Origin and hazard of CO2 and H2S emissions in the Lavinio-Tor Caldara zone (Metropolitan City of Rome Capital, Italy)

Abstract

Gas hazard was evaluated at Lavinio-Tor Caldara, the southernmost gas-discharging zone of the quiescent Albani Hills volcano in central Italy. Also this zone, like the other gas discharges of this volcanic complex, is located above a structural high of the buried Mesozoic carbonate basement, which represents the main reservoir for gas rising from depth. All extensional faults affecting the carbonates are leaking pathways along which gas may rise to the surface creating hazardous conditions. Gas is dominated by CO2 (>90 vol.%) and the second component at Lavinio-Tor Caldara is H2S that displays the highest content (4.0-6.3 vol.%) of all gas manifestations of the Rome region. This H2S enrichment corresponds to a marked decrease in 3He/4He (R/Ra) isotopic ratio suggesting that gas was contaminated in an upper crustal environment. The main gas discharge occurs at the natural reserve of Tor Caldara, in zones where past sulphur mining excavations removed the surficial impervious cover, or along a ditch. Comparison of the results of four soil CO2 flux surveys carried out in 2005-2018 at Miniera Grande within Tor Caldara, indicates that the highest soil CO2 release occurs shortly after local earthquakes. Continuous monitoring of CO2 and H2S air concentration and of wind speed has been carried out for four months in twelve anomalous gas realising sites of Tor Caldara. Results indicate that only H2S reaches lethal concentration (>250 ppm) near the soil in no wind nights, explaining the presence of small dead animals. At Lavinio, the main soil gas release occurs near old water wells that likely produced a gas blowout during drilling. A total release of over 20 tons/day from 2.93 km2 of gas of endogenous origin, has been estimated for the Lavinio-Tor Caldara area by a detailed soil CO2 flux survey (2,572 measurement points over an area of 3.65 km2). The main structural lineaments of the area have N-S and W-E directions, but also NE-SW and NW-SE directions are well represented. Some sectors of the investigated area are exposed to a severe gas hazard for people and animals and precautionary measures should be adopted.