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A Conceptual and Numerical Model for Groundwater Management

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  • A conceptual and numerical model for groundwater management:a case study on a coastal aquifer in southern Tuscany, Italy

    Piero Barazzuoli & Monica Nocchi & Roberto Rigati &Massimo Salleolini

    Abstract Ongoing hydrogeological research aims todevelop a correct management model for the Plio-Pleistocene multi-aquifer system of the Albegna Rivercoastal plain (southern Tuscany, Italy); overexploitation ofthis aquifer for irrigation and tourism has caused seawaterintrusion. The conceptual model is based on eld andlaboratory data collected during the 19952003 period.Meteoric inltration and ows from the adjoining carbon-ate aquifer recharge the aquifer. Natural outow occursthrough a diffuse ow into the sea and river; articialoutow occurs through intensive extraction of groundwa-ter from wells. Water exchanges in the aquifer occurnaturally (leakage, closing of aquitard) and articially(multiscreened wells). The aquifer was represented by athree-dimensional nite element model using theFEFLOW numerical code. The model was calibrated forsteady-state and transient conditions by matching com-puted and measured piezometric levels (February 1995February 1996). The model helped establish that seawaterintrusion is essentially due to withdrawals near the coastduring the irrigation season and that it occurs above all inthe Osa-Albegna sector, as well as along the river that attimes feeds the aquifer. The effects of hypothetical aquiferexploitation were assessed in terms of water budget andhydraulic head evolution.

    Keywords Groundwater modeling . Coastal aquifer .Salinization . Water budget . Italy

    Introduction

    In many coastal areas the growth of human settlements,together with the development of agricultural, industrialand tourist activities, has led to the overexploitation ofaquifers. Such overexploitation induces a rise in thefreshwatersaltwater interface (seawater intrusion) andthus the degradation of the chemical quality of ground-water; the problem will be aggravated by the expected risein sea level associated with global warming (IPCC 2007).This situation occurs in several areas of the Mediterraneanand will worsen due to the increase in the residentpopulation and in coastal tourism (Lpez-Geta et al.2003). The quality and quantity of groundwater resourcesalong the Italian coasts has been degrading for some time;the impact of the growing population is alarming,especially in the southern regions, where 45% of the totalresident population lives in coastal zones (Barrocu 2003).

    The southern coast of Tuscany (central Italy) is largelyaffected by seawater intrusion (Bencini and Pranzini 1992,1996; Barazzuoli et al. 1999; Angelini et al. 2000; Benciniet al. 2001) and by the consequent deterioration of thequality of groundwater and the local anomalous accumu-lation of heavy metals (Grassi and Netti 2000; Protano etal. 2000; Agati et al. 2001); this is due to intense pumpingfor different purposes (above all irrigation and domesticuse), especially during summer, when the water demandfor agriculture and tourism is highest and the naturalavailability of water is lowest. The problem has beenaggravated in the last few decades by the progressivedecrease in the potential renewable water resources ofsouthern Tuscany (Barazzuoli et al. 2002) due to areduction in total annual precipitation. The coastal plainof the Albegna River is currently experiencing seawaterintrusion owing to an irrational exploitation of the aquiferthrough hundreds of wells of different types and depthsand with different pumping rates. The deterioration ofgroundwater quality is currently a limiting factor for localeconomic growth; agriculture has either been completelyabandoned or has been directed towards crops which cantolerate saltwater but are of inferior quality.

    The intrusion of seawater in coastal aquifers was rstconceptualized independently by Badon-Ghijben (1889)and Herzberg (1901) assuming hydrostatic equilibrium,immiscible uids and the existence of a sharp interface

    Received: 2 March 2007 /Accepted: 16 May 2008Published online: 1 July 2008

    * Springer-Verlag 2008

    P. Barazzuoli :M. Nocchi :R. Rigati :M. Salleolini ())Department of Earth Sciences,University of Siena,Via Laterina 8, 53100 Siena, Italye-mail: [email protected]: +39-577-233811Fax: +39-577-233938

