IDROGEOLOGIA E IDROGEOCHIMICA DELLE ACQUE MINERALIZZATE DELL’AVANFOSSA ABRUZZESE MOLISANA

Hydrogeology and hydrochemistry of the mineralised waters of the Abruzzo and Molise foredeep (Central Italy). A large number of mineral water springs occur in the Pliocene- Pleistocene foredeep of Abruzzo and Molise regions (central-southern Italy). The waters are characterised by sodium-chloride, sulphate- sodium chloride, sodium bicarbonate, sodium sulphate, with no dominant ions. Water has been analysed from 880 wells, 3 mud volcano fields and 28 springs occurring along alluvial valleys. High-salinity water can be found in 50 sites, including 39 wells, 8 springs and 3 small mud volcanoes. Water salinity ranges from 0.8 to 38.6 microS/cm, depending on seasons and on geological setting of the water discharge. Waters from Pliocene-Pleistocene clay outcrops and from springs or wells in complex formations of Varicolor clay, generally show higher salinity than those occurring in the alluvial plains, due to the lower dilution with alluvial aquifers. A preliminary comparative analysis of hydrochemical data (PIPER, 1944; D’AMORE et alii, 1983), geological setting and isotopic data of 18O and 2H suggested 4 main water groups. Group «A» includes sodium-chloride rich waters with Na/Cl and Br/Cl ratio close to seawater, with low sulphate concentration. These waters usually discharge in correspondence with structural highs and nappes (DESIDERIO et alii, 2001a) where Pliocene compressive stresses produced brines. In some cases waters reach the surface through outcropping or blind tectonic discontinuities. The Pliocene-Pleistocene sequence is, in fact, characterised by NW-SE (Apennine) and NE-SW faults which may act as preferential pathways of fluid migration (saline waters and gases; NANNI & VIVALDA, 1999a; ETIOPE et alii, 2003). Group «B» includes sodium-chloride water with high sulphate content, generally above the typical concentration of seawater. Sulphate enrichment is due to mixing processes between Pliocene brines and shallower gypsum deposits. Group «C» includes calcium-sulphate waters with no dominant ions. These waters have been detected in wells and springs occurring south of Pescara river and in correspondence with the Alanno structural high, where gypsum deposits either outcrop or occur at shallow depth. The water chemistry results from the mixing of shallow and deep circuits, where the high calcium-sulphate content is related to Messinian evaporites occurring at different depths, also within the complex formation of Varicolor clay. Group «D» includes sodium-bicarbonate waters resulting from the mixing between saline waters and surface calcium-bicarbonate waters, deriving from circulation in eluvial, colluvial and alluvial deposits. All the waters investigated fall along the theoretical curve of seawater evaporation-dilution (COLLINS, 1970; 1975; CARPENTER, 1978). Deviations from the curve can be related to geological and hydrogeological factors, at the discharge sites, facilitating the mixing between deep saline and surface waters. The first isotopic results confirm the recognition of the four groups in terms of different mixing degree. In particular, higher 18O and 2H values are related to «A» waters coming from the deep brines. «D» waters show the lowest isotopic values as a result of shallower and surface water input. Isotopic signatures of «B» and «C» are intermediate. KEY WORDS: Hydrogeology, Hydrogeochemistry, mineralised water, Plio-Pleistocene foredeep, Italy.