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Relations between stratigraphy, groundwater flow and hydrogeochemistry in Poirino Plateau and Roero areas of theTertiary Piedmont Basin, Italy / Vigna B.; Fiorucci A.; Ghielmi M.. - In: MEMORIE DESCRITTIVE DELLA CARTAGEOLOGICA D'ITALIA. - ISSN 0536-0242. - XC(2010), pp. 267-292.

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Relations between stratigraphy, groundwater flow and hydrogeochemistry in Poirino Plateau and Roeroareas of the Tertiary Piedmont Basin, Italy

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ABSTRACT - The study concerns the hydrogeologic setting ofa vast portion of land, of about 1000 km2, between thePoirino Plateau, the thalweg of the Tanaro River and the hillsbetween the towns of Bra and Asti.

The stratigraphic framework of the Messinian-to-Pleistocene succession of this area has recently been redefinedon the basis of a multidisciplinary study carried out on the en-tire western Tertiary Piedmont Basin (TPB). This study, per-formed by a group of researchers from ENI, the Politecnico diTorino and the University of Turin, was based on the analysisand interpretation of biostratigraphic, sedimentologic, struc-tural data from both outcrops and subsurface (ENI deep wellsand seismic sections). The main result of the stratigraphicanalysis of the Messinian to Pleistocene succession of thewestern TPB was the recognition of three main tectono-strati-graphic units or allogroups, bounded at base and top by majortectonically-induced unconformities. These allogroups havebeen named: Late Messinian Allogroup (LM), Early PlioceneAllogroup (EP) and Late Pliocene Allogroup (LP). Each ofthese allogroups is made up of informal lithostratigraphic unitscharacterised by relative lithologic homogeneity and referableto one or to a set of marine or continental genetically relateddepositional environments (e.g. fluvial, deltaic, shelfal, slope,basinal depositional systems).

In this work, the correlation between the outcrop andsubsurface data has made it possible to define a detailed hy-drostratigraphic scheme of the entire area under examina-tion. In fact, the informal lithostratigraphic units recognised

in the multidisciplinary study have been correlated to theirrespective hydrogeologic units (aquifer analogues), referred towith their same nomenclature. The hydrogeologic units con-sist of different sedimentary facies. Hydraulic conductivityvalues have been assigned to the recognized sedimentary fa-cies on the basis of direct measurements or of bibliographicdata. Then different aquifer systems have been identified onthe basis of the geometry of the aquifer, semi-permeable(aquitards) and impermeable (aquicludes) layers, of the typeand geometry of their contacts, and of the hydrodynamicaland hydrogeochemical data.

The main aquifer system is located in the basal part ofthe Villafranchiano B Hydrogeologic Unit and in the perme-able sand layers of the Sabbie d’Asti B Hydrogeologic Unit. Inthe southwestern sector the piezometry of this aquifer sys-tem is strongly influenced by the geometry of the unconfor-mity that separates the EP Allogroup from the underlyingLM Allogroup and, in the remaining sector, by the geometryof the synclinal structure that involves the Sabbie d’Asti B andthe Argille Azzurre B. This piezometry indicates groundwaterflows from the Poirino Plateau and Bra Hills towards the VersaValley (close to Asti) where, in the past, there was an impor-tant spring which was tapped for drinking water purposes.At present several wells are in operation and are over-ex-ploiting the aquifer under examination causing a large de-pression cone. The aquifer system is confined and, below theEastern Escarpment, is generally artesian. The chemistry ofthese waters is substantially homogeneous with bicarbonate-

Relations between stratigraphy, groundwater flow and hydrogeochemistry in Poirino Plateau and Roero areas of the Tertiary Piedmont Basin, ItalyRapporti 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. (**)

(*) 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]

Mem. Descr. Carta Geol. d’It.XC (2010), pp. 267-292,

figg. 16

calcic and bicarbonate-calcic-magnesium facies with decid-edly low nitrate contents and iron and manganese levelswhich are often above the maximum admissible concentra-tions established by the Italian Law.

A second aquifer system, which at present is not very im-portant but in the past was the only water resource availablefor the local population, is found in the hilly sector north ofthe Tanaro River between the villages of Santa Vittoria d’Albaand San Damiano d’Asti. This aquifer system, located in thesand bodies of the Argille Azzurre A Hydrogeologic Unit, isartesian along the main valley bottom and is under pressurein the other zones. The groundwater flow is completely dif-ferent from the main aquifer system with prevalent directionstowards the west and a rather complex recharge mechanismdue to recurrent leakage from the aquifers above. This aquifersystem is intercepted by deep wells that reach the water-logged sand bodies below a thick succession of silty clays.

The chemical facies of the waters from this aquifer sy-stem are very different one from the other. The presence ofchloride-sodic facies shows the existence of marine waterthat was trapped in the sediment at the moment of the dep-osition and which still has not been completely displaced bycontinental water. These latter have been identified in severalwells and have bicarbonate-calcic and bicarbonate-magne-sium-calcic facies. Bicarbonate-alkaline facies have also beenfound that can be explained by to cationic exchange with theclayey confining layers favoured by long permanence times inthe aquifer. The quality of the water is generally poor becauseof the ammonium, iron and manganese ion contents.

Other less important aquifer systems are present in thePoirino Plateau sector in the Villafranchiano C HydrogeologicUnit and in the Terraced Quaternary Alluvium Hydrogeo-logic Unit. The piezometry of the latter indicates flow direc-tions opposite to the main aquifer system below and poorquality waters because of high nitrate contents.

KEY WORDS: aquifers, hydrogeochemistry, hydrogeology,stratigraphy, Tertiary Piedmont Basin.

RIASSUNTO - Lo studio interessa l’assetto idrogeologico diuna vasta area (circa 1000 km2) compresa tra l’Altopiano diPoirino, il fondovalle del Fiume Tanaro e la zona collinare trale città di Bra e Asti.

L’assetto stratigrafico di quest’area, relativo alla succes-sione plio-pleistocenica, è stato ridefinito da uno studio mul-tidisciplinare che ha interessato l’intero settore occidentaledel Bacino Terziario Piemontese. Per lo studio, condotto daun gruppo di ricercatori di Eni, Politecnico di Torino e Uni-versità di Torino, sono stati utilizzati dati di superficie e disottosuolo che hanno permesso di definire uno schema stra-tigrafico di dettaglio della successione studiata. Al di sopradei depositi messiniani della Formazione della Vena delGesso, sono state riconosciute tre unità stratigrafico-sequen-ziali principali separate da importanti discontinuità di naturatettonica così denominate: Allogruppo Late Messinian (LM),Allogruppo Early Pliocene (EP) e Allogruppo Late Pliocene(LP). Tali allogruppi sono costituiti da differenti unità strati-grafico-deposizionali riconducibili a uno o più ambienti dideposizione marini o continentali (sistemi deposizionali flu-viali, deltizi, di piattaforma, ecc.). Tali unità si caratterizzanoper una relativa omogeneità litologica e sono state elevate alrango di unità litostratigrafiche informali.

La correlazione tra la geologia di superficie e i dati di sot-tosuolo ha permesso di definire un modello idrostratigraficodell’intera area in esame, in cui le unità litostratigrafiche in-formali riconosciute nello studio multidisciplinare sono statecorrelate alle corrispondenti unità idrogeologiche, indicate

ove possibile con la stessa nomenclatura utilizzata per le unitàlitostratigrafiche informali. Alle diverse associazioni di faciescostituenti le varie unità idrogeologiche sono stati assegnativalori di conducibilità idraulica desunti da prove in situ o daletteratura. Sulla base della geometria dei livelli acquiferi, se-mipermeabili ed impermeabili, della natura e geometria deiloro contatti, dei dati idrodinamici e idrogeochimici sonoquindi stati identificati una serie di sistemi acquiferi.

Il sistema acquifero principale è impostato negli orizzontibasali dell’Unità Idrogeologica del Villafranchiano B e dei li-velli permeabili dell’Unità Idrogeologica delle Sabbie d’Asti B.La piezometria di tale sistema acquifero è fortemente condi-zionata, nel settore sud-occidentale dell’area studiata, dallagiacitura dell’unconformity che separa l’Allogruppo EP dalsottostante Allogruppo LM e, nel restante settore, dalla geo-metria sinclinalica che caratterizza il contatto stratigrafico trale Sabbie d’Asti B e le Argille Azzurre B. L’assetto piezome-trico presenta linee di flusso idrico sotterraneo dirette dal set-tore dell’Altopiano di Poirino – Colline di Bra verso la ValleVersa (in prossimità di Asti) dove in passato era presenteun’importante sorgente captata ad uso idropotabile e attual-mente sono in funzione numerosi pozzi che sovrasfruttanol’acquifero in esame provocando un vistoso cono di depres-sione. Il sistema acquifero è in pressione e a valle della “Scar-pata Orientale” dell’altopiano di Poirino è generalmenteartesiano. La chimica di queste acque è sostanzialmente uni-forme con facies bicarbonato-calcica, bicarbonato-calcico-magnesiaca, contenuti di nitrati decisamente bassi e tenori diferro e manganese sovente più alti della concentrazione mas-sima ammissibile per la normativa italiana vigente.

