Continuous monitoring of CO2 and H2S air concentration and soil CO2 flux survey for gas hazard assessment at Tor Caldara nature reserve (Anzio, Italy)

Tor Caldara natural reserve hosts the southernmost discharge of endogenous gas of Colli Albani volcano (mostly CO2 with a relevant H2S content up to 6.3 vol.%). Gas discharges in zones where past sulfur mining removed the impervious surficial cover (e.g. Miniera Grande and Miniera Piccola) and along tectonic fissures. A structural study of the reserve has shown the presence of two zones with different characteristics: prevailing directions N-S and N30 degrees in the northern zone; E-W and N60 degrees in the southern one. In March-July 2012 a geochemical study was carried out, including a soil CO2 flux survey and continuous monitoring (from 2 to 11 days) of air concentration of CO2 and H2S in 12 sites of the reserve. Environmental parameters were also monitored. Total diffuse soil flux of endogenous CO 2 was estimated to 17.48 ton*day(-1) from 1,259 measurements over a 0.47 km 2 surface, with 6.56 ton*day(-1) only from Miniera Grande. This is the second highest value of soil CO2 flux at Miniera Grande, after that of 2005 (9.25 ton*day(-1)) and is significantly higher than in 2009 (1.20 ton*day(-1)). As both the 2005 and 2012 surveys were made shortly after earthquakes with epicentres near to Tor Caldara (max ML= 4.7 in 2005 and 3.5 in 2012), data confirm that soil CO2 flux increases during earthquakes because of seismic rock microfracturing and soil shaking. Hazardous air concentrations have been found only for H2S, up to immediately lethal values (565-1,124 ppm) and with potentially lethal values (>= 250 ppm) long persisting (up to 12h27') in several no wind nights. Instead, the CO2 air concentration remained always well below dangerous levels (maximum recorded value = 2.1 vol.%). The most hazardous gas releasing sites were found in Miniera Grande and in a small pond NE of Miniera Piccola, where the carcasses of mammals and other small animals are frequently found. The killer gas is H2S, and the dangerous sites should be appropriately fenced to prevent access to people and animals.