The most important soil CO₂ flux anomalies are displayed in the vicinities of the main fumarolic fields and an important soil diffuse degassing area extends below Furnas village dwellings. From the pattern of the main soil CO₂ anomalies it is possible to define degassing lineaments with general NW-SE trend.

 

In the early nineties and during 2002-2003 soil CO₂ concentration surveys were performed in an area focused on Furnas village. When comparing the previous maps with the soil CO₂ flux anomalies obtained in this study, it is possible to observe that the degassing anomalies seem to have remained quite stable along the last 15 years. This constitutes an evidence of the properness of these soil diffuse degassing surveys for seismo-volcanic monitoring purposes. Furthermore, considering that at Furnas village many dwellings are built over significant soil degassing areas and that the CO₂ may upflow indoors, this kind of maps are particularly important for public health risk assessment and should be taken into account by the land-use planners in active volcanic areas or in soil degassing prone areas resulting from other sources.