Un secondo sistema acquifero, ora di modesta impor-tanza ma che in passato ha rappresentato l’unica risorsa idricadisponibile per la popolazione locale, è presente nel settorecollinare in sinistra Tanaro tra i centri abitati di Santa Vitto-ria d’Alba e San Damiano d’Asti. Tale sistema acquifero, im-postato nei livelli sabbiosi di mare profondo dell’UnitàIdrogeologica delle Argille Azzurre A, è artesiano in corri-spondenza dei principali fondovalle e in pressione nelle re-stanti zone. Il flusso sotterraneo presenta un andamento deltutto differente rispetto al sistema acquifero principale condirezioni prevalenti verso ovest e una ricarica piuttosto com-plessa legata ad una serie di travasi provenienti dai sistemi ac-quiferi sovrastanti. Questo acquifero è intercettato da pozziprofondi che attraversano potenti intervalli di argille siltoseper raggiungere i corpi sabbiosi acquiferi con acque aventifacies chimiche molto diverse tra loro. La presenza di faciescloruro-sodiche evidenzia l’esistenza di acque marine intrap-polate nel sedimento all’atto della deposizione e non ancoracompletamente sostituite da quelle della circolazione attiva.Queste ultime sono state individuate in numerosi pozzi e pre-sentano facies bicarbonato-calcica e bicarbonato-magnesiaco-calcica. Si rinvengono, inoltre, facies bicarbonato-alcalineimputabili a fenomeni di scambio cationico con i sedimentiargillosi e quindi caratterizzate da tempi di permanenza in ac-quifero decisamente lunghi. La qualità delle acque è general-mente scadente a causa degli alti contenuti di ione ammonio,ferro e manganese.

Nel settore dell’Altopiano di Poirino sono presenti altri si-stemi acquiferi di secondaria importanza impostati nell’UnitàIdrogeologica del Villafranchiano C e nell’Unità Idrogeolo-gica Alluvionale dei Terrazzi Alti. La piezometria di quest’ul-tima presenta linee di flusso diametralmente opposte (versoovest) rispetto a quelle dell’acquifero profondo principale eacque di scarsa qualità a causa degli alti contenuti di nitrati.

PAROLE CHIAVE: acquiferi, Bacino Terziario Piemontese, idro-geochimica, idrogeologia, stratigrafia.

268VIGNA B. - FIORUCCI A. - GHIELMI M.

1. - INTRODUCTION

The examined area covers a vast portion of thePiedmont territory (about 1000 km2) and fallsroughly between the Turin Hill reliefs to the north,the western sector of the Turin-Cuneo plain to thewest, the Tanaro River to the south-east and theAsti reliefs to the north-east (fig. 1).

Faced with the necessity of examining the hy-drogeology of an area characterised by a rathercomplex geological and structural framework asthe Piedmont Tertiary Basin (PTB), the work hasbeen performed in different stages in order to cor-relate the tectono-stratigraphic model with the hy-drogeologic context, applying the principle of“aquifer analogues” proposed by BERSEZIO (2007).

First, all the data obtained from a multidisci-plinary study related to the Pliocene-Pleistocenesuccession of the western sector of the PiedmontTertiary Basin (PTB), in which some sequence-stratigraphic (i.e. allogroups and sequences) andlithostratigraphic units were identified (GHIELMIet alii, 2002; GHIELMI et alii, in preparation), havebeen analysed. The informal lithostratigraphicunits of the geological model has been ascribed tothe respective hydrogeologic unit. Finally the iden-tification of the main aquifer systems in the stud-ied succession has been carried out on the basisof the geometry of the different aquiferous hori-zons, their connection, their lower and upperboundaries (identified by the presence ofaquicludes or aquitards), the piezometric levelsthat has been measured in the wells and the che-mistry of the waters.

2. - GEOGRAPHIC SETTING

From the morphological point of view (fig. 2),this area includes the entire Poirino Plateau, whichconstitutes a sub-level portion of the same area. Itslopes slightly towards the west and is borderedon the north by the Turin Hill reliefs and on thesouth by the Bra Hills with topographical altitudesthat vary between 265 (near Dusino) and 230 (nearSantena) m a.s.l. This area has a surface drainagenetwork that collects the water from the TurinHills, the Braidese reliefs and from the plateau it-self through two main collectors (the BannaStream and the Melletta Stream) and sends it to-wards the Turin-Cuneo Plain. The eastern margin ofthe plateau is represented by a clear morphologicalescarpment (referred to as the Eastern Escar-pment) that separates it from the Asti reliefs.These reliefs are characterized by a lower topo-graphical altitude (between 260 and 150 m a.s.l.)

than that of the plateau. They are connected tothe diversion of the Tanaro River and therefore areinterested by the consequent deepening and reju-venation of its catchment as far as the Eastern Escarpment. The remaining part of the territory ismade up of a wide hilly area, deeply incised byseveral streamlets that flow into the Tanaro Riverthat comprises the Asti, Roero and Bra hills. Geo-graphically speaking, the Roero Hills correspond toa vast portion of land between Bra, Montà and theTanaro River.

3. - METHODOLOGY

In the areas characterised by a remarkable strati-graphic and tectonic complexity, hydrogeologicstudies should involve a specific sequence of ope-rative phases. First, it is of fundamental importanceto reconstruct in detail the stratigraphic setting ofthe area through classical geological studies basedon biostratigraphic, sedimentologic, structural andsequence stratigraphic analysis of the sedimentarysuccession, in order to recognize the main strati-graphic units, their sedimentary environments andassociated facies, their lateral and vertical strati-graphic and tectonic relationships. Each strati-graphic unit is converted in an hydrogeologic unit(aquifer analogues). The proper hydraulic con-ductibility value is attributed to each constituent fa-cies of an hydrogeologic unit through the use ofliterature data or direct in situ measurements (per-meability tests, grain-size analysis, slug-tests, aquifertests). Therefore, different permeability values canbe assigned to each hydrogeologic unit and thesevalues can identify aquifers, aquitards, aquicludesand aquifuges.

An aquifer system is characterised by a hydro-geologic structure of known geometry, includingthe unsaturated zone, the saturated zone and itsrecharge area (CIVITA, 2005) and it is made up ofa set of different hydrogeologic units hydraulicallyconnected to each other. Each aquifer system istherefore recognised on the basis of its hydrody-namic situation and of its hydrogeochemical char-acterisation and is bounded by aquicludes oraquifuges. Aquifer systems can also be made up ofone or several hydrogeologic units characterised bythe presence of aquitards and numerous aquifersof small volume so that they can hardly bemapped.

The studied area has proved to be particularlysuitable for the application of this approach thatalso benefitted of the detailed multidisciplinarystudies carried out in the western TPB by GHIELMIet alii, 2002 and in preparation.

269STRATIGRAPHY GROUNDWATER FLOW AND HYDROCHEMISTRY TERTIARY PIEDMONT BASIN


Fig. 1 – Location of the study area on the Lithologic Unit map of Regione Piemonte (Italy) (modified after Regione Piemonte C.S.I., 1990).– Ubicazione dell’area in studio sulla Carta delle Unità Litologiche della Regione Piemonte (Regione Piemonte C.S.I., 1990 - modificata).

LEGEND

RiversStudy areaLakePleistocene-Olocene DepositsPliocene DepositsOligo-Miocene DepositsAlpine Basament

4. - TECTONO-STRATIGRAPHIC MODELOF THE PLIO-PLEISTOCENE SUCCESSIONOF THE WESTERN TERTIARY PIEDMONTBASIN

The Pliocene-Pleistocene succession of the Ter-tiary Piedmont Basin (TPB) has been the subjectof numerous geological studies, starting from theworks of SACCO (1890, 1912, 1924, 1933), until therecent monographic revision of the Villafranchi-ano in the type-area by CARRARO (1996). The TPBis considered as an epi-sutural basin, according tothe definition given by BALLY & SNELSON (1980),formed in a collisional regime behind the front ofthe Monferrato Front overthrusting the Insubricforeland (GELATI & GNACCOLINI, 1988; BOC-CALETTI & MARTELLI, 2004). The oligo-miocenicsedimentary succession of the TPB rests uncon-formably upon the Alps and the Northern Apen-nines structural units, hiding their complexgeometrical relationships.

The Plio-Pleistocene strata of TPB are bor-dered towards south and west by the Alpine Unitsand towards east and northeast by the Oligo-Miocene successions of the TPB (the Turin, Mon-ferrato and Langhe Hills). The Pliocene andPleistocene sediments outcrop principally in thearea between the towns of Asti, Villafranca and Bra.

They are also exposed on the southern limbof the Turin Hills, on the edges of the Langhehills, along the valley bottoms of the Tanaro Riverand of its main tributary streams as far as thetown of Mondovì. In the Cuneo plain and in theplain to the south of Turin, the Pliocene depositsare overlain by a thin succession of Pleistocenedeposits.

