Caratterizzazione idrogeochimica delle acque sotterranee abruzzesi e relative anomalie

Hydrogeochemical characterization of Abruzzo groundwaters and relative anomalies. The geological and stratigraphical sequence of the Abruzzo region (Central Italy) is dominated by the Jurassic to Oligocene succession, constituted mainly by limestone, marley limestones, calcareous sandstones, calcareous marls, marls and clayey marls along the Apennines, hilly areas and shoreline. The Miocene sequence, outcropping along the Apennines front, is constituted of limestones, marley limestones, calcareous marls and marls. The top of the succession, in the central and northern areas, is constituted by Messinian deposits of the marley-arenaceous turbidite formation, while, in the southern area delimited by Pescara river at north, by the evaporitic terrigeneous succession. The Pliocene-Pleistocene sequence represents the greater part of the hill domain, mainly constituted by marley clays and clayey marls surrounding arenaceous and conglomerate bodies. A gravity flow (Oligocene-Miocene Molisan sequence) originated from the Apennines is located at south of Maiella within the Pliocene-Pleistocene sequence; it is mainly constituted by clays and marls, with smaller proportions of limestones, calcareous sandstones, calcareous marls, Cretaceous and Miocene reworked marls and chalks, with maximum thicknesses up to approximately 2000 m. Lacustrian deposits are found in the intra-mountain basins, while Quaternary alluvial deposits lay on top of the marine deposits along main rivers and streams. The alluvial domain is mainly constituted by continental terraced deposits in the fluvial plains, while intramountain basins by fluvial-lacustrian deposits, continental head deposits and in a lesser extent by Pliocene-Pleistocene sandstones and conglomerates. Aquifers, mainly recharged by rainfall river contribution; groundwater flow is significantly influenced by paleo-river bodies and bedrock geometries (DESIDERIO et alii, 1999). These mostly non-confined aquifers, mainly constituted by gravelly-sandy deposits, are locally multilayered and semi-confined, especially in the intra-mountain and coastal zones. Exchange between surface waters and groundwater can be significant. The Abruzzo main aquifers are located in the Appennine carbonate domain (CELICO, 1983; BONI et alii, 1986; CONESE et alii, 2001; PETITTA & TALLINI, 2002; NANNI & RUSI, 2003; DESIDERIO et alii, 2002). The major springs (Q > 50 l/s) are located at the border of the carbonate hydro-structures along the contact with less permeable deposits that constitute the aquiclude (Miocene, Pliocene and Pleistocene terrigenous deposits) or aquitards (continental Pliocene and Pleistocene deposits). Minor springs (Q < 20 l/s) are controlled by local factors as head deposits, impermeable layers within the lithological sequence and tectonic discontinuities. Perennial springs are seldom found in the Appennines foreareas and hilly areas, constituted by calcareous sandstones, marls, clays, sandstones and chalks originated from the alloctonous units, Miocene turbidite and evaporitic successions, including clayey marls and clays from Pliocene and Pleistocene pelagic successions. The few springs are related to aquifers within the more permeable deposits, mainly recharged by rainfall. Surface and buried bituminous and evaporitic sediments sometimes increase the spring waters salinity (DESIDERIO & RUSI, 2004). Groundwater classification of the four hydrogeological domains has been accomplished within this study on the basis of 690 chemical analysies of 325 monitoring points. Piper diagram heighlights that spring waters from the Apennine carbonate domain are classified as calcium bicarbonate facies; groundwaters from the intra-mountain domain are slightly shifted, while those from springs and wells in the hilly domain, thus originating from terrigenous deposits, show a greater dispersion with a slight prevalence of alkaline earth-bicarbonate and sulphatechloride-alkaline earth waters. Alluvial domain groundwaters are even more disperse with a prevalence of calcium bicarbonate facies. The mother groundwaters in the Apennine carbonate domain undergo a first mixing within the intra-mountain basins with waters flowing in the alkali rich mainly lacustrian continental deposits. More significant mixing occurs when the Apennine groundwater flow into the alluvial domain. Greater residence times and alluvial lithological heterogeneity imply greater groundwater salinity with a significant increase in alkali elements, sulphates and chlorides. Saline intrusion in coastal areas are related to groundwater withdrawals causing significant variations of original facies. Groundwater flowing within the terrigenous domain do not undergo mixing due to aquifer geometries and small discharges. The hydrochemical facies dispersion is related to the extreme lithological heterogeneity of this domain. A modest circulation of mineralised waters originates from groundwater flow within the Adriatic Pliocene-Pleistocene foredeep, allochthonous deposits and Miocene evaporites, The hydrochemical facies are sodium-chloride, sodium-chloride-sulphate, sodium-bicarbonate, sodium-sulphate and without dominant ions; these groundwaters undergo different degrees of mixing with those of the main domains, thus modifying their characteristics. KEY WORDS: Hydrogeochemistry, groundwater chemistry, hydrochemical anomalies, mineralised waters, Abruzzo, Italy.