    Hydrogeology Journal (2008) 16: 15571576 DOI 10.1007/s10040-008-0324-z

  • between fresh- and saltwater in a homogeneous uncon-ned aquifer. They found that the depth of the freshwatersaltwater interface below sea level (zs) is given by:

    zs hf fs f

    where f is the density of freshwater, s is the density ofsaltwater, and hf is the elevation of the water table abovesea level. When the equation is applied correctly, theestimated depth closely approximates the real one (Chengand Ouazar 1999); it is still widely used to simulatesaltwater intrusion (Essaid 1990; Cheng and Chen 2001)and, especially for educational purposes, to gain clearinsight into the behaviour of fresh and saline groundwaterin coastal aquifer systems (Oude Essink 2003). Due tomolecular diffusion and hydrodynamic dispersion, freshand salt water are actually miscible liquids: the contactbetween the two uids is therefore a transition zone ratherthan a sharp interface (Gambolati et al. 1999; Cheng andChen 2001). The situation is further complicated by thefact that the saltwater intrusion itself changes the uiddensity, so that this parameter varies in space and time as afunction of changes in concentration, temperature andpressure in the uid. Furthermore, the porous mediumitself is usually stochastically heterogeneous. In order toproperly reproduce the mechanism of saltwater encroach-ment, a variable density ow and transport modellingapproach is therefore currently adopted (Voss and Souza1987; Koch and Zhang 1992; Diersch 1998b; Holzbecher1998; Bear et al. 1999; Diersch and Kolditz 2002).

    The medium- and long-term effects of land manage-ment policies are difcult to foresee due to interactionamong numerous elements and variables of differentnature, especially as far as seawater intrusion is concernedbecause many aspects of this problem are not completelyunderstood (Custodio and Bruggeman 1987; Custodio andGalofr 1993; FAO 1997; Bear et al 1999; Cheng andOuazar 2004). Groundwater management thus requires theuse of numerical models to test present and alternativeexploitation scenarios taking into account not onlytechnical aspects but also economic, legal, social andpolitical ones (Wang and Anderson 1982; Bear andVerruijt 1987; Emch and Yeh 1998; Custodio and Galofr1993; van Dam 1999; Maimone et al. 2004; Bear 2004).

    In this context, the authors developed a long-term hydro-geological research program, the preliminary results of whichwere published by Angelini et al. (2000) and Barazzuoli etal. (2003, 2004). This work presents a conceptual andnumerical model for simulating the hydrodynamics of themulti-aquifer system of the lower Albegna River valley. Themodel will be used for hydrochemical simulations andthe correct management of local water resources. Thisaquifer system provides a good example of the situation inrecent coastal plains (Custodio 2002; Morell 2003).

    The study area

    The Albegna River coastal plain, located in southernTuscany, consists of aeolian and alluvial sediments covering

    an area of about 100 km2 (Fig. 1). The river crosses the plainfrom ENEWSW with an average annual discharge ofabout 5 m3/s at the mouth. The area north of the Osa Riveris characterized by gentle hills alternating with short atsectors where there are sometimes both natural and articialditches resulting from land reclamation works. The highestpeaks, no more than 354 metres above sea level (m asl), arein the south-eastern sector (Poggio del Leccio hill).

    The average annual precipitation in the plain is about630 mm, and the average annual temperature is about 16C.The effective precipitation is rather low, varying from150 mm/year in the low plain to 250 mm/year in thesurrounding hilly areas. As occurs along the entire coastlineof southern Tuscany, most precipitation is returnedto the atmosphere through actual evapotranspiration, withan average of more than 70% (Barazzuoli et al. 1993).According to the climatic classication proposed byThornthwaite (1948), the investigated area can be consid-ered subarid C1 (moisture index from 33.30).

    This area is a tectonic depression made up of continental,transition andmarine sediments (MioceneQuaternary). Thesecover a pre-Neogene substratum composed of Liguride units(an argillaceous-calcareous-ysch complex), cropping out onthe northern and eastern borders of the plain, as well asTuscan units on the southeastern and northern borders(Mancini 1960; Tozzi and Zanchi 1987; Bonazzi et al.1992; Bossio et al. 2004). Miocene (essentially Messinian)sedimentary sequences consist of strongly eroded con-glomerates that can be found only in the eastern sector.Pliocene sediments prevalently consist of clay, togetherwith regressive sands, gravel and conglomerates. In theeastern sector, the limestones, lacustrine clays andtravertine deposits formed in the Pleistocene are inter-ngered with the terraced Albegna River deposits consist-ing of gravels, sands and conglomerates. The aeoliansands and nest uvial deposits (clays and silts) date backto the Holocene period.

    The outcropping rocks can be divided into two maingroups with different permeability through a qualitativeclassication according to formation:

    Quaternary and Neogene complexes. These depositshave weak or non-existent cementation, and showpredominantly primary permeability due to interstitialporosity. The degree of permeability varies: it ismoderate-high in the Neogene conglomerates, sand-stone and sand, travertine, terraced alluvial deposits,shores, and in the horizons of coarse aeolian sediments,but zero to low in the Pliocene cl