Starting from the Upper Miocene and going onuntil the Pliocene and part of the Pleistocene, var-ious severe phases of structural deformation, con-nected in the western sector of TPB to somecompressive fronts with northwest vergence, ledto the creation of a wide sedimentary basin filledwith a succession of both marine and continentaldeposits. The thickness of these deposits reaches1200-2200 m in the main depocenters of the basin(in the Moretta and Savigliano sectors) (GHIELMI etalii, 2002). As a result of the phases of growth ofthe compressive fronts, the western TPB is subdi-vided in a few sub-basins referred to as the Savi-gliano Sub-basin, comprising the central-southernpart of the western TPB, the Moretta Sub-basin, inthe north-western part, and the Asti Sub-basin inthe north-eastern part (GHIELMI et alii, 2002). Thenorthernmost Turin Hills fronts separate thesesub-basins from the coeval Western Po Plain Fore-deep located in the central Piedmont (MINERVINI etalii, 2008). The Pliocene-Pleistocene succession is

represented by an overall transgressive-regressivecycle made up of (from base to top) relatively deepmarine clays (Argille Azzurre or Lugagnano Forma-tions), shelfal to nearshore sands (Sabbie d’Asti For-mation), deltaic and continental sands, gravels andclays (Villafranchiano). A major unconformity, theCascina Viarengo Surface, subdivides the Vil-lafranchiano succession into two sedimentary units,which are referred to as the Lower Complex andUpper Complex (CARRARO, 1996). The LowerComplex includes two units (from base to top): theFerrere Unit made up of delta-front sands withtidal influence which, according to BONI et alii(1970), belongs to the Sabbie d’Asti Formation, andthe San Martino Unit referred to a delta plain envi-ronment (CARRARO, 1996). Also the Upper Com-plex includes two units: the Cascina Gherba Unit,represented by fluvial deposits and the MarettoUnit, made up of continental flood-plain deposits(CARRARO, 1996).

In the past the stratigraphic relationships be-tween the different Pliocene-Pleistocene lithos-tratigraphic units of the western TPB were alwaysconsidered the record of the aggradational infill-ing of the basin expressed by a layer-cake stratig-raphy. The study presented by GHIELMI et alii(2002), based on both outcrop and subsurfacedata, highlighted, for the first time, the existenceof lateral stratigraphic relationships between theArgille Azzurre Fm., the Sabbie d’Asti Fm. and theVillafranchiano deposits. Moreover these lithos-tratigraphic units, that seem to show significantlydifferent ages in the different parts of the basin,are framed into different sequence-stratigraphicunits (allogroups). As documented in the 2002study and particularly in a new publication whichis presently in preparation by the same authors,during the Messinian and Pliocene the westernTPB underwent a few severe phases of compres-sive and transpressive Apennine structural defor-mation as suggested by the presence of majorunconformities of tectonic origin. Therefore, inthese studies, the sequence-stratigraphic analysiswas based on the recognition of stratigraphicunits bounded at base and top by tectonically-in-duced major unconformities: the allogroups. Twomajor unconformities subdivide the Pliocene-Pleistocene succession of the western TPB intothree different allogroups: the Late Messinian Al-logroup (LM), the Early Pliocene Allogroup (EP)and the Late Pliocene Allogroup (LP). The suc-cession of each allogroup, deposited in relativecontinuity of sedimentation and attributed to awell defined stratigraphic interval on the basis ofthe biostratigraphic data, is made up of sedimentsreferred to genetically related depositional envi-ronments.



Fig. 2 - Morphologic setting of the study area (processing by Digital Terrain Model of Regione Piemonte 1985 - 1994).- Assetto morfologico dell'area di studio (elaborazione da modello altimetrico digitale della Regione Piemonte 1985 - 1994).

4.1. - LATE MESSINIAN ALLOGROUP (LM)

The LM Allogroup includes the post–evapor-itic Messinian and Lower Pliocene sediments. Theallogroup boundary is represented by a major tec-tonic unconformity referred to as the “Intra-Messinian Unconformity”. The sediments of theallogroup unconformably overlie the pre- and syn-evaporitic Messinian succession or older Miocenedeposits. The Intra-Messinian severe phase ofstructural deformation caused, in some marginalsectors of the basin, the re-sedimentation of partor, sometimes, of the entire evaporitic succession(with blocks and olistoliths of gypsum and car-bonates incorporated in a fine matrix) into thelower part of post-evaporitic succession. Thesemass-deposits, already described in the outcropsof the southern limb of the Turin Hills front andof south-western Langhe Hills as the Chaotic Com-plex of the Versa Valley (DELA PIERRE et alii, 2002,2003), have also been recognized in the subsurfaceof other sector of the western TPB (GHIELMI etalii, in preparation). The post-evaporitic Messiniansuccession is made up of gravel, sand and silt ofthe Cassano Spinola Formation (BONI & CASNEDI,1970). These sediments, characterized by very vari-able thicknesses, have been interpreted as delta-fandeposits associated to lacustrine and palustrine fa-cies (GHIBAUDO et alii, 1985). The “lago-mare facies”,deposited in a brackish environment and predom-inantly made up of greenish clay, belong to thesame formation.

The regressive post-evaporitic Messinian suc-cession is overlain, through an sharp lithologic, bytransgressive Lower Pliocene deposits mostly rep-resented by relatively deep marine clays with anabundant marine microfauna. These sedimentsshow an abrupt return, in most of the basin, to arather deep marine sedimentation in consequenceof the Pliocene transgression.

Once the basal Pliocene transgressive phasehad run out, an important regressive phase started(referred to as “Pliocene progradation” byGHIELMI et alii, 2002). This phase gradually led toa partial filling of the western TPB during the firstpart of Lower Pliocene. Towards the end of theLM Allogroup the basin was characterised by asomewhat complex environmental framework. Inthe southernmost portion of the western PTB,close to the Alpine basement (between the townsof Cuneo and Mondovì), a deposition of alluvialfan, fluvial and flood-plain sediments took place.These continental sediments have been attributed,by GHIELMI et alii, (2002 and in preparation), tothe informal Villafranchiano A Unit. Towards thenorth, in the area roughly between Fossano and

Salmour, the continental deposits of the allogroupfirst grade laterally into deltaic and shelfal faciesmostly made up of sands, that have been includedin the informal Sabbie d’Asti A Unit, then intoouter shelf, slope and basinal deposits of the in-formal Argille Azzurre A Unit (GHIELMI et alii,2002). The slope deposits consist of clays, locallychaotic, with intercalations of sands and gravels(outcropping above all in the area between thetowns of Morozzo and Cherasco). The basinal sedi-ments (exposed in particular in the study area be-tween Bra and Canale), are represented by 5-30 mthick sandy bodies alternating with 10-80 m thickclay and silty clay layers. The sand bodies aremostly represented by packages of amalgamatedmedium to thick sandy beds deposited in a rela-tively deep water environment by sediment ofgravity flows. Moving towards the town of Asti,these sediments grade into distal fine-grainedbasin plain deposits made up of clay with interca-lations of thin-bedded silt.

4.2. - EARLY PLIOCENE ALLOGROUP (EP)

This allogroup consist of Lower and MiddlePliocene sediments. The lower boundary of the al-logroup corresponds to an important tectonicmodification phase of the basin and to a growthphase of the main structural fronts. Throughoutthe studied area, the boundary is characterized bya pronounced angular unconformity (Pocapaglia un-conformity) and by a sharp change in the sedi-mentary facies indicating an abrupt reduction ofwater depth in consequence of the uplift of theeastern margin of the basin. In the area betweenthe towns of Bra and Monteu Roero, the continentaland deltaic gravels and sands, belonging to the Vil-lafranchiano B informal unit, unconformably over-lie the shelfal sands and silts attributed to the Sabbied’Asti A informal Unit (Allogroup LM). To thesouthwest of the town of Asti yellow-brown lam-inated silty clays of the Argille Azzurre B informalunit overlie the deeper typically grey-blue clay ofthe Argille Azzurre A, interpreted as basinal de-posits. The EP Allogroup is also made up of de-posits that ranges from continental to coastal andmarine facies. Moving from the southwest towardsthe northeast in a basinward direction, continen-tal, fluvio-deltaic and tide-dominated deltaic de-posits of the informal Villafranchiano B Unit(corresponding to the “Complesso Inferiore” of theVillafranchiano type-area; CARRARO, 1996), pre-dominantly sandy shelfal deposits of the informalSabbie d’Asti B Unit, outer shelf, slope and basinalsilty shales of the informal Argille Azzurre B Unitoutcrop between Bra and Asti.


4.3. - LATE PLIOCENE ALLOGROUP (LP)

The LP Allogroup is made up of UpperPliocene and Pleistocene continental sediments.The allogroup boundary corresponds to a severephase of compressive deformation of the westernTPB. A dramatic uplift (referable to the LatePliocene) interested many sectors of the basin, in-cluding also the study area, which were conse-quently interested by a long period of nodeposition and of sub-aerial erosion of the under-lying older successions. In the Villafranchiano type-area, the allogroup boundary should correspond tothe “Cascina Viarengo surface”, angular unconfor-mity identified in the area by CARRARO (1996). Thisunconformity often represents an important hia-tus that can locally encompass the entire UpperPliocene and part of the middle Pliocene and Pleis-tocene (CARRARO, 1996; GHIELMI et alii, 2002 andin preparation). The sedimentation of upperPliocene-lower Pleistocene fluvial and flood-plaindeposits of the informal Villafranchiano C Unittook place only in the westernmost Moretta sub-basin, and in a little sector of the Asti sub-basin(GHIELMI et alii, 2002 and in preparation). In thetype-area of the Villafranchiano, these strata wereincluded by CARRARO (1996) into the “Complesso Su-periore” (Cascina Gherba and Maretto Units).

