Hazardous gas blowouts from shallow wells in the Colli Albani volcanic complex (Rome, Italy).

Colli Albani quiescent volcano is affected by huge degassing of likely magmatic or mantle origin as suggested by the gas He isotopic composition. Gas rising from a buried calcareous aquifer gives rise to several surface emissions and accumulates also in permeable layers at various depths, from 10 to 350 m, beneath impervious caps. When reached by wells, these pressurized gas pockets cause hazardous blowouts of CO2 and H2S often in inhabited zone. Remedy requires the restoration of the drilled cap by cement injections whose effectiveness can be conveniently assured by CO2 soil diffuse flux and CO2 and H2S air concentration monitoring.