Caratterizzazione del flusso in acquiferi carbonatici fratturati dell'Italia centrale per la gestione e la protezione delle risorse idriche sotterranee

The peculiarities of karst aquifers make them strategic resources which, however, are not yet exploitable because of objective difficulties that are found in their study. Indeed, it is recognized the importance of carbonate aquifers for water sourcing (BAKALOWICZ, 2005; CIVITA, 2008; GOLDSCHEIDER & DREW, 2007; VIGNA, 2002), but often the inadequacy of the investigation methods together with their inaccessibility due to morphological factors, does not allow an adequate investigation. For this reason, a study was carried out with a multidisciplinary approach, which made possible to compare, analyze and validate the results obtained by the different techniques of investigation. Final result is the characterization of fractured aquifers and of their baseflow, as well as the evaluation of the influence of the variability of recharge on the groundwater regime. This study describes the results obtained in two areas of the central Apennines, which are representative of the Umbria-Marche and Lazio-Abruzzi geological domains. The upper basin of Aso river is located in the south-eastern area of “Sibillini Mounts National Park”, surrounded by some of the higher reliefs of Sibillini Mounts: Mt. Vettore, Mt. Porche and Mt. Sibilla. Here outcrop the pelagic Mesozoic–Cenozoic Umbria-Marche succession, whose formations are involved by the Olevano-Antrodoco-Monti Sibillini or OAMS Thrust (PIERANTONI et alii, 2005) and have a structural setting characterized by an asymmetric, east-verging anticline with box-fold geometry. Groundwater of the Aso River has been evaluated between 1400 L/s (BONI et alii, 2010, NANNI et alii, 2010) and 1800 L/s (MASTRORILLO et alii, 2012) and about 530 L/s of those (Foce spring) are tapped for drinking use by a tunnel drainage. The Sagittario river is located in the Northern Montagna Grande (Eastern Marsica, Abruzzi) where the Cavuto springs represent the main discharge of Basal aquifer, with an average flow rate evaluated in 1800 L/s (BONI & RUISI, 2005). Here outcrop Mesozoic and Cenozoic carbonate deposits, belonging to different palaeogeographic domains (from platform to slope-to-basin); this unit is placed in contact with synorogenic flysch deposits by significant tectonic lines that are predominantly N-S and NNW-SSE oriented. The work has been carried out by a multidisciplinary approach including geological, hydrogeological, hydrochemical-isotope and climatic aspects. In particular, the analysis and elaboration of latest geological-stratigraphic and structural data allowed to define the major regional carbonate aquifers, to locate structural elements that can affect groundwater circulation and to identify stratigraphic elements leading to the presence of local or regional flow. The hydrogeological analysis is based on the direct measurements of discharge, on the baseflow recession analysis, on the evaluation of annual changes in piezometric levels and on the implementation of vertical log with Flowmeter. The results obtained have allowed to evaluate the flow rate and the seasonal and annual variability, also related to climate change, and the intrinsic characteristic of the aquifer systems; in addition, it was possible to determine vertical component of groundwater flow and to detect any interactions with adjacent aquifers and/or to differentiate separate flow coming from aquifers shallow. The chemical and isotopic data were useful to individuate a sole circulation or to confirm the presence of separate groundwater flows, identified on the basis of geological and hydrogeological data. Furthermore, the isotopic data allow to define the average elevation of recharge area of main springs, while the results of chemical analysis have provided information about the hydrodynamic conditions in the aquifers and the different circulation systems drained by nearby springs which have or not similar chemistry. Finally, in the investigated areas, the detailed analysis of temperature, rainfall and snow data (in terms of thickness variation of snowpack or amount felt on the ground) and, then, of “Effective Rainfall”, have allowed to obtain important considerations on the meteoric inflows and its relations with the variability of the groundwater regime. The total amount of base flow of the Aso River in 2009-2012 has been evaluated in about 1900 L/s, ranging between about 2200 and 1500 L/s. The multidisciplinary approach made it possible to identify three regional groundwater flows inside the “Basal”, “Maiolica” and “Scaglia calcarea” aquifers. The flow of the river is mainly supported by Foce and linear springs of the Basal aquifer (more than 70%), whose recharge area has been evaluated in about 40 Km2 by geological-tectonic and isotopic results: underground drainage occurs mainly in a direction NNW-SSE, from South to North. Shallow alluvial-detritic aquifer of “Gardosa Plain”, connected with Basal aquifer, supplies the spring of Aso and Foce lake: this happens only in the wettest years, when the rise of the groundwater table feeds a flowpath from Basal to shallow aquifer. Recharge of the Basal aquifer is due to melting snows and subordinately to rainfall: the delay between the beginning of the snowmelt and discharge increase was estimated on about one month. The baseflow recession analysis also shown that the basin of the Aso river is the main destination of the Basal groundwater flow in the south-eastern part of the Sibillini Mountains ridge. Maiolica and “Scaglia calcarea” aquifers also contribute to the discharge of Aso river with, respectively, 240 L/s and 290 L/s (referred to the 2009-2012 period): the recharge areas outcrop northwards and their extension depends on the variation of meteoric inflow, that affects the involvement of more extensive areas of aquifers when the recharge is greater. Influence of the snow on the recharge of aquifers of Sibillini has also been confirmed by the negative rainfall gradient (-41,3 mm/100 mt), because as the altitude increases the rains are replaced by snow, and by coefficient of Lauscher (LAUSCHER, 1954) that is over 50% above 1300 meters a.s.l. The analysis of the Effective Infiltration, evaluated on the basis of the temperature, rainfall and snow data, showed a great variability depending on the areal extension, the local relief and annual climatic variations: a mean value for 2008-2012 period of about 1020 mm/year has been calculated, ranging between 630 and above 1400 mm/year. In addition, the water budget has provided a mean net infiltration of 990 mm/year, ranging between 735 and 1220 mm/year, values comparable with those obtained from the analysis of climate data. Measurements of discharge in the Sagittario River were carried out in 2009-2010, between San Domenico dam and Cavuto spring group, that have shown how the flow rate in this area is strongly influenced by the presence of hydrodiversion. In fact, the volumes of water released immediately and about 1,5 Km downstream of the dam, evaluated in about 50 and 320 L/s, move through the soil surface into the ground, leaving part of the riverbed dry. The Cavuto spring group, whose average base flow in 2009-2010 has been evaluated in 1860 L/s, is fed both by meteoric recharge due to rainfall and snow, but also by approximately 225 L/s of groundwaters that infiltrate directly into the riverbed. The baseflow recession analysis provided that the recession coefficient α increases with time: this has been interpreted as temporarily flooded caves in the northern karst region of Montagna Grande. Indeed, analysis of climatic data referred to 2002-2012, returned a wide range of effective rainfall with an average value of about 670 mm/year; instead, for 2009-2010 the average value is 925 mm/year, reflecting that these two years have been characterized by particularly high recharge rate. In addition, the water budget carried out by assuming an area of 53 Km2, identified on the basis of geological-structural setting, allowed to determine an average value of Effective Infiltration for the 2009-2010 of almost 970 mm/year, very close to that calculated by the “Indirect method” (BONI et alii, 1986). The study conducted in the carbonate basins of the rivers Aso and Sagittario, based on multi-disciplinary methodologies, allowed the detailed evaluation of the annual and multiannual range of the “Effective Rainfall”, especially with regard to snow, and the validation of Conceptual Hydrogeological Models by comparing the Direct and Indirect hydrogeological method of budget analysis.