The LP Allogroup also includes the middle-upper Pleistocene coarse-grained fluvio-glacial de-posits of the “Quaternary Alluvium” (alternativelyreferred to as the “Terraced Fluvial Deposits” fol-lowing the terminology proposed by CARRARO(1996). The “Quaternary Alluvium” sedimentation,predominantly made up of gravels, mostly devel-oped in the Cuneo and Turin plain. Fluvio-glacialdeposits are also present, to a certain extent, in thePoirino Plateau area and in the valley floors of thewatercourses. In these areas it is possible to recog-nize at least three main morphological units: 1) thehigh terraces, 2) the principal plain (Cuneo andTurin plain), 3) the present valley floor depositsand the suspended terraces (CAVALLI & VIGNA,1992). The three units, bounded at the base by ero-sional surfaces, are interpreted as the result of asmany major phases of erosion and subsequent flu-vio-glacial deposition controlled by Middle Pleis-tocene climatic cyclicity.

5. - HYDROGEOLOGIC FRAMEWORK

The first hydrogeologic entry to describe thearea was the study by BORTALAMI et alii, (1989). Theauthors showed how the Cassano-Spinola conglom-erates constitute the deepest aquifer complex, withmodest aquifers and rather poor-quality water qual-

ity, while the clays of the Lugagnano Formation rep-resent a thick impermeable succession with thepresence, on the top of the formation of somesandy intercalations that house limited confinedaquifers of limited production. The Sabbie d’Astiresult to be an aquifer with a remarkable produc-tion and which, in the Versa Valley zone, house im-portant aquifers under pressure. The LowerVillafranchiano deposits also house aquifers ofsome importance, while the Upper Villafranchianoones have a very low production. The deep wells inthe Poirino Plateau zone exploit the Lower Vil-lafranchiano and the Sabbie d’Asti layers and havefair flows. The ancient alluvial sediments (UpperVillafranchiano) constitute an impermeable com-plex with totally negligible water reserves. In theaforementioned work, there is no mention of thepiezometry of the different aquifers, but the qual-ity of the water of the different aquifers wasanalysed and the results have shown the presenceof several very different hydrogeochemical facies.

A subsequent contribution, by CANAVESE et alii(1999), analysed the stratigraphy and the aquiferdistribution in the subsurface of the central sectorof the Poirino Plateau. The authors identified thepresence of two main aquifers. The first is a shal-low aquifer located in the terraced fluvial deposits(Term 4) and, in the southern sector, in the UpperVillafranchiano deposits (Maretto Unit) with a flow,strongly conditioned to a great extent by the mor-phology of the area, from south-east towardsnorth-west and from east towards west in the di-rection of the Turin plain. A second deeper aquiferis located in the Term 1 (Sabbie d’Asti and FerrereUnit) and Term 2 (San Martino Unit) deposits aswell as in the lower part of the Term 3 (CascinaGherba Unit). The deep water flow is in a west toeast direction, that is, overall in an opposite direc-tion compared to the shallow aquifer. The twoaquifers are probably separated by an impermeablelayer belonging to the upper part of Term 3(Maretto Unit). Chemical analyses were also carriedout on the groundwater and different chemicalcharacteristics were encountered for the twoaquifers.

5.1. - HYDROGEOLOGIC UNITS

In this study several hydrogeologic units havebeen identified in the test area on the basis of thepreviously briefly described stratigraphic model ofthe Pliocene-Pleistocene succession of the west-ern PTB (aquifer analogues). These hydrogeologicunits show rather different hydrogeologic charac-teristics in comparison with those recognised in theprevious investigations. An example of the manydifferences concerns the Argille Azzurre Formation


(or Lugagnano Formation). In the study area, theclayey succession of the formation is intercalatedwith numerous sandy aquifer layers previouslynever recognised. In former studies, this hydroge-ologic unit was in fact considered completely im-permeable and to play the role of an aquiclude.In the present work, the main sedimentary faciesof the different hydrostratigraphic units have beendistinguished. Then a value of hydraulic con-ductibility has been attributed to the sedimentaryfacies on the basis of direct measurements or ofbibliographic data. The hydrogeologic characteris-tics of the main stratigraphic units identified in thethree allogroups are here briefly described (fromthe bottom to the top):

Cassano-Spinola Hydrogeologic Unit;Argille Azzurre A Hydrogeologic Unit;Sabbie d’Asti A Hydrogeologic Unit;Argille Azzurre B Hydrogeologc Unit;Sabbie d’Asti B Hydrogeologic Unit;Villafranchiano B Hydrogeologic Unit;Villafranchiano C Hydrogeologic Unit;

Ancient terraced Pleistocene Alluvium Hydro-geologic Unit;

Fluvial channel Holocene Alluvium Hydroge-ologic Unit.

The Hydrogeologic Units map and the Hydro-geologic Units cross-section are shown in figures 3and 4 respectively.

5.1.1. - Cassano-Spinola Hydrogeologic Unit

The unit is made up of alternating sand, graveland clay of the Cassano-Spinola Formation that out-crop in a narrow belt between the villages of SantaVittoria d’Alba and Magliano Alfieri, close to theTanaro River valley floor. The thickness of the for-mation is variable, with an average value in the out-crops of some tens of metres. The sands andgravels, interpreted as fan-delta deposits, are char-acterized by a rather low permeability due to thepresence of a fine matrix, while the fine-grainedfacies referred to brackish lagoon, lacustrine andswamp environments are completely impermeable.


Fig. 3 – Hydrogeologic Units map.– Carta delle Unità Idrogeologiche affioranti nell’area in esame.

Fluvial channel Holocene AlluviulPlain Quaternary AlluviumAncient terraced Pleistocene AlluviumVillafranchiano CVillafranchiano BSabbie d’Asti BArgille Azzurre BSabbie d’Asti AArgille Azzurre ACassano-SpinolaGessoso-solfiferaMiocenic Sequence

LEGEND

Hydrogeological Units

Cascina Viarengo Unconformity Pocapaglia UnconformityIntramessinianUnconformityTrack section

Unconformity

5.1.2. - Argille Azzurre A Hydrogeologic Unit

The Argille Azzurre A Hydrogeologic Unit isrepresented by a thick succession of alternatingclay and fine- to medium-grained sand of theArgille Azzurre A informal unit which outcropsbetween the villages of Santa Vittoria d’Alba-Pol-lenzo and the towns of Canale and Priocca. Thesand bodies, with individual thickness between 5and 30 m, are mostly represented by packages ofamalgamated medium to thick sand beds (fig. 5).From the Borbore Stream to the northeast, the ex-amined unit, masked by the overlying Argille Az-zurre B deposits, is not exposed. The sand bodies,that reach their maximum thickness in the sectorbetween Monticello d’Alba and Vezza d’Alba, grad-ually thin and shale out in a northeast direction

towards the city of Asti, where the succession isrepresented by clays with intercalations of thin-bedded very fine-grained sands. The thickness ofthe entire hydrogeologic unit ranges between 300m, in correspondence to the eastern margin ofthe basin, and some tens of metres in the TurinHill sector.The permeability of the sands, that depends onthe grain-size (ranging predominantly betweenfine- and medium-grained sand), the compaction(generally elevated) and the presence of silt ma-trix, usually shows quite low values. A series ofhydraulic jump tests carried out in wells of thearea has supplied transmissivity values of between10-5 and 10-6 m/s, and hydraulic conductivity val-ues, obtained using the QSPEC calculation code(CIVITA, 2005), of between 10-4 and 10 -5 m/s. The


Fig. 4 – Hydrogeologic Units cross-sections. Location in figure 3.– Sezioni schematiche delle Unità Idrogeologiche. Ubicazione in figura 3.

Plain Quaternary AlluviumHydrogeologic Unit

Ancient terraced Pleistocene AlluviumHydrogeologic Unit

Villafranchiano CHydrogeologic Unit

Villafranchiano B Hydrogeologic Unit (San Martino Unit)

Villafranchiano B Hydrogeologic Unit (Ferrere Unit)Sabbie d’Asti BHydrogeologic Unit

Argille Azzurre BHydrogeologic Unit

Sabbie d’Asti AHydrogeologic Unit

Argille Azzurre A Hydrogeologic Unit (a: resedimentary sand)

Cassano-SpinolaHydrogeologic Unit

Miocenic SequenceHydrogeologic UnitCascina Viarengo Unconformity

Pocapaglia Unconformity

Intramessinian Unconformity

LEGENDSECTION A - A’

SECTION B - B’

SECTION C - C’

clay intercalations and two thicker clay layers atthe top and bottom of the unit are completely im-permeable and play an aquiclude role, separatingthe different sandy aquifer layers.

