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POLITECNICO DI TORINO Repository ISTITUZIONALE Relations between stratigraphy, groundwater flow and hydrogeochemistry in Poirino Plateau and Roero areas of the Tertiary Piedmont Basin, Italy Original Relations between stratigraphy, groundwater flow and hydrogeochemistry in Poirino Plateau and Roero areas of the Tertiary Piedmont Basin, Italy / Vigna B.; Fiorucci A.; Ghielmi M.. - In: MEMORIE DESCRITTIVE DELLA CARTA GEOLOGICA D'ITALIA. - ISSN 0536-0242. - XC(2010), pp. 267-292. Availability: This version is available at: 11583/2423949 since: Publisher: Istituto Superiore per la Protezione e la Ricerca Ambientale Published DOI: Terms of use: openAccess This article is made available under terms and conditions as specified in the corresponding bibliographic description in the repository Publisher copyright (Article begins on next page) 07 October 2021 Mem. Descr. Carta Geol. d’It. XC (2010), pp. 267-292, figg. 16 Relations between stratigraphy, groundwater flow and hydrogeochemistry in Poirino Plateau and Roero areas of the Tertiary Piedmont Basin, Italy Rapporti tra assetto stratigrafico, idrogeologia e idrogeochimica nel settore compreso tra l’Altopiano di Poirino e il Roero (Bacino Terziario Piemontese, Italia) VIGNA B. (*), FIORUCCI A. (*), GHIELMI M. (**) ABSTRACT - The study concerns the hydrogeologic setting of in the multidisciplinary study have been correlated to their a vast portion of land, of about 1000 km2, between the respective hydrogeologic units (aquifer analogues), referred to Poirino Plateau, the thalweg of the Tanaro River and the hills with their same nomenclature. The hydrogeologic units con- between the towns of Bra and Asti. sist of different sedimentary facies. Hydraulic conductivity The stratigraphic framework of the Messinian-to- values have been assigned to the recognized sedimentary fa- Pleistocene succession of this area has recently been redefined cies on the basis of direct measurements or of bibliographic on the basis of a multidisciplinary study carried out on the en- data. Then different aquifer systems have been identified on tire western Tertiary Piedmont Basin (TPB). This study, per- the basis of the geometry of the aquifer, semi-permeable formed by a group of researchers from ENI, the Politecnico di (aquitards) and impermeable (aquicludes) layers, of the type Torino and the University of Turin, was based on the analysis and geometry of their contacts, and of the hydrodynamical and interpretation of biostratigraphic, sedimentologic, struc- and hydrogeochemical data. tural data from both outcrops and subsurface (ENI deep wells The main aquifer system is located in the basal part of and seismic sections). The main result of the stratigraphic the Villafranchiano B Hydrogeologic Unit and in the perme- analysis of the Messinian to Pleistocene succession of the able sand layers of the Sabbie d’Asti B Hydrogeologic Unit. In western TPB was the recognition of three main tectono-strati- the southwestern sector the piezometry of this aquifer sys- graphic units or allogroups, bounded at base and top by major tem is strongly influenced by the geometry of the unconfor- tectonically-induced unconformities. These allogroups have mity that separates the EP Allogroup from the underlying been named: Late Messinian Allogroup (LM), Early Pliocene LM Allogroup and, in the remaining sector, by the geometry Allogroup (EP) and Late Pliocene Allogroup (LP). Each of of the synclinal structure that involves the Sabbie d’Asti B and these allogroups is made up of informal lithostratigraphic units the Argille Azzurre B. This piezometry indicates groundwater characterised by relative lithologic homogeneity and referable flows from the Poirino Plateau and Bra Hills towards the Versa to one or to a set of marine or continental genetically related Valley (close to Asti) where, in the past, there was an impor- depositional environments (e.g. fluvial, deltaic, shelfal, slope, tant spring which was tapped for drinking water purposes. basinal depositional systems). At present several wells are in operation and are over-ex- In this work, the correlation between the outcrop and ploiting the aquifer under examination causing a large de- subsurface data has made it possible to define a detailed hy- pression cone. The aquifer system is confined and, below the drostratigraphic scheme of the entire area under examina- Eastern Escarpment, is generally artesian. The chemistry of tion. In fact, the informal lithostratigraphic units recognised these waters is substantially homogeneous with bicarbonate- (*) Dipartimento di Ingegneria del Territorio, dell’Ambiente e delle Geotecnologie, Politecnico di Torino, Corso Duca degli Abruzzi 24, 10129 Torino, Italy. E-mail: [email protected]; [email