The aquifers of the umbria- marche adriatic region: relationships between structural setting and groundwaters chemistry.

ABSTRACT A hypothesis on the hydrogeochemical evolution of the groundwaters present in the Apennine limestones, Mio-Pliocene terrigenous deposits and alluvial plain aquifers of the Adriatic-Umbria-Marche area is discussed here. The mineralised springs of the region, their origin and relationships with the regional aquifers and tectonic setting are also analysed. The mineralised waters are sulphureous with Ca-sulphate composition and saline with Na-Cl composition; they also comprise Na-Cl-sulphate and Ca-sulphate -bicarbonate facies with various degrees of salinity. In this paper, the role of the Umbria-Marche tectonics and structural setting is considered a fundamental factor for the hydrogeological conditions, which favour mixing of groundwaters from different aquifers. The structural setting, for example, affords hydraulic contact between deep and shallow aquifers, allowing deep waters to rise upward, at the same time encouraging the movement of mineralised waters through fractured zones in relation to tectonic lines. This phenomenon hence causes mixing between Ca-bicarbonate superficial waters and deep waters, which are frequently very mineralised. In the present paper we relate the hydrogeochemical differentiation of the aquifers to their different recharge conditions and hydraulic circulation. From this viewpoint, the chemistry of the basal aquifer waters of the limestone ridges is analysed, and its variation from the Apennines to the Adriatic coast is related to the structural setting of the limestone ridges. Lastly, the chemistry of the waters in the alluvial plain aquifers is discussed, and the different hydrogeochemical facies in many large areas of the aquifers are shown. Key Words: Hydrogeology, hydrochemistry, structural setting, Umbria –Marche area