5.1.3. - Sabbie d’Asti A Hydrogeologic Unit

This hydrogeologic unit outcrops between thetowns of Pocapaglia and Montà, along the EasternEscarpment of the Poirino Plteau, locally known asthe “Rocche di Pocapaglia”. In the staudy area, the suc-cession consist of prevailing yellowish bioturbatefine-grained sand and clayey silt (fig. 6), interpretedas outer shelf deposits and characterised by a verylow permeability. The thickness of the sequenceranges between 60 and 100 m. In the area of mainplain close to the town of Bra, the unit, which un-derlies a thin sequence of quaternary alluvium,seems to be made up of coarser-grained and rela-tively permeable sand.

This unit therefore takes on the role of ac-quitard in the Roero area and controls the waterflow trend of the main aquifer system which is lo-cated in the overlying aquifer layers of the Vil-lafranchiano B and Sabbie d’Asti B HydrogeologicUnits.

5.1.4. - Argille Azzurre B Hydrogeologic Unit

In the study area this hydrogeologic unit is madeup of a monotonous succession of clay and silty claybelonging to the informal Argille Azzurre B informalunit. These deposits, interpreted as outer shelf, slopeand basin deposits, are characterised by a nil per-meability, with the exception of thin-bedded, fine-grained sand layers, that can host aquifers. The unitoutcrops in the eastern sector between San Damianod’Asti and the town of Asti, by the side of the val-leys of the main streams of Asti area (Borbore Valley)and on the southern flank of the Turin Hill. Thethickness of the sequence is of the order of somehundreds of meter. The hydrogeologic unit, whichassumes the role of an aquiclude, influences in thecentral-eastern sector the circulation of the mainaquifer system located in the Villafranchiano B –Sabbie d’Asti B hydrogeologic units.

5.1.5. - Sabbie d’Asti B Hydrogeological Unit

The Sabbie d’Asti B outcrop in the entire sectorin the Asti hills, between the villages of Cisternad’Asti on the south, the town of Asti, the villageof Montechiaro d’Asti on the north and on the sides


Fig. 5 – Argille Azzurre A Hydrogeologic Unit: grey -blue silty clays alternating with yellowish sand bodies. – Unità Idrogeologica delle Argille Azzurre A: alternanze di argille siltose grigio-blu e bancate di sabbie giallastre.

of the reliefs of the Turin Hill. The unit is made upof yellowish, medium to coarse-grained sand in-tercalated with silt. The sand is locally cementedand is characterized by the presence of layers withhigh concentrations of macrofossils. The Sabbied’Asti B sequence reaches a thickness of less than100 m (fig. 7). These sediments were deposited bythe catastrophic floods of the Villafranchiano flu-vio-deltaic system in a shelf environment. The per-meability of the different sandy layers results to besomewhat variable and ranges from relatively highvalues, for the coarser-grained sand (deposited byhigh-energy and large-volume floods), to lower val-ues for the finer-grained sand (deposited by rela-tively low-energy and small-volume floods) withhydraulic conductivity between 10-4 and 10-6 m/s.

5.1.6. - Villafranchiano B Hydrogeologic Unit

The succession of this unit is represented by rel-atively coarse sediments. It consists, in the lowerpart, of gravel and medium-grained sand of the Fer-

rere Unit (CARRARO, 1996) interpreted as mouth-bardeposits (Pocapaglia – Montà area) and tide-dominateddelta-front deposits (Cantarana area), and in theupper part, of delta-plain fine-grained sand andgravel (fig. 8) intercalated with lacustrine clay of theSan Martino Unit. The thickness of the unit rangesbetween some tens of metres, in the examined area,and up to more than 300 m in the buried basin de-pocenter (area between the villages of Sommariva delBosco and Poirino). The succession outcrops from thehills close to the town of Bra as far as the Vil-lafranchiano type-area (Villafranca d’Asti, Cantarana).The permeability of the hydrogeologic unit is vari-able: it is quite high for the coarser and well-sortedmouth-bar deposits (hydraulic conductivity between10-3 and 10-4 m/s), decreases for the delta plain sandand gets nil for the fluvio-lacustrine clay.

5.1.7. - Villafranchiano C Hydrogeologic Unit

The unit is made up of alternating lacustrine clay,prevalently, intercalated with fluvial thick-bedded


Fig. 6 – Sabbie d’Asti A Hydrogeologic Unit: burrowed fine-grained sands and silts. – Unità Idrogeologica delle Sabbie d’Asti A: sabbie fini e silt bioturbati.

sand and gravel belonging to the Upper Vil-lafranchiano sequence (Cascina Gherba and MarettoUnits, CARRARO, 1996). These deposits only outcropalong the syncline axis of the Asti sub-basin andpresents a reduced thicknesses of the order of sometens of metres. They are still present in the core ofthe same syncline in the Poirino Plateau sector, wherethey are overlain by a thin sequence of Quaternaryalluvial sediments. The coarser layers show a rela-tively low permeability, because of the abundant finematrix, while the thick sequence of clay and silt playthe role of either aquitard or aquiclude.

5.1.8. - Ancient terraced Pleistocene Alluvium Hydrogeologic Unit

This unit, outcropping only in the PoirinoPlateau, is represented by a thin sequence (about10 m) of prevailing fine-grained deposits (i.e. fine-grained sand, silt and clay) intercalated with thinlayers of gravel. The permeability of the sedimen-tary succession is somewhat low because of the

presence of an abundant fine matrix. A thickclayey-silt horizon, produced by Pleistocene weath-ering processes, overlies the whole plateau sector.

5.1.9. - Fluvial channel Holocene Alluvium Hydrogeologic Unit

The unit is prevalently made up of fine sedi-ments (fine-grained sand and silt). It outcrops alongthe incised valleys of the numerous secondary waterflows that drain the Poirino Plateau (belonging to thehydrographical networks of the Banna and Mellettastreams) and the Asti hills (belonging to the hydro-graphical networks of the Traversola, Triversa, Cortaz-zone, Val Andona, Stanavasso, Maggiore, Borbore andVersa streams). The deposits are only some metresthick and their permeability is low.

5.2. - AQUIFER SYSTEMS

On the basis of the permeability of the mainsedimentary facies forming the hydrogeologic


Fig. 7 – Sabbie d’Asti B Hydrogeologic Unit: medium- and coarse-grained sands and silts. – Unità Idrogeologica delle Sabbie d’Asti B: alternanze di sabbie medie e grossolane e silt.

units, and of their geometric relationships, it hasbeen possible recognize the presence of differentof aquifer systems in the Pliocene-Pleistocene suc-cession of the study area. These aquifer systems,that can involve one or a part of a hydrogeologicunit, or a complex of two or more of them, arecharacterised by a piezometric network, a rechargearea and specific hydrogeochemical characteristics(fig. 9). They are interested by numerous wells usedfor drinking water, irrigation and industrial uses.

5.2.1. - Villafranchiano B – Sabbie d’Asti B aquifer system

This aquifer system is located in the lower partof the Villafranchiano B Hydrogeologic Unit (Fer-rere Unit) and in the coarser layers of Sabbie d’AstiB whose facies are heteropic. The deep water cir-culation of the aquifer is strongly influenced by asyncline, with a east-west axes that gently dips to-wards the west, involves the whole Pliocene-Pleis-tocene succession. In the south-eastern sector of

the area, corresponding to the escarpment of the“Rocche di Pocapaglia”, the direction of the waterflow is controlled by the geometry of the uncon-formity that separates the EP and LM allogroups(fig. 9) with the basal Villafranchiano aquifer layersin contact with the silty sand of the Sabbie d’Asti AHydrogeologic Unit (aquitard). The deep water cir-culation in the Asti area is controlled by the het-eropy contact between the Sabbie d’Asti BHydrogeologic Unit and the time-equivalent ArgilleAzzurre B Hydrogeologic Unit which representsthe main aquiclude. The aquifer system is presentin almost the whole study area, starting from thePoirino Plateau and Bra hills, where it is interceptedby deep wells at a depth of more than 250 m. Inthis sector the aquifer is in pressure and is borderedby thick clayey layers of the middle-upper part ofVillafranchiano succession (lacustrine and swampdeposits of the San Martino, Gherba and MarettoUnits), with risings of some tens of meter. In thevalley bottoms of the Asti hills sector, the aquifersystem is reached by shallower drillings (between


Fig. 8 – Villafranchiano B Hydrogeologic Unit: sands and gravels interpreted as tide-dominated delta-front deposits (Ferrere Unit).– Unità idrogeologica del Villafranchiano B: sabbie e ghiaie e di fronte deltizio dominato dalle maree (Unità di Ferrere).

40 and 80 m) and show artesian conditions in cor-respondence to the Stavanasso, Traversola and Cor-tazzone Valleys sectors. In the Maggiore Valley area,before the drilling (in 1933) of the numerous wellsfor human supply of the many municipalities in thearea, a spring was present (Cascina Bonoma Spring)with a somewhat abundant discharge which wastapped to supply the town of Asti (SACCO, 1933).

