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 (stron g diffuse emission and vents) and by high-CO2 groundwater. He-3/He-4 data a nd the carbon isotopic composition of CO2 are compatible with derivation fr om 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 flui d exceeds hydrostatic pressure, a free gas phase forms gas reservoirs withi n the permeable host rocks from which gases may escape toward the surface. This process generates both the focused vents and the CO2-rich springs whic h characterise the study area. The storage and expulsion of CO2 is controll ed by fractures and faults and/or structural highs of permeable carbonate f ormations. Influx of deep CO2 into the overlying groundwater yields a wides pread elevated pCO(2) anomaly in the Tyrrhenian Central Italy aquifers, The se 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 geoch emistry with isotopic and hydrogeological data. The resulting production ra te of deep CO2 ranges from 4 x 10(5) to 9 x 10(6) mol y(-1) km(-2). In conc ert with the regional geologic setting, the deep CO2 production rate increa ses estward. In the aquifers with anomalously high pCO(2) the average CO2 i nflux rate of the anomalous areas is several times higher than the value de rived by Kerrick et al. [Kerrick, D.M., McKibben, M.A., Seward, T.M., Calde ira, K., 1995. Convective hydrothermal CO2 emission from high heat flow reg ions. Chem, Geol., 121 (1995) 285-293.] as baseline for CO2 emission from a reas of high heat flow. The flux of CO2 lost to the atmosphere from water e mitted 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.