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

In Central Italy non-volcanic CO2 is discharged by focused degassing (strong diffuse emission and vents) and by high-CO2 groundwater. He-3/He-4 data and the carbon isotopic composition of CO2 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 pCO(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 CO2-rich springs which characterise the study area. The storage and expulsion of CO2 is controlled by fractures and faults and/or structural highs of permeable carbonate formations. Influx of deep CO2 into the overlying groundwater yields a widespread elevated pCO(2) anomaly in the Tyrrhenian Central Italy aquifers, These aquifers release CO2 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 CO2 ranges from 4 x 10(5) to 9 x 10(6) mol y(-1) km(-2). In concert with the regional geologic setting, the deep CO2 production rate increases estward. In the aquifers with anomalously high pCO(2) the average CO2 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 CO2 emission from high heat flow regions. Chem, Geol., 121 (1995) 285-293.] as baseline for CO2 emission from areas of high heat flow. The flux of CO2 lost to the atmosphere from water emitted from springs is of the same order of magnitude as the influx of deep CO2 into the aquifer. (C) 1999 Elsevier Science B.V. All rights reserved.