This spring was located roughly in correspon-dence to the stratigraphic contact between theaquiferous layers of the basal Villafranchiano BUnit and the Sabbie d’Asti B Unit. The wells reachdepths of 100 m and intercept several aquiferouslayers in the Sabbie d’Asti B Unit, which were arte-sian in the past (up to 18 m above ground level)and now, in consequence of their over-exploita-tion, level out at about 40 m below the groundlevel. The aquifer system also extends in the Asti

hills sector where is only located in the Sabbie d’AstiB Unit in consequence of the complete absence ofthe Villafranchiano deposits.

The deep water circulation was reconstructedthrough the measurement of the piezometric lev-els in more than 190 wells that exclusively involvethe tested aquifer system. Other 200 wells can befound in the area which usually interconnect dif-ferent aquifers of the Upper Villafranchiano suc-cession. These wells were therefore excluded fromthe calculations relative to the reconstruction ofthe piezometric network.

The piezometric network of the deep aquifersystem (fig. 10) was reconstructed starting from thesector of the Turin-Cuneo Plain between the townsof Bra and Cambiano, where a series of deep wellsthat reach the aquifer under examination was en-countered. In the hilly zone between Bra, Montà


Fig. 9 – Major aquifer systems cross-sections. Location in figure 3.– Sezioni schematiche dei sistemi acquiferi maggiori. Ubicazione in figura 3.

SECTION A - A’

SECTION B - B’

SECTION C - C’

Aquifer System in the Plain Quaternary Allu-vium Hydrogeologic Unit

Aquifer System in the Ancient terraced Plei-stocene Alluvium Hydrogeologic UnitAquifer System in the Villafranchiano C -Villafranchiano B Hydrogeologic Unit

Aquifer System in the Villafranchiano B -Sabbie d’Asti B Hydrogeologic UnitAquifer System in the Argille Azzurre A Hy-drogeologic Unit

Aquitard

Aquiclude

Cascina Viarengo UnconformityPocapaglia Unconformity

Intramessinian Unconformity

LEGEND

and Pralormo, the direction of the undergroundflow is towards northeast. As far as Montà the flowis conditioned by the contact between the Vil-lafranchiano deposits and the underlying Sabbied’Asti A sediments below, which play the role ofaquitard. Instead, from Montà to the Maggiore Val-ley, the flow is conditioned to a great extent by thedrawing action exerted by the Maggiore Valley wellfield, by the geometry of the unconformity be-tween the LM and EP allogroups and by the localhydrostratigraphic layout.

In the area between the village of Sommariva delBosco and the town of Poirino, the prevalent direc-tion of the flow is roughly towards the north. Inthis area, there are two underground water dividesthat direct the flow towards the zone between thetowns of Santena and Cambiano where there is a sec-tor that is characterised by relatively homogeneouspiezometric levels of between 226 and 228 m a.s.l.Towards the east, starting from the Riva villagezone, an evident underground drainage axis can beidentified, which directs the flow towards the Tra-versola and Triversa valleys, where there is a series ofartesian wells, and then towards Villafranca d’Asti,where several wells are tapped for human supply.

On the basis of the available data at present, it re-sults that this aquifer system is supplied in part bythe zenithal recharge in the hilly area between Braand Montà, where the Lower Villafranchiano de-posits outcrop, and in part by the outflows fromthe extended Turin-Cuneo Plain unconfined aquifer,which is located in the Quaternary Alluvium de-posits that rest unconformable on the Vil-lafranchiano succession. The contribution from thehilly sector to the northeast between the villagesof Montechiaro d’Asti and Settime, where the Sabbied’Asti B sequence outcrops, is absolutely negligible.The aquifer system in the Villafranchiano B – Sab-bie d’Asti B Hydrogeologic Units is the main sourceof underground water tapped for human supplyby numerous waterworks organisations. In theRoero hills, well fields are located in the villages ofPocapaglia, Sommariva Perno and Santo Stefano Roero.These wells reach depths of more than 150 m withdepth-to-water tables between 60 and 95 m. In thePoirino Plateau, the aquifer is exploited by the Mu-nicipalities of Chieri, Cambiano, Buttigliera d’Asti, San-tena and Poirino with wells that reach to 300 m ofdepth with depth-to-water tables between 30 and70 m. In the Asti zone, the aquifer is tapped at dif-


Fig. 10 – Piezometric networks of the main aquifer system in the Villafranchiano B Hydrogeologic Unit – Sabbie d’Asti B Hydrogeologic Unit and of the aquifer system in the Argille Azzurre A Hydrogeologic Unit.

– Piezometria del sistema acquifero principale delle Unità Idrogeologiche del Villafranchiano B – Sabbie d’Asti B e del sistema acquifero dell’Unità Idrogeologica delle Argille Azzurre A.

Fluvial channel Holocene AlluviulPlain Quaternary AlluviumAncient terraced Pleistocene AlluviumVillafranchiano CVillafranchiano BSabbie d’Asti BArgille Azzurre BSabbie d’Asti AArgille Azzurre ACassano-SpinolaGessoso-solfiferaMiocenic Sequence

LEGEND


Cascina Viarengo Unconformity Pocapaglia UnconformityIntramessinianUnconformity

Unconformity

WellsFlow directionPiezometric contour line

WellsFlow directionPiezometric contour line

Aquifer System in theVillafranchiano B-Asti Sands B Hydrogeological Unit

Aquifer System in the Azure Clay A Hydrogeological unit

ferent points, in particular by the Maggiore Valleywell field which supplies the Municipalities of Asti,Ferrere, Cantarana, Valfenera, San Damiano d’Asti andMonale. The examined aquifer system is also ex-ploited by numerous irrigation and industrial usewells. Such an important water resource, inter-cepted by numerous artesian wells (Triversa, Sta-vanasso and Traversola Valleys), is often not utilisedand left to flow towards the secondary water flows.

5.2.2. - Argille Azzurre A aquifer system

This aquifer system, which at present is not veryimportant from a production point of view, was inthe past the only underground water resource inthe Roero area used for both human supply and ir-rigation purposes. The aquifer system, housed inthe sandy layers of the Argille Azzurre A Hydro-geologic Unit, is confined and is generally artesianin correspondence to the valley bottoms. The dis-charges from the wells are at present very low alsoin consequence of the way they were built. Thesewells, which are known as “calandre”, were con-structed starting from the end of the nineteenthcentury through striking of small diameter pipesperforated for a length of 1 to 2 m at the point, orthrough a rotation method without the lining pipesystem and filter. A maximum depth of over 200 mcould be reached. The calandre were located close tothe inhabited areas and were used to supply smallcommunities and are still used today to supply themany fountains of those towns. The Argille Az-zurre A Hydrogeologic Unit, which houses theaquifer system, outcrops in the hills between thevillages of Santa Vittoria d’Alba–Pollenzo and Canale-Govone. The wells in this sector usually have depthsof between 100 and 120 m and only exploit a fewaquifer layers.

Moving towards north-northeast, the welldrillings have to pass a thick clayey succession(Argille Azzurre B) before reaching the aquifer lay-ers at a depth of between 160 and 240 m. Thesandy layers are almost non existent under thetown of Asti, due to the diminishing of both thegrain-size and the thickness of these levels, wherethere is an absence of water resources.

The examined aquifer system seems to be lo-cated in different sand bodies aquifers separatedfrom each other by thick clayey layers that assumethe role of aquiclude.

Through a series of water level measurementscarried out in the different wells, it was possible toreconstruct the piezometric network of the aquifersystem, which showed a direction of the under-ground flow roughly from west to east (fig. 10), di-rection completely different from that encounteredin the shallower main aquifer. The trend of the

piezometric lines is rather regular and this showsthe existence of a multi-layer that is probably sup-plied by leakage from the Sabbie d’Asti A whose fa-cies are in heteropy with the Argille Azzurre Adeposits. The Sabbie d’Asti A deposits, which out-crop in correspondence to the “Rocche di Pocapaglia”sector, consist of silty facies and play the role ofaquitard, while at a greater depth, towards the de-pocenter of the basin, they are represented bycoarser facies and are therefore waterlogged. Thesupply of this aquifer system therefore seems tobe provided by a rather complex leakage mecha-nism: fresh water has been intercepted also in theAGIP petroleum exploration wells Sommariva delBosco 1, in the south-western sector of the studyarea, in correspondence to aquiferous layers of theArgille Azzurre A levels at depths of more than 600m. The displacement phenomenon of the originalsalty water with the fresh water coming from thesectors below the Turin-Cuneo Plain, connected tothe remarkable hydraulic charge in this area (morethan 270 m a.s.l.), compared to that found in theSanta Vittoria d’Alba – Pollenzo and Canale – Govonesector (from 190 to 150 m a.s.l.), and to the lateraland vertical continuity of the various water bod-ies, has played an important role.

5.2.3. - Villafranchiano C - Villafranchiano B aquifer system

A multi-level aquifer system under pressure canbe found in the Villafranchiano C and in part ofthe Villafranchiano B (San Martino Unit) Hydroge-ologic Units. This multi-level aquifer system is lo-cated in relatively permeable sandy-gravelyhorizons of modest thickness. The permeable lay-ers are intercalated with fine-grained sediments,silty sand, sandy silt and clay, which act as anaquitard. It is intercepted by relatively deep wells,prevalently for irrigation or industrial use, in thePoirino Plateau which often interconnect the morepermeable horizons encountered at differentdepths. It has a rather poor productivity and thewater quality is heavily conditioned by the elevatedconcentrations of nitrates. The piezometric levelhas very similar heights to those encountered in theunderlying principal aquifer (The aquifer system inthe Villafranchiano B – Sabbie d’Asti B Hydrogeo-logic Units), which is directly in contact with theone under examination because of the lack of alaterally continuous seal.

