Hydrogeology of the «montagna della Majella» carbonate massif (Central Apennines-Italy) [Idrogeologia del massiccio carbonatico della montagna della Majella (Appennino centrale)]

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 hydrostructures. 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. The recharge of the Majella hydrostucture takes place exclusively by precipitation, without any water yield from the adjacent structures. Recharge of the basal aquifer is due to melting snows and subordinately to rainfall. The perched aquifers are recharged both by melting snow and by rainwater. The snow-covered zones at high altitude are characterized by extensive plains, with less intense karstic phenomena (PARATORE, 1972; AGOSTINI & ROSSI, 1992) and the greater abundance of detritus. This influences recharge, with two consequences: 1) it reduces infiltration in winter and early spring, in turn reflecting on the basal emergence hydrographs, unaffected in these periods (figs. 11÷14); 2) it causes marked spring and summer infiltration due to the slow melting of snow in zones with elevated infiltration capacity. The effect of the melting snow can be clearly seen both qualitatively and quantitatively in the rise of the discharge maxima and minima recorded in 1998-’99 with respect to 1997-’98 (fig. 6). Only the Acquevive and Lavino springs are affected by the restarting of the autumn rains, which can cause infiltration before the snow cover is able to prevent it. The remarkable ramification of the superficial circuits due to fissuring and karst is demonstrated by the presence of over 240 springs (D’AMICO, 1991). The recharge circuits are very rapid, with delays of less than 15 days between the rainy or melting event and the relative discharge increment (figs. 18 and 20). The rapid circuits are highlighted by the ever-present seasonal thermal signal, the limited turnover times and the emptying capacity. Within this general trend, however, it is possible to characterize various kinds of circuits depending on whether the spring comes from the hydrogeological complex of the Cretaceous-Paleocene limestones (fig. 18) or from the extended detrital covering (fig. 20). The basal springs of the hydrostucture (plate 2) are located in the eastern and northern slopes, with average discharges varying from 0.6 to 2.6 m3/s. All the registered parameters indicate that they are recharged by circuits pertaining to a single aquifer, whose dimensions, volumetric capacity and depth increase northward. In fact the more southern springs (figs. 7 and 8), even if they involve elevated volumes of water (average discharge 0.6-1 m3/s), are characterised by active circulation in which the delay between recharge event and discharge increase was estimated at 15-30 days. In particular, the analysis of the hydrographs established the presence of karstic conduits overlapping with fissured circuits, giving delay times of 15 days for the former and 1 or 2 months for the latter. The active circulation is confirmed by the inverse correlation between electrical conductivity and discharge, due to the poor mixing with deep waters and dilution of nearly all the chemical-physical parameters. The central northern springs involve considerable water volumes (average discharge 0.6-2.6 m3/s), and are characterised by slow circulation zones. The increased discharge (figs. 11 and 13) of the springs here – closely correlated to snow melting – is caused by piezometric lifting in the emergence zone. This phenomenon is in turn due to a transfer of pressure resulting from a rapid increment of hydraulic charge in the central area of the basal aquifer, due to a very fast infiltration (fig. 26) of melted snow. This kind of circulation is confirmed also by the constancy of chemical-physical parameters (NANNI & RUSI, 2001) and by the elevated turnover time (4-6 years). The northern part of the basal aquifer emerges from the Lavino spring at Decontra through a N-S system of faults that allows the water to rise through the Bolognano and Gessoso Solfifera Formations (fig. 22). The circulation of the Lavino spring is more complex than the other emergences because of the crossing of evaporitic units, with consequent mineralization, and the superposition of a shallow circuit and basal circuit. In conclusion, a single basal aquifer (plates 1 and 2) was recognised with maximum extension and elevated hydrodynamic characteristics in the central part of the hydrostructure. The hydrogeologic balance (tab. 8) excludes both the presence of water yields from the borderland structures, in particular from the Porrara M., or the presence of great flows towards the buried northern continuation of the structure.