Le acque sotterranee della Pianura e della Collina Cuneese

In the Piedmont region (NW Italy) between the SW Alps (Paleozoic - Mesozoic) and the hills of the Langhe domain (Tertiary) there is an extensive flood plain (3.000 km2) consisting of a series of overlapped alluvial fans originated by the quaternary geological activity produced by the evolution of several rivers which drain the Alps. These alluvial deposits overlap a Tertiary sedimentary succession through a series of erosional unconformity surfaces. The quaternary deposits highlight a variable thickness ranging from 80 to 100 m in the foothills of the mountains up to a few meters in the more distal portion of the plain. In these deposits there are several unconfined aquifers which are not respectively interconnected from the hydraulic point of view due to the deep fluvial incisions that reach the underlying tertiary substrate. This plain is intensively populated and lot of villages and farms characterize the landscape. In the overall area it is present an intensive agricultural and livestock activity predominantly represented by crops of wheat and corn and farms of cattle and pigs. All these activities require a considerable amount of groundwater which is withdrawn from the quaternary aquifers by means of thousands of water wells. These aquifers are generally very productive due to the high permeability of the alluvial deposits and the large water losses from many rivers which drain the mountain sectors of the SW Alps. The depth to groundwater ranges from 60 m in the foothills of the mountains to few meters in the more distal plain sector where it is possible to observe the groundwater outcropping through several springs. The groundwater flow nets highlight the mechanisms of the aquifers recharge and their evolutive dynamics. Rivers lost most of their water directly in the aquifers during the limited discharge flow periods (summer and winter) in the area located at the foothills of the mountain sectors. On the contrary, in the more distal portion of the plain the rivers highlight significant increases in their surface flow due to the contribution of groundwater supplied by the quaternary aquifers. Effectively, several springs which supply the rivers are present in the river valleys incisions in correspondence of the separation surfaces between the quaternary alluvial aquifer and the underlying less permeable tertiary substrate. The groundwater circulating in these aquifers has a predominantly calcium-bicarbonate hydrochemical facies with a significant presence of sulphates in some areas. The groundwater quality is strongly influenced by the content of nitrates and manganese. The nitrates are certainly linked to pollution due to agricultural activities and livestock and increase along the groundwater flow lines. The manganese content is quite high in the foothills of the mountains and in some restricted areas of the plain and appears to be linked to the natural lithological composition of the aquifers. Below the alluvial deposits of the Quaternary succession there is the Plio-Pleistocene succession, it is outcropping in the Roero hills, in some sectors of the Monregalese and next to the valley edge of the main streams. Three main tectonic sequences have been recognised and they are separated by important unconformities which identify three principle allogroups. These allogroups have been named: Late Messinian allogroup (LM), Early Pliocene allogroup (EP) and Late Pliocene allogroup (LP. These allogroups are characterised by a succession of sediments that have been deposited over different periods and in different environments and to which different informal stratigraphic units have been attributed. In this work, the units that have been recognised have been correlated to their respective hydrogeological units. These units, which are characterised by different facies, have been assigned their hydraulic conductibility values. The correlation between the surface data and the underground data has made it possible to define a hydrostratigraphical scheme of the entire area under examination. On the basis of the geometry of the aquifer levels, which are both semi-permeable and impermeable, of their contacts, and of the hydrodynamical and hydrogeochemical data, it has been possible to identify a series of aquifers. The main aquifer is located in correspondence to the basal horizons of the Villafranchiano hydrogeological Unit B and the permeable levels of the Asti Sands hydrogeological Unit B. This aquifer has a piezometry that is highly conditioned, in the south-western sector, by the geometry of the unconformity that separates the LM allogroup from the EP allogroup and, in the remaining sector, by the geometry of the synclinal structure that involves the Asti Sands B and the Azure Clay B. The aquifer is under pressure and, below the eastern escarpment, it is generally artesian. The chemistry of these waters is substantially uniform with bicarbonate-calcic and bicarbonate-calcic-magnesium facies with decidedly low nitrate contents and iron and manganese levels which are often above the maximum admissible concentrations established by the Italian Laws presently in force. A second aquifer, which at present is not very important but in the past was the only water resource available for the local population, can be found in the hilly sector on the left of the Tanaro River between the villages of Santa Vittoria d’Alba and Govone. This aquifer is located in the re-sedimentary sandy levels of the Argille di Lugagnano frm.; it is artesian in correspondence to the main valley bottoms and is under pressure in the other zones. The underground flow is completely different from the previously mentioned aquifer with directions prevalently towards the Est and a rather complex recharge mechanism related to a series of overflows arriving from the aquifers above. This aquifer is intercepted by rather deep wells that cross thick clayey-silty layers and reach the aquiferous sandy bodies. The chemical facies of the waters belonging to this aquifer are different, in fact, it is possible to recognize alkaline-chloride, bicarbonate-alkaline and bicarbonate -calcium facies. The water quality is usually impaired by high concentrations of iron and manganese and in some cases the water quality is also compromised by chlorides and sulfates. On the hills of the Langhe outcrops the oligo-Miocene succession. It consists of a series of stratigraphic units. Starting from the base there is a sequence of gravels, continental and marine-marginal sands correspondent to the Molar Unit. This aspect is discordant with the rocks of the mountain base. Overlying these sediments there are powerful set of platform deposits, slope and basin with predominantly arenaceous-marly succession, this is alternating to powerful pelitic horizons. At the end of the Miocene cycle there are a set of evaporite deposits and a series of synevaporitic referring to Messinian. The permeability of the deposits of the Miocene sequence is rather low. In the oligo-Miocene series there are several aquifer, they are, currently, of limited importance but, in the past, they constituted the only source of available underground water for drinking water, zootechnical and agricultural sector of the Langhe hills. The water from this aquifer has chemical facies that are very different one from the other. The presence of chloride-sodic facies shows the existence of marine water that was trapped in the sediment at the moment of deposition and which still has not been substituted by the active circulating ones. These latter waters have been identified in numerous wells and have bicarbonate-calcic and bicarbonate-magnesium-calcic facies. Bicarbonate-alkaline facies have also been found and these have been put down to cationic exchanges with the clayey sediments and therefore with decidedly long permanence times in the aquifer. The quality of the water from this aquifer is generally poor because of the ammonium, iron and manganese ion contents. The vulnerability to pollution of the plain area aquifers was determined using the method SINTACS, while on the hillsides of the Langhe and Roero was applied the CNR Basic Method. The results show that the vulnerability of the plain is very high, with a grade between High and Elevated. This is due to the high permeability of sediments at the foot of the alpine areas where the depth to be quite high or in areas corresponding to the highlands where there is a powerful paleosols. In the remaining portions of the plain, characterized by a reduced depth and in main valley areas the vulnerability reaches extremely high values. On the hillsides of the Langhe and Roero the vulnerability of major aquifers is quite small except in the areas of the valley bottom of secondary streams where it reaches extremely high values.