In the available stratigraphic data, obtainedfrom drillings, the sandy-silty-clayey lithologies areusually indicated as clay thus generating erroneousinterpretations of the hydrostratigraphic situation.Only a few continuous coring geognostic surveys,carried out to depths of more than 200 m, pro-


vided useful indications on the true hydrogeologiccharacteristics of these deposits. The role ofaquitard that has been assigned to the fine sandy-silty deposits highlights the complexity of the hy-drogeologic framework of the tested aquifersystem, which seems to be partially supplied by thevertical recharge from the Quaternary aquifer sys-tem above, and partially by the overflows from theunconfined aquifer system of the main Turin-Cuneo Plain. The wells that intercept the studiedaquifer (usually for irrigation and industrial use) canreach depths of more than 100 m and often inter-connect this system with that one located in theoverlying Quaternary alluvial deposits. For this rea-son, it has not been possible to reconstruct thepiezometric network or identify the main rechargeareas. The elevated concentrations of nitrates en-countered in the wells that intercept the aquifersystem and the reduced productivity of the aquiferlayers would seem to indicate a supply that preva-lently comes from the sector above relative to thePoirino Plateau.

5.2.4. - Ancient terraced Quaternary Alluvium aquifersystem

An aquifer system exists in the Poirino Plateausector. This system is located mostly in the Ancientterraced Quaternary Alluvium HydrogeologicUnit, in part in the upper layers of the Vil-lafranchiano C Hydrogeologic Unit and in part inthe Fluvial channel Holocene Alluvium Hydroge-ologic Unit. The aquifer system, which is locallyconfined by a thick layer of clay and silt connectedto important weathering processes during thePleistocene, is intercepted by numerous shallowwells that reach depths of between 2 and 20 mused in the past for domestic and/or zootechnicaluse. They have a very low productivity, while thepiezometric network indicates a flow direction thatis strongly influenced by the morphology of theslopes and by the surface drainage network andwhich shows a predominant direction from east towest (fig. 11). This direction is therefore oppositethat of the principal deep aquifer. The recharge ofthe aquifer system under examination is providedby local infiltration phenomena which heavily con-ditions the quality of the water with high nitrateconcentrations.

5.2.5. - Other aquifer systems

Apart from those previously described, in thearea under examination there are other aquifer sys-tems of limited importance which are located inthe shallower part of the previously describedPliocene Units and in correspondence to the thal-

wegs of the different rivers and streams that canbe found in the Asti hills area. These aquifer sys-tems are usually intercepted by very shallow wellsfor domestic use.

Deeper wells intercept an aquifer system underpressure located in the gravel and sand of theUpper Messinian Cassano-Spinola HydrogeologicUnit and which is characterized by very reduceddischarges.

6. - GEOCHEMISTRY

The data relative to the water of the deepaquifer systems and in particular those located inthe Villafranchiano B – Sabbie d’Asti B Hydrogeo-logic Units and the Argille Azzurre A Hydrogeo-logic Unit are examined in this chapter.

Although a large quantity of chemical analyseswas carried out on samples drawn from over 200wells that intercept the more superficial aquifer sys-tems, these data are here not described or com-mented on as they are characterised by remarkablehuman induced impact phenomena, as shown bythe high nitrate contents. These wells often have el-evated depths, but they also intercept and inter-connect different aquifer systems. Thus the drawnsamples cannot be considered representative. Asthe conditioning of most of the wells, in particularthe location of the drains and of the plugged lev-els, was not known, only the chemical analyses en-abled us to verify the mixing of the different watersand, when this occurred, to exclude these samples.

6.1. - VILLAFRANCHIANO B – SABBIE D’ASTI B HYDROCHEMISTRY

The aquifer system located in the Villafranchi-ano B Hydrogeologic Unit and Sabbie d’Asti B Hy-drogeologic Unit is intercepted by several wells andthe sampling of the water was carried out in manyof these together with the relative chemical analy-ses. Numerous wells, even though reaching re-markable depths, intercept and interconnect thewaters from the overlying aquifer systems (the Vil-lafranchiano C Hydrogeologic Unit aquifer systemand the Upper Terrace Quaternary Alluvium Hy-drogeologic Unit) which are generally characterisedby the presence of elevated nitrate contents. Forthis reason, 72 different analyses carried out in thesame number of wells that only intercept theaquifer system under examination and which had anitrate content of below 5 mg/l (that is, consid-ered without signs of human induced impact) wereconsidered for the geochemical characterisation.

The waters in the Villafranchiano aquifer systemhave a specific electric conductibility that varies be-


tween 284 and 854 µS/cm with a mean value of 417± 104 µS/cm. These values make it possible to clas-sify these waters as medium-mineral and are there-fore, at least for this parameter, suitable for humanconsumption. The total hardness values, which fallbetween 13 and 53 French degrees with a meanvalue of 22 ± 6 French degrees, also underline a po-tentially utilizable underground water resource forhuman supply. Instead, either the iron or manganesevalues, and in some cases both, are higher than themaximum admissible concentrations for water des-tined for human consumption, according to the Ital-ian laws currently in force.

Taking into consideration the fact that a suffi-cient number of data were available, a simple sta-tistical analysis was performed for the principalions (calcium, magnesium, sodium, bicarbonate,sulphate and chloride), in which not only the meanand standard deviation were evaluated, but also themost frequent values. For this purpose, a range ob-tained dividing each parameter into ten classes ofequal extent was considered and then the percent-age of cases that fell into each single class was as-sessed. The thus obtained frequency diagrams areshown in fig 12. The common factor of all theconsidered parameters is that more than 75% of

the cases fall into three or four contiguous classesthat are positioned towards lower values. The onlyexception is represented by the sodium which pres-ents a typically Gaussian distribution, althoughwith a maximum frequency which is not in the cen-tral position but moved slightly towards the lowervalues. Overall, the values indicate a generallypoorly mineralised aquifer with limited cases ofhigher mineralisation, but always contained withinvalues that can be considered not excessive, asshown by the overall range.

The type of water of all the samples isCa2+>Mg2+>(Na+ + K+) – HCO3

->SO42->Cl-.

More information can be obtained from somecharacteristic ratios calculated by expressing the ionconcentrations in meq/1.

The Mg2+/Ca2+ ratio varies from 0.151 to 0.796,thus showing a greater presence of the calcium ionthan the magnesium ion. However, in 86% of thecases, this ratio is below 0.500; therefore the faciesis generally of a calcium type and calcic-magne-sium only to a lesser extent.

The Mg2+/(Na+ + K+) ratio is always in favourof the magnesium ion as the values are between1.072 and 7.538. However, the values are not welldistributed, considering that 83% of the cases fall


Fig. 11 – Piezometric network of the aquifer system in the Hydrogeologic Unit of the Ancient Terraced Pleistocene Alluvium.– Piezometria del sistema acquifero dell’Unità Idrogeologica dei Terrazzi Antichi Pleistocenici.

Fluvial channel Holocene AlluviulPlain Quaternary AlluviumAncient terraced Pleistocene AlluviumVillafranchiano CVillafranchiano BSabbie d’Asti BArgille Azzurre BSabbie d’Asti AArgille Azzurre AGessoso-solfiferaMiocenic Sequence

LEGEND


Cascina Viarengo Unconformity Pocapaglia Unconformity

Unconformity

WellsFlow direction

Piezometric contour line

Aquifer System in theHydrogeological Unit ofthe Ancient terraced Pleistocene Alluvium

into the first third of the range (1.072 – 3.227).This shows that the alkaline ions, in the cationcomponent, are not generally negligible.

As far as the alkaline ions are concerned, the Cl-/(Na+ + K+) ratio, which varies between 0.019 and1.267, is particularly interesting. This ratio is abovethe unit only in two points (V19 and V41) andbelow 0.500 in about 88% of the cases. The datumthat emerges is that the alkaline ions dissolved inthe waters in this aquifer system generally cannotbe attributed to the presence of chlorides; there-fore the alkaline value should be attributed to otherfactors such as ionic exchange phenomena or a rel-evant presence of alkaline feldspatos in the lithiccomponent of the aquifer system.

The HCO3-/SO4

2- ratio varies from 1.572 to

37.668 and is widely distributed as it has a meanvalue of 13.283 ±9.152. As far as the characteristicratio is concerned, the spatial trend described by theisovalue lines is of particular interest (fig. 13). In factit can be noted that the values are lower in the south-ern area of the aquifer and increase following theflow lines of the aquifer. The highest values are dis-tribuited in correspondence to the principal drainageaxis. The obtained data are plotted in a Durov dia-gram (figs. 14 and 15) where a substantial differenceis not clearly shown. The individual points in thecation triangle are concentrated towards the calciumion vertex, with the exception of four samples (V01,V02 V03 e V04) which are moved slightly towardsthe centre of the triangle, in consequence of agreater presence of magnesium and alkaline ions. In


Fig. 12 – Frequency diagrams of the main parameters relative to water chemistry of the aquifer system in the Villafranchiano B Hydrogeologic Unit – Sabbied’Asti B Hydrogeologic Unit (Values of each class expressed by mg/l).

