Idrogeologia degli acquiferi carbonatici, terrigeni ed alluvionali tra i fiumi Cesano e Potenza (Marche centrali). Sintesi dei risultati

The results of hydrogeological researches on the aquifers of the limestone ridges of the Marche region, are synthesized in this work. The study at first aimed to analyse the hydrodynamic characteristics of the springs emerging from the limestone hydrogeological complexes, then it was extended to “sample” hydrostructure and to the analysis of the river discharge. The research was however aimed at the definition of the hydrodynamic characteristics of the supply basins of the springs emerging from the limestone complexes, of the water circulation, of the hydrogeological balance of the “sample” hydrostructures and at the evaluation of the contribution of limestone aquifers on the fluvial discharge during the dry season. From the analysis of the spring hydrograms, compared with the rainfall pattern and variability of chemical and physical parameters, the presence of a double circuit in the spring basins emerges. One is shallow and rapid, through conduits and macrofissures, which allow the rapid infiltration of the rainwater through the unsaturated area of the aquifer and its transport to the spring. In some springs, fed by surface aquifers, small drops of discharge are recorded only a few hours after the rainfall. This circuit is always present in springs fed by the Scaglia and Maiolica aquifers, generally at high altitude. The other circuit is the slow basal flow of the springs, through micro fractures. This is essentially typical of the Massiccio aquifer, in which however, the rapid circuit is also present. This study has also underlined a considerable difference in the hydrogeological setting of the two limestone ridges in the Marche region. The Umbria-Marche ridge is constituted of hydrostructures isolated in anti-apennine direction. In fact they are in part overlapped and hydraulically separated by lithotypes with low hydraulic conductivity on the front of the Apennine thrusts. Hydraulic contacts between the different hydrogeological complexes of a same hydrostructure can be possible through faults and fractured zones also connected with the Jurassic tectonic lines. The Marche ridge consists of one complex hydrostructure, bordered by the lithotypes of Pliocene on the east, and of the Miocene on the west. Hydraulic contacts between the different aquifers may occur, as is shown in discharge measurements in the Esino river (gola della Rossa) in presence of fractured zones and faults. In the study the role of the lithostructural setting of the region is also analysed, as fundamental element that produces the hydrogeological conditions for the groundwater mixing. It allows the hydraulic contacts between deep and surface aquifers: the deep and frequently mineralised waters rise along fractured zones related to tectonic lines and mix with the surface waters, generally with calcium-bicarbonate facies and low saline content. The groundwater mineralization, especially in the basal aquifer in contact with the Anidriti di Burano formation, is characterised by different enrichments in chloride, sodium and sulphates, proceeding from the Umbria-Marche ridge to the Marche ridge. This phenomenon is correlated to the overthrusts of the limestone structures on the terrigenous Miocene deposits for the Umbria-Marche ridge and minor limestone ridges, and on Messinian and Pliocene deposits for the Marche ridge.