Natural Gas Hazard (CO2 and radon) in the Colli Albani volcano, central

Abstract

We present a very detailed study focused on the CO2 and radon content in the groundwater circulating in the Colli Albani volcano and surroundings. A total of 650 water sites (wells and springs) were visited, with a more detailed sampling in the calderic and extra-calderic sectors of the volcano, acquiring new and more complete data with respect to the past. This study was aimed at highlighting the areas characterised by high CO2 and radon contents in groundwater in order to: i) deepen their relationships with the tectonic features of the area; ii) evaluate the present level of Natural Gas Hazard (NGH) and iii) start a continuous radon indoor survey and pursue discrete measurements. In particular, new radon detectors have been built and tested in the frame of an agreement between INGV and DINCE (Department of Nuclear Engineering and Conversions of Energy) of the “La Sapienza” University of Rome. Large areas with CO2 and radon-rich groundwater were recognised. They correspond to the well known main tectonic and volcanic structures affecting the area (horsts, regional faults, caldera rim). Existing faults and associated fracture networks allow deep gases to upraise and dissolve in shallow aquifers, generally over-saturated in CO2, allowing a free gas phase to reach the surface, thus affecting soils and causing a widespread degassing (Figg. 1, 2, 3). Moreover, CO2 can act as carrier for radon, also produced by quaternary volcanic rocks rich in U and Th-bearing minerals. As a consequence, the detailed mapping of both CO2 and radon contents in groundwater can be a powerful tool in order to assess and evaluate the NGH-prone areas (Figg. 4, 5). Our new data showed the existence of NGH-prone sectors, so far unknown, very close to the city of Rome. As a consequence, the detailed mapping of both CO2 and radon contents in groundwater can be a powerful tool in order to assess and evaluate the NGH-prone areas (Figg. 4, 5). Our new data showed the existence of NGH-prone sectors, so far unknown, very close to the city of Rome.