– Diagrammi di frequenza dei principali parametri relativi alle acque del sistema acquifero delle Unità Idrogeologiche del Villafranchiano B – Sabbie d’Asti B (I valori delle singole classisono espressi in mg/l).

the anion triangle, the points are distributed alongthe bicarbonate-sulphate side, because of the lowvalues of the chloride ions found in all the samples.The points are concentrated towards the vertex ofthe bicarbonate, with the exception of four sampleslocated in the southern part of the aquifer (V49,V62, V63 and V68) which are moved more towardsthe sulphates.

6.2. - ARGILLE AZZURRE A HYDROCHEMISTRY

In order to geochemically characterise theaquifer system located in the Argille Azzurre A Hy-drogeologic Unit, chemical analyses were carriedout on 29 samples drawn from the same numberof wells that only intercept the studied aquifer sys-tem. In consequence of the limited number ofsamples, a statistical analysis was not carried out.

The obtained data describe a very complex geo-chemical situation. Six different types of waterwere encountered:Type 1 Ca2+>Mg2+>(Na+ + K+) – HCO3

->SO42-

>Cl- (14 samples);Type 2 Mg2+>Ca2+>(Na+ + K+) – HCO3

->SO42-

>Cl- (3 samples);Type 3 (Na+ + K+)>Ca2+>Mg2+ – HCO3

->SO42-

>Cl- (2 samples);Type 4 (Na+ + K+)>Mg2+>Ca2+ – HCO3

->SO42-

>Cl- (5 samples);Type 5 (Na+ + K+)>Ca2+>Mg2+ – HCO3

->Cl->SO4

2- (2 samples);Type 6 (Na+ + K+)>Mg2+>Ca2+ – Cl->HCO3

-

>SO42- (3 samples).

The six types of water that were encounteredcan be grouped together into three basic hydro-geochemical facies: a bicarbonate calcic facies towhich types 1 and 2 belong, a bicarbonate-alkalinefacies to which types 3, 4 and 5 belong and a chlo-rine-alkaline facies to which type 6 belongs.

The obtained data, which are reported in theDurov diagram (fig. 16), clearly shows the presenceof the six types of water and the three facies. Inthe diagram it is also possible to note that inter-mediate facies exist between the bicarbonate-cal-


Fig. 13 – Isovalue lines of the HCO3-/SO42- ratio relative to the aquifer system in the Villafranchiano B Hydrogeologic Unit – Sabbie d’Asti B Hydrogeologic Unit.– Carta degli isovalori del rapporto HCO3-/SO42- relativo al sistema acquifero delle Unità Idrogeologiche del Villafranchiano B – Sabbie d’Asti B.

Fluvial channel Holocene AlluviulPlain Quaternary AlluviumAncient terraced Pleistocene Alluvium

Villafranchiano CVillafranchiano BSabbie d’Asti BArgille Azzurre B

Sabbie d’Asti AArgille Azzurre A

Cassano-SpinolaGessoso-solfiferaMiocenic Sequence

LEGEND


Cascina Viarengo Unconformity Pocapaglia UnconformityIntramessinianUnconformity

Unconformity

Villafranchiano wellsBicarbonates/Sulphates

cic facies and the bicarbonate-alkaline facies, dueto the variation in the main characteristic ratios.

The deep marine sand bodies alternated withthick layers of silty clay that characterise the stud-ied aquifer system must originally have been satu-rated by marine water (chloride-sodium). After theremarkable deformation of the Argille Azzurre Asuccession during the Pliocene, a water flow wasactivated that progressively substituted the marinewater with fresh water from the aquifer systemrecharge areas. As many of these sand bodies arelaterally and vertically sealed by clay, the marine

water could not have been washed away completelyand some remained trapped within them. The ma-rine water began to be progressively substituted byfresh water only when the sand bodies saturated bymarine water were intercepted by the drilling andthey were triggered into circulation, even thoughwith slow flow. Documents are available whichdemonstrate that this salt water was used to makebread during de II World War, when it was partic-ularly difficult to find normal salt. The differentchemical facies encountered in the various samplesmight be therefore connected to the mixing phe-


Fig. 14 – Durov diagram of groundwater chemistry in aquifer system of Villafranchiano B Hydrogeologic Unit – Sabbie d’Asti B Hydrogeologic Unit (samples fromV01 to V35).

- Diagramma di Durov delle acque del sistema acquifero delle Unità Idrogeologiche del Villafranchiano B – Sabbie d’Asti B (campioni V01 – V35).

nomena generated by the excavation of the wellswhich interconnect several aquifer levels with dif-ferent types of water.

The displacement phenomenon is surely fun-damental to justify the presence of the bicarbon-ate-calcic water, but not for the presence ofbicarbonate-alkaline water. It is therefore necessaryto consider the permanence times of the neo-in-filtration water in the aquifer and the cationic ex-change phenomena. It is possible that thebicarbonate-calcic water in these sediments pro-gressively become bicarbonate-alkaline because of

the presence of clayey deposits with which thecationic exchange processes occur. The cationic ex-changes should have been favoured by the ex-tremely slow velocity of the underground flow andby the consequent long permanence times.

The quality of the water extracted from theaquifer system for drinkable purposes is poor be-cause of the high salt content and, above all, due tothe abundant presence of iron, manganese andammonium ions which are very often above themaximum admissible concentrations established bythe Italian laws in force for drinkable water.


Fig. 15 – Durov diagram of groundwater chemistry in aquifer system of Villafranchiano B Hydrogeologic Unit – Sabbie d’Asti B Hydrogeologic Unit (samples from V36 to V72).

- Diagramma di Durov delle acque del sistema acquifero delle Unità Idrogeologiche del Villafranchiano B – Sabbie d’Asti B (campioni V36 – V72).

7. - CONCLUSIONS

The geological model of the upper Messinianto Pleistocene succession is based on a set of in-formal stratigraphic units corresponding to hydro-geologic units that have been associated to theirconstituent hydrofacies. The hydrogeologic unitsthat have been recognized in the area are:

Cassano-Spinola;Argille Azzurre A;Sabbie d’Asti A;Argille Azzurre B;Sabbie d’Asti B;Villafranchiano B;

Villafranchiano C;Ancient terraced Pleistocene Alluvium;Fluvial channel Holocene Alluvium.

The aquifer systems have been identified by the hy-drogeologic correlations of many wells, the analy-sis of deep subsurface data (ENI deep wells andseismic sections), in situ surveys and the analysisof the water levels measured in numerous wells inthe area. On the basis of this large data-base, it hasbeen possible to reconstruct the piezometric net-work of the major aquifer systems and to definetheir hydrogeochemical cha-racteristics.

The main aquifer system, which is widely ex-ploited by numerous wells for drinkable water and


Fig. 16 – Durov diagram of groundwater chemistry in aquifer system of Argille Azzurre A Hydrogeologic Unit.- Diagramma di Durov delle acque del sistema acquifero dell’Unità Idrogeologica delle Argille Azzurre A.

irrigation use, is located in the Villafranchiano BHydrogeologic Unit and the Sabbie d’Asti B Hy-drogeologic Unit, units which are in hydraulic com-munication. The main aquifer system is boundedon the top by another aquifer system mainly lo-cated in the Villafranchiano C Hydrogeologic Unitand made up of a set of aquifers and aquitards.The hydrodynamic data did not allow a clear dis-tinction between the two superimposed aquifersystems. A distinction between the wells that onlyintercept the first aquifer system from those thatinterconnect the two systems has been possiblethanks to chemical analysis of groundwater (in par-ticular of the nitrate contents). In fact the waters inthe main aquifer system show a rather homoge-neous hydrochemical facies, regardless of whichhydrogeologic unit is locally intercepted.

A second aquifer system, of local importance, islocated in a series of aquiferous horizons separatedby thick aquicludes but characterised by a singlepiezometric network, that have been included intothe Argille Azzurre A Hydrogeologic Unit. Thechemical analyses carried out on the waters sam-pled in the different horizons have shown the pres-ence of different hydrogeochemical facies. Thechlorine-alkaline hydrogeochemical facies are con-nected to the partial and local displacement of an-cient saltwater of marine origin by the flow of thepresent-day water circulation; the bicarbonate-al-kaline ones are due to long permanence times ofthe neo-infiltration water in the aquifer where it hasundergone cationic exchange phenomena, whilebicarbonate-calcic and calcic-magnesium facies aretypical of the main water flow.

From the results here presented, it is possibleto deduce that multi-disciplinary studies, such asthose described in this paper, are important toclearly understand water circulation modalities indeep aquifers characterised by a remarkable geo-logical-structural complexity.

Acknowledgements

The Authors are indebted to the Referees Renzo Valloni and StefanoMargiotta for their careful revision of the manuscript.

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