Dangerous emissions of endogenous CO2 and H2S from gas blowouts of shallow wells in the Rome Municipality (Italy).

The southeastern zone of Rome city is located at the northwest periphery of the quiescent Colli Albano volcano. This zone is characterized by the presence of a shallow (depth ~ 45–50 m) gas pressurized aquifer that produces gas blowouts when it is reached by wells. Three gas blowouts occurred in this zone in 2003, 2008 (another one was discovered during the present study) and 2016 and in this paper we describe in detail the latter two. The emitted gas consists mostly of CO 2 (>90 vol%) and contains a low but significant quantity of H 2 S (0.3–0.5 vol%) and it has the highest helium isotopic R/Ra value (1.90) of all Colli Albani natural gas discharges, suggesting its likely magmatic origin. In both the described gas blowouts, dozens of families had to be prudentially evacuated from their houses and the emitted gas killed some animals. We monitored, continuously or by discrete surveys, the soil CO 2 flux, the indoor and outdoor air concentration of CO 2 and H 2 S, the environmental parameters and we checked whether the cementation of the gas releasing wells had been effective. In both cases, the upper part of the wells had been partly closed with an inflating packer to avoid free gas dispersion in atmosphere; as a consequence gas diffused laterally from the wells into the permeable surficial soil up to reach the nearest houses creating hazardous indoor conditions, particularly for CO 2 in some basements. During the well cementation operations, and in one case because of the packer rupture, gas and nebulized water were freely discharged from the wells into the atmosphere, and high air CO 2 and H 2 S concentrations were found. Fortunately gas was quickly dispersed by strong winds. The positive results obtained in all the studied gas blowouts demonstrate that our applied geochemistry approach represents a model of intervention useful for the assessment of the hazard associated to accidental endogenous gas release. This model is of fundamental importance also to overcome the risk problems created by accidental gas blowout from wells in an urbanized environment, up to the safe return of the people in their evacuated houses. • Gas blowouts from water wells are frequent in the southeastern zone of Rome • Emitted gas killed some pets and families had to be evacuated for security reasons • Gas has a magmatic origin with the highest helium R/Ra of Colli Albani gas discharges • Monitoring of soil CO 2 flux and air gas concentration allowed to assess gas hazard • Gas diffusing in soil reached nearby houses creating dangerous indoor conditions [ABSTRACT FROM AUTHOR]