Structural-Stratigraphic Setting of Middle Adriatic Alluvial Plains and its Control on Quantitative and Qualitative Groundwater Circulation.

Sedimentary bodies of the Middle Adriatic alluvial plains are generally laid out on transverse tectonic lines that have strongly conditioned the Pleistocenic evolution. They comprise gravelly and gravelly-sandy lenticular bodies and variously extended lenses of fine silty-sandy and siltyclay deposits with thicknesses varying appreciably in the different plains. They are supported in general by the aquiclude comprising Plio-Pleistocenic marly clays, while in southern Abruzzo and in Molise, south of the Aventino-Sangro (Maiella) line, the Plio-Pleistocenic aquiclude is substituted in the piedmont part of the plains by gravitational flow clays and marls. The most important water beds are contained in the middle-high Pleistocene and Holocene low terrace deposits, while the size of the resources in the high terrace deposits is smaller. The sedimentological layout of the alluvial deposits is such that in the upper part of the valleys gravelly and sandy lithotypes prevail, giving rise to monostrate aquifers. In the lower part there are many different situations so that in some plains, and in particular in the larger ones, the presence of fine deposits denote water beds with multistrate characteristics. Alluvial deposit transmissivity values range from 10-2 to 10-4 m2/s and permeability varies from 10-3 a 10-4 m/s. Underground circulation is conditioned by the presence of high permeability paleo-beds that allow identification of drainage axes that have generally moved in relation to the present river course, and have sometimes been exploited for drinking water. The groundwater flow rate through these may reach a level of over 0.6 m3/s. Supply to the aquifers is essentially due to the fluvial waters of the main watercourses and recharge takes place especially in correspondence with the paleo-beds. A considerable contribution to supplying the plain aquifers is also made by the sub-bed waters of the larger tributaries, while a modest contribution is given by mineralized waters rising from the succession of Plio-Pleistocenic pelites along fracture zones and by the gravitational flow pelites (Varicolor Clay formation). The hydrochemical facies of the underground waters of the plain aquifers is generally bicarbonate-calcic. However, the presence of waters with different chemical composition, characterized by greater mineralizations, has been identified. Waters with sodium bicarbonate facies are present in numerous zones, while in others, there are waters with sulphate-calcic facies, without dominant ions, sodiumchloride facies and sodium-chloride enriched with sulphates. The presence of waters with anomalous chemism is justified by the rise of mineralized waters (both connate and from solubilisation of evaporitic rocks) through tectonised belts with Apenninic and anti-Apenninic orientation or where they cross, whether outcropping or buried. Emergences of mineralized waters are often located in correspondence with structural highs and outcropping and buried overthrusts where the original brines have accumulated following compressive Pliocenic phenomena. Waters with different chemical compositions are present in the coastal areas of the alluvial valley floors because of marine intrusion. The latter phenomenon is also influenced by the size, geometry and permeability of the alluvial deposits. KEY WORDS: Hydrogeology, Alluvial Plain, Hydrodynamic Parameters, Mineralized Waters, Chemical Composition, Salt-Fresh Water Ratio.