222Rn monitoring in soil gas before, during and after the 2002-2003 and 2007 Stromboli eruptions.

Reykjavík - ICELAND

Seven 222Rn soil gas surveys have been accomplished throughout the N-E area of Stromboli Island. The former survey has been performed in July 2001, in a quiescent period since 1985, throughout a grid of 50 sites. The same grid has been repeated, in January 2003, soon after December, 28, 2002 eruption onset. The other surveys have been carried out in March 2003, in June 2003 after the April, 5, 2003 paroxysm event, and in July 2004 and March 2005 (quiescent periods). Finally, CO2 fluxes and soil gas surveys (Rn, CO2, CH4, H2, O2, N2, He) have been performed in March 2007 after the slightly February 2007 eruption onset. Results have highlighted correlated and synchronous 222Rn soil gas changes throughout the grid, allowing to discriminate the sectors of maximum increased 222Rn emission at surface, with a volcano-tectonical significance, mostly in January and March 2003 surveys. The positive NE-SW anomalies have been found maximum up to 13.000 Bq/m3 in January 2003, while after the Rn values returned to pre-eruptive values. Plotting single point concentration versus monitoring time, it is well evident that, in proximity of a paroxysm event, always the same points undergo to a radon concentration increasing. Data set of the investigated area has been divided in eight zones (on the basis of geographic sectors of the island). In every zone maximum Rn values correspond to the paroxysmal phase of the Stromboli eruptions. In particular, highest radon values are found in the “Lampara – COA” and “Town Hall” zones located along the known N40° lineament. After the eruptive phase, concentrations start to decrease. The NW-SE extension could be linked to the ongoing volcano collapse toward the NW located Sciara del Fuoco slope. Also the SE sector could undergo to collapses, along weakness lines (faults and fractures) whose prolongation in the Mole-S14 zone could release anomalous CO2-fluxes and anomalous soil gas concentrations (up to 10-20 % in CO2) dangerous for human health. The importance of geochemical monitoring together with volcanic time series, is a valid tool for evaluating the volcanic hazard.