Toxic Gas Emanations In Populated Areas: Construction Of A Gis Gas Risk Map For Two Communities Built On The Quiescent Alban Hills Volcanic Complex (Ciampino And Marino, Near Rome Italy)

Italy is geologically young and still active, being characterised by a high level of seismicity and by active or quiescent volcanic structures. The combination of these two features can cause elevated toxic-gas concentrations in the near surface environment, with gases produced at depth migrating upwards along highly permeable fault zones. The elevated concentrations of these naturally-occurring toxic gases in the shallow environment results in a health risk to the people living in the affected areas. In particular, the urban expansion of densely populated cities above active/quiescent volcanic complexes, such as Rome onto the Alban Hills volcanic district, dramatically increases the number of people exposed to this risk. In general this problem receives little attention from local governments, although public concern is raised periodically when anomalous concentrations suddenly kill humans or livestock. Of principle concern in this regard is the long-term risk of developing lung cancer due to extended exposure to high Rn values and the short term risk of exposure to deadly concentrations of H2S and CO2. This paper describes the use of geostatistical and GIS techniques to assess toxic gas risk. GIS provides a powerful tool for the management and integration of georeferenced information to assess natural hazards. This method allows the organization and collection of data using software designed to efficiently capture, store, update, manipulate, analyse, and display all forms of geographically referenced information. In this work a number of geological factors were studied which relate to the production, migration and accumulation of hazardous gases in the shallow environment. The approach used consists of: a) organization of data into geognostic layers; b) application of software functions in the GIS program which allow the management, analysis and integration of these various layers, including merging each data set with a user-defined grid; c) application of geostatistical techniques to calculate a weighting factor for each layer-grid which assesses the importance of its contribution to hazardous gas emanation; d) overlaying the grid-layers to obtain an algebraic sum of the weighted single cells; and e) comparison and validation of the obtained TGR (Toxic Gas Risk) map with the geochemical information provided by indoor measurements. The produced risk map is presently being used by the affected communities in land-use planning and safety assessments.