Natural Gas Hazard (CO<Emphasis Type="Bold">2, <Superscript> <Emphasis Type="Bold">222 </Superscript>Rn) within a Quiescent Volcanic Region and Its Relations with Tectonics: The Case of the <Emphasis Type="BoldItalic">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, <Superscript>222</Superscript>Rn, alkalinity and by calculating the pCO<Subscript>2</Subscript>, throughout the Ciampino and Marino towns in the Alban Hills quiescent volcano (Central Italy). Following several episodes of dangerous CO<Subscript>2</Subscript> 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<Subscript>2</Subscript> was found to be a ``carrier'' for the other poisonous minor and in trace components (H<Subscript>s</Subscript>S, CH<Subscript>4</Subscript>, <Superscript>222</Superscript>Rn, etc.). This assessment was performed by extrapolating in the aquifer CO<Subscript>2</Subscript> and <Superscript>222</Superscript>Rn conditions, and discriminating sectors where future CO<Subscript>2</Subscript> 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<Subscript>2</Subscript> and high <Superscript>222</Superscript>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