Modelling of CO2 circulation in the Colli Albani area
The Colli Albani is a quiescent volcano located near the city of Roma, characterized by the presence of an active geothermal system, periodic seismic swarms and intense diffuse degassing. Several accidents, some lethal, have occurred in recent years associated with episodes of more intense releases and outbursts of volcanic gases, predominantly CO2 and H2S. Gas emissions presently comprise the most hazardous phenomenon for the highly populated Colli Albani area, and there is always the potential for the occurrence of seismic activity. This chapter presents numerical modelling of heat and fluid circulation, applied to study the mechanisms that control the diffuse degassing at Colli Albani volcano. Multi-phase and multi-component simulations were carried out using the TOUGH2 geothermal simulator in a realistic geological context, which includes all available information on the stratigraphy and structure of the Colli Albani substrate, together with data on the total gas flux, the local geothermal gradient, the local hydrogeology and the thermal characteristics of the rocks. The geothermal reservoir at Colli Albani is hosted by the 2000–3000-m-thick Mesozoic–Cenozoic carbonatic succession, capped by Pliocene clays that act as an aquiclude and are few hundreds to over 1000 m thick, which are in turn covered by continental sedimentary and volcanic deposits that host the shallow hydrogeological system. Numerical simulations evaluate the effects associated with the thickness of the carbonatic basement and its cap rock, the role of CO2 supply rate at depth, and the influence of permeable channel-ways through the cap rocks. Numerical simulations show that the thickness of the geothermal reservoir hosted by the carbonatic basement and of its impervious cover control the vigour of the convection, the extent and depth (and hence temperature) of the lateral recharge area, and the distribution of the CO2 within the system. This result suggests that the temperature distribution and diffuse degassing at the surface reflect not only the characteristics of the heat and fluid source at depth, but also the specific structure and hydrological properties of the site where they are measured.
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The Colli Albani volcano (also Alban Hills volcano) is the large quiescent volcanic field that dominates the Roman skyline. The Colli Albani is one of the most explosive mafic calderas in the world, associated with intermediate to large volume ignimbrites. At present it shows signs of unrest, including periodic seismic swarms, ground uplift and intense diffuse degassing, which are the main short-term hazards. New studies have discovered deposits related to previously unknown pre-Holocene and Holocene volcanic and phreatic activity. In the fourth Century B.C.E. Roman engineers excavated a tunnel through the Albano maar crater wall to keep the lake from breaching the rim and flooding the surrounding countryside, events that had previously destroyed this region several times.
The Colli Albani Volcano contains 21 scientific contributions on stratigraphy, volcanotectonics, geochronology, petrography and geochemistry, hydrogeology, volcanic hazards, geophysics and archaeology, and a new 1:50 000 scale geological map of the volcano. The proximity to Rome and the interconnection between volcanic and human history also make this volcano of interest for both specialists and non-specialists.