Natural Gas Hazard (CO 2, 222 Rn) within a Quiescent Volcanic Region and Its Relations with Tectonics: The Case of the Ciampino-Marino Area, Alban Hills Volcano, Italy

Groundwater surveys were performed by detailed(around 300 sites) grid-analysis of water temperature, pH, redox potential, electrical conductivity, 222 Rn, alkalinity and by calculating the pCO 2 , throughout the Ciampino and Marino towns in the Alban Hills quiescent volcano (Central Italy). Following several episodes of dangerous CO 2 exhalation from soils during the last 20 years and earlier ashistorically recorded, the work aimed at assessing the Natural Gas Hazard (NGH) including the indoor-Rn hazard. The NGH was defined as the probability of an area to become a site of poisonous peri-volcanic gas exhalations from soils to the lower atmosphere (comprising buildings). CO 2 was found to be a ``carrier'' for the other poisonous minor and in trace components (H s S, CH 4 , 222 Rn, etc.). This assessment was performed by extrapolating in the aquifer CO 2 and 222 Rn conditions, and discriminating sectors where future CO 2 flux in soils as well as indoor-Rn measurements have to be noted. A preliminary indoor-Rn survey was performed at about 200 sites. The highest values were found in the highest pCO 2 and high 222 Rn values in groundwater. This indicates convection and enhanced permeability in certain sectors of the main aquifer, i.e., along the bordering faults and inside the gas-trap of the Ciampino Horst., where ``continuous gas-phase micro-macro seepage mechanism'' is invoked to explain the high peri-volcanic gases flux. Copyright Kluwer Academic Publishers 2002