Gas blowout from shallow boreholes near Fiumicino International Airport (Rome): Gas origin and hazard assessment

In summer 2013 a toxic and polluting gas blowout (19 tonnes day−1 CO2, 95 kg day−1 CH4) occurred from two
shallow boreholes drilled at only 50 m from the International Airport of Rome (Italy), in the town of Fiumicino.
Another gas blowout occurred in the same period from a borehole located offshore, 2 km away, also generating
sea-water acidification; it lasted only a couple of days. Onshore, CO2was also diffusing fromholes within the soil,
particularly toward the airport, generating a soil flux up to 1.8 tonnes day−1. In 3.5 months ~1500 tonnes of CO2
and 5.4 tonnes of CH4 were emitted in the atmosphere. Temporal monitoring of gas geochemistry indicates that
in this area a mixing occurs between shallow and pressurized gas pockets, CO2-dominated, but with different
chemical (i.e., He/CH4 ratio) and isotopic (3He/4He, δ13C-δDCH4) characteristics. Numerical simulation of CO2 dispersion
in the atmosphere showed that dangerous air CO2 concentrations, up to lethal values, were only found
near the vents at a height of 0.2 m. Fiumicino is a high blowout risk area, as CO2 rising through deep reaching
faults pressurizes the shallowaquifer contained in gravels confined underneath shales of the Tiber delta deposits.
The Fiumicino blowout is a typical example of dangerous phenomenon that may occur in urban context lying
nearby active or recent volcanoes and requires quick response on hazard assessment by scientists to be addressed
to civil protection and administrators.

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