Quantification of deep CO2 fluxes from Central Italy. Examples of carbon balance for regional aquifers and of soil diffuse degassing

Abstract In Central Italy non-volcanic CO 2 is discharged by focused degassing (strong diffuse emission and vents) and by high-CO 2 groundwater. 3 He / 4 He data and the carbon isotopic composition of CO 2 are compatible with derivation from mantle degassing and/or metamorphic decarbonation. The gases produced at depth accumulate in permeable reservoirs composed of Mesozoic carbonates. When total pressure (roughly corresponding to p CO 2 ) of the reservoir fluid exceeds hydrostatic pressure, a free gas phase forms gas reservoirs within the permeable host rocks from which gases may escape toward the surface. This process generates both the focused vents and the CO 2 -rich springs which characterise the study area. The storage and expulsion of CO 2 is controlled by fractures and faults and/or structural highs of permeable carbonate formations. Influx of deep CO 2 into the overlying groundwater yields a widespread elevated p CO 2 anomaly in the Tyrrhenian Central Italy aquifers. These aquifers release CO 2 to the atmosphere when groundwater is discharged at the surface from springs. The groundwater degassing flux is estimated from the carbon balance of regional aquifers computed by coupling aquifer geochemistry with isotopic and hydrogeological data. The resulting production rate of deep CO 2 ranges from 4×10 5 to 9×10 6 mol y −1 km −2 . In concert with the regional geologic setting, the deep CO 2 production rate increases westward. In the aquifers with anomalously high p CO 2 , the average CO 2 influx rate of the anomalous areas is several times higher than the value derived by Kerrick et al. [Kerrick, D.M., McKibben, M.A., Seward, T.M., Caldeira, K., 1995. Convective hydrothermal CO 2 emission from high heat flow regions. Chem. Geol., 121 (1995) 285–293.] as baseline for CO 2 emission from areas of high heat flow. The flux of CO 2 lost to the atmosphere from water emitted from springs is of the same order of magnitude as the influx of deep CO 2 into the aquifer.