Hydrogeological analysis of carbonate structures can be undertaken in various ways: in recent years, descriptive studies (Celico, 1978 & 1983; Boni et alii, 1986) have led to adequate knowledge of regional models. These provided an excellent starting-point for more detailed analysis aimed at drawing up quantitative hydrogeological models and understanding the hydrodynamics of the structures. An essential condition for specific quantitative analysis is a knowledge of the structural geologic setting, not only on a regional scale, but also for each structure. The absence of experimental verifications on the geology of depth or their different interpretation is evident in the difficulty of correctly locating the limits of the hydro-structures. The difficulties of locating the limits of hydrostructures correctly stem from either lack of experimental proof of the geology at depth or the possibility of alternative interpretations. These limitations can be overcome through geologic-structural verification in specific zones and through experimental measurements. The well-known hydrogeological studies of the Majella Mountain considered single sources for water supply (Manfredini, 1989). The main studies on a regional scale are those by Celico (1978) and Boni et alii (1986). No systematic study of the whole structure has yet been attempted, aimed at understanding its subterranean circulation and the relationships between the various outflows. Moreover, no systematic data are available on the discharges or the hydrodynamic, hydrochemical and hydrologic parameters. This work describes the hydrogeology of the Majella Mountain, the most external carbonate unit outcropping in the Central Apennines, particularly in terms of its stratigraphic and tectonic setting (Crescenti et alii, 1969; Catenacci, 1974; Donzelli, 1998; Vezzani & Ghisetti, 1998), which significantly affect the subterranean water circulation. Studies were made of the hydrodynamics of the recharge basins, recharge modalities through the unsaturated zone, stored water volumes, discharge and temperature hydrographs in relation to rainfall and snow melting, the variability of water chemistry, and groundwater quality. Monthly measurements of discharge, temperature, electrical conductivity and the main chemical parameters were carried out on all the basal springs and on the springs of some perched aquifers. The basal springs were also monitored continuously in order to record the chemical-physical parameters. The structure of the Majella (plate 2) is hydraulically isolated on the surface on all sides, while in depth it is limited on three fronts (E, S and W). The northernmost front of the structure extends below the Mio-Pleistocene terrigenous units of the Pescara valley (fig. 3), and does not exclude its hydraulic continuity in this direction. Analysis of the lithofacies, hydrogeological balance and river discharges of the northern streams has made it possible to locate a new northwestern hydraulic limit, corresponding to a marly member of a terrigenous, mostly calcarenitic formation. Within the Majella structure the following hydrogeological complexes can be recognized (plate 1 and 2): A) a hydrogeological complex of Jurassic-Paleocene limestone characterized by high permeability due to karst and fissuring; B) an aquiclude of the Bolognano Formation consisting of marly limestone and marlstone; C) a hydrogeological complex of calcarenites of the Bolognano Formation characterized by variable permeability, decreasing northward, caused by fracturing and porosity; D) an aquiclude of terrigenous and evaporitic formations consisting of clay, marl and marly clay; E) a hydrogeological complex of highly permeable continental detritus.

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