protected] (**) ENI S.p.A., Exploration & Production Division, ESEI, Via Emilia 1, 20097 San Donato Milanese (MI), Italy. E-mail: [email protected] 268 VIGNA B. - FIORUCCI A. - GHIELMI M. calcic and bicarbonate-calcic-magnesium facies with decid- ove possibile con la stessa nomenclatura utilizzata per le unità edly low nitrate contents and iron and manganese levels litostratigrafiche informali. Alle diverse associazioni di facies which are often above the maximum admissible concentra- costituenti le varie unità idrogeologiche sono stati assegnati tions established by the Italian Law. valori di conducibilità idraulica desunti da prove in situ o da A second aquifer system, which at present is not very im- letteratura. Sulla base della geometria dei livelli acquiferi, se- portant but in the past was the only water resource available mipermeabili ed impermeabili, della natura e geometria dei for the local population, is found in the hilly sector north of loro contatti, dei dati idrodinamici e idrogeochimici sono the Tanaro River between the villages of Santa Vittoria d’Alba quindi stati identificati una serie di sistemi acquiferi. and San Damiano d’Asti. This aquifer system, located in the Il sistema acquifero principale è impostato negli orizzonti sand bodies of the Argille Azzurre A Hydrogeologic Unit, is basali dell’Unità Idrogeologica del Villafranchiano B e dei li- artesian along the main valley bottom and is under pressure velli permeabili dell’Unità Idrogeologica delle Sabbie d’Asti B. in the other zones. The groundwater flow is completely dif- La piezometria di tale sistema acquifero è fortemente condi- ferent from the main aquifer system with prevalent directions zionata, nel settore sud-occidentale dell’area studiata, dalla towards the west and a rather complex recharge mechanism giacitura dell’unconformity che separa l’Allogruppo EP dal due to recurrent leakage from the aquifers above. This aquifer sottostante Allogruppo LM e, nel restante settore, dalla geo- system is intercepted by deep wells that reach the water- metria sinclinalica che caratterizza il contatto stratigrafico tra logged sand bodies below a thick succession of silty clays. le Sabbie d’Asti B e le Argille Azzurre B. L’assetto piezome- The chemical facies of the waters from this aquifer sy- trico presenta linee di flusso idrico sotterraneo dirette dal set- stem are very different one from the other. The presence of tore dell’Altopiano di Poirino – Colline di Bra verso la Valle chloride-sodic facies shows the existence of marine water Versa (in prossimità di Asti) dove in passato era presente that was trapped in the sediment at the moment of the dep- un’importante sorgente captata ad uso idropotabile e attual- osition and which still has not been completely displaced by mente sono in funzione numerosi pozzi che sovrasfruttano continental water. These latter have been identified in several l’acquifero in esame provocando un vistoso cono di depres- wells and have bicarbonate-calcic and bicarbonate-magne- sione. Il sistema acquifero è in pressione e a valle della “Scar- sium-calcic facies. Bicarbonate-alkaline facies have also been pata Orientale” dell’altopiano di Poirino è generalmente found that can be explained by to cationic exchange with the artesiano. La chimica di queste acque è sostanzialmente uni- clayey confining layers favoured by long permanence times in forme con facies bicarbonato-calcica, bicarbonato-calcico- the aquifer. The quality of the water is generally poor because magnesiaca, contenuti di nitrati decisamente bassi e tenori di of the ammonium, iron and manganese ion contents. ferro e manganese sovente più alti della concentrazione mas- Other less important aquifer systems are present in the sima ammissibile per la normativa italiana vigente. Poirino Plateau sector in the Villafranchiano C Hydrogeologic Un secondo sistema acquifero, ora di modesta impor- Unit and in the Terraced Quaternary Alluvium Hydrogeo- tanza ma che in passato ha rappresentato l’unica risorsa idrica logic Unit. The piezometry of the latter indicates flow direc- disponibile per la popolazione locale, è presente nel settore tions opposite to the main aquifer system below and poor collinare in sinistra Tanaro tra i centri abitati di Santa Vitto- quality waters because of high nitrate contents. ria d’Alba e San Damiano d’Asti. Tale sistema acquifero, im- postato nei livelli sabbiosi di mare profondo dell’Unità KEY WORDS: aquifers, hydrogeochemistry, hydrogeology, Idrogeologica delle Argille Azzurre A, è artesiano in corri- stratigraphy, Tertiary Piedmont Basin. spondenza dei principali fondovalle e in pressione nelle re- stanti zone. Il flusso sotterraneo presenta un andamento del RIASSUNTO - Lo studio interessa l’assetto idrogeologico di tutto differente rispetto al