IAVCEI 2008 - General Assembly, August 17th - 22nd - Reykjavik, Iceland 222Rn monitoring in soil gas before, during and after the 2002-2003 and 2007 Stromboli eruptions. Sciarra A., Quattrocchi F., Cantucci B., Cinti D., Pizzino L., Voltattorni N. Istituto Nazionale di Geofisica e Vulcanologia, Sezione di Roma1, Italy corresponding author: alessandra.sciarra@ingv.it From Finizola et al., 2003 Seven 222Rn soil gas surveys have been accomplished throughout the N-E area of Stromboli Island. The former survey was performed in July 2001, in a quiescent period since 1985, throughout a grid of 50 sites. The same grid was repeated in January 2003, soon after 28 December 2002 eruption onset. Other surveys were carried out in March 2003, June 2003 after the 5 April 2003 paroxysm event, July 2004 and March 2005 (quiescent periods). Finally, CO2 fluxes and soil gas surveys (Rn, CO2, CH4, H2, O2, N2, He) were performed in March 2007 after the slightly February 2007 eruption onset. Bq/m3 July 2001 – Rn concentration Decades of quiescence Bq/m3 January 2003 – Rn concentration Soon after paroxysmal phase of the December 2002 onset of Eruption Bq/m3 March 2003 – Rn concentration Before the 5 April 2003 strong explosion Bq/m3 June 2003 – Rn concentration Before the end of the 2002-2003 eruptive crisis Bq/m3 July 2004 – Rn concentration Well after the 2002-2003 eruptive crisis Bq/m3 January 2005 – Rn concentration Quiescence or preparation of the 2007 eruption ? Anomalies far from the NE-SW main system Bq/m3 March 2007 – Rn concentration Soon after the slighter February 2007 eruption Istituto Nazionale di Geofisica e Vulcanologia Radon data set of the investigated area has been divided in eight zones (on the basis of the 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. 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 reached a maximum of 13.000 Bq/m3 in January 2003, then 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 a radon concentration increasing. From Finizola et al., 2003 ppm v/v March 2007 - He concentration 5 Km March 2007 – H2 concentration Tyrrhenian Sea %v/v March 2007 – CH4 concentration % v/v March 2007 – CO2 flux g/m2day March 2007 – CO2 concentration 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 collapse, 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.