Interaction between deep and shallow groundwater systems in areas affected by Quaternary tectonics (Central Italy): a geochemical and isotope approach

Abstract

Hydro-and isotope geochemistry are used to refine groundwater conceptual models in two areas of central Italy (Acque Albule Basin and Velino River Valley) affected by extensional Quaternary tectonics, where deep and shallow groundwater flow systems are interacting. The role of geology, of recent deposits filling the plains and of main tectonic features controlling groundwater flowpaths and deep-seated fluids emergences are investigated and discussed. Environmental isotopes ( 2 H and 18 O) confirm recharge in the surrounding carbonate aquifers, and meteoric origin of both shallow and deep groundwater. Major ion chemistry indicates a mixing between shallow Ca-HCO 3 groundwater from carbonate aquifers and deep Ca-HCO 3 -SO 4 groundwater, characterised by higher salinity and temperature and high concentration in sulphates. Isotopic composition of dissolved sulphates (d 34 S and d 18 O) and dissolved inorganic carbon (d 13 C), henceforth indicated as DIC, are used to verify the presence of different sources of groundwater, and to validate the mixing model suggested by the major ion analyses. Sulphate isotope composition suggests a marine origin for the groundwater characterised by elevated sulphate concentration, whose source is present in the deep buried sequences. Carbon isotope composition confirms the role of a DIC source associated to CO 2 degassing of a deep reservoir. Groundwater conceptual models are improved underlining the importance of Plio-Pleistocene sequences filling the tectonic depression. In the Acque Albule area, the travertine plateau represents a mixing stratified aquifer, where deep groundwater contribution is spread into the shallow aquifer. The alluvial-clastic-lacustrine leaky aquifer of Velino Valley enables a complete mixing of shallow and deep groundwater allowing spot-located discharge of deep groundwater along tectonic patterns and facilitating sulphate reduction in the lacustrine sediments, explaining locally the presence of H 2 S.

Groundwater flow and geochemical modeling of the Acque Albule thermal basin (Central Italy): a conceptual model for evaluating influences of human exploitation on flowpath and thermal resource availability

“…In the Acque Albule area, tectonics led to the formation of a pull-apart subsiding basin characterized by a north-striking, right-lateral, seismically-active fault system and N40°E-N60°E extensional structures. The fault system is partially overburdened by travertine deposits and crops out in the carbonate Cornicolani Mountains (Maiorani et al 1992;Faccenna et al 1994;Sagnotti et al 1994;Gasparini et al 2002;Florindo et al 2004;Billi et al 2006;Faccenna et al 2008;Petitta et al 2011;Carucci et al 2012).…”

“…De Rita et al (1995) dated the volcanic products between 500 and 350 ka. Travertine emplacement began during the late Pleistocene just after the end of Colli Albani volcanic activity (Faccenna et al 2008) and occurred intermittently between about 115 and 30 ka BP (Petitta et al 2011).…”

“…The carbonate ridges (Lucretili-Tiburtini-Cornicolani Mountains) have been recognized as the recharge areas for the Tivoli Plain, including for the travertine aquifer (Petitta et al 2011). The infiltration rate is very high because of the widespread karst features: 70%-80% of the annual 1000-1200 mm rainfall.…”

“…These layers cover the deep confined aquifer contained in the downthrown Meso-Cenozoic carbonates, in which mineralized hot water and gas fluids circulate. This aquifer is fed by the same carbonate ridge (Petitta et al 2011). In fact, it is difficult to quantify what the contribution of the two flowpaths to spring discharge is.…”

“…Many conceptual models have been proposed for the study site (Petitta et al 2011;La Vigna et al 2011;Carucci et al 2012;Brunetti et al 2013;La Vigna et al 2013), analyzing the area based on hydrogeologic, hydrodynamic and hydrogeochemical data. These studies have substantially contributed to addressing several aspects concerning groundwater flow, interaction between different aquifers, mixing of waters of different origin and water-rock interactions.…”