Environmental modelling in the Gulf of Cadiz: Heavy metal distributions in water and sediments

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    km

    ectnumpichicrsith

    pplrianrt models have been compared with measurements in the GoC. In particular, the

    b-basiraltar,northetic coas

    Science of the Total Environment 407 (2009) 33923406

    Contents lists available at ScienceDirect

    Science of the Tot

    l semechanisms of water exchange between the Atlantic Ocean and theMediterranean Sea; as well as the behaviour of the dense plume ofMediterranean water (Ambar and Howe, 1979; Garca-Lafuente et al.,2006; Criado-Aldeanueva et al., 2006; Machn et al., 2006; Garca-Lafuente and Ruz, 2007). The distribution of suspended matter andsediment transport in the GoC has also been investigated (Gonzalezet al., 2007; Freitas and Abrantes, 2002; Lobo et al., 2004; Cravo et al.,2006; Palanques et al., 19861987).

    The freshwater inputs of rivers discharging in the GoC are relatively

    ecosystem functioning. Thus, several papers concerning the distributionof metals in the GoC have been published in the last years (Sainz andRuiz, 2006; Morillo et al., 2004; Elbaz-Poulichet et al., 2001; Fernndez-Caliani et al.,1997). Indeed, studies on the distribution of tracemetals incoastal waters are frequently published in recent times (see for instanceLi et al., 2007; Suntornvongsagula et al., 2007; Cuong et al., 2008; Chenand Jiao, 2008; Valds et al., 2008; Marn-Guirao et al., 2008).

    The objective of this work consists of studying the dynamics ofheavy metals in the northern GoC by means of numerical modelling.small. However, the Guadalquivir, Guadiana athe southern Iberian Peninsula (Fig.1), presentmetal concentrations since they drain the IberRuiz, 2006), one of the most important mini

    Fax: +34 954486436.E-mail address: rperianez@us.es.

    0048-9697/$ see front matter 2009 Elsevier B.V. Adoi:10.1016/j.scitotenv.2009.01.023f oceanographic studiesed at understanding the

    ecological interest. Consequently, it is relevant to study and understandthe geochemistry and dispersion patterns of heavy metals in the GoCsystem, since thiswill help assessing the potential inuence ofmetals onRecently, the GoC has been the subject odealing with surface and deep circulation, aimthe Strait of Gibraltar. The western bou9W meridian (Fig. 1).ndary is usually dened by the1. Introduction

    The Gulf of Cadiz (GoC) is the suwhich is nearest to the Strait Of GibOcean and the Mediterranean Sea. Itsboundaries are, respectively, the AtlanMediterranean Sea. 2009 Elsevier B.V. All rights reserved.

    n of the Atlantic Oceanconnecting the Atlanticrn, southern and easternts of Spain, Morocco and

    Europe. Mineral resources have been extracted in the last 5000 yearsduring two main periods: the Roman age and the last two centuries.During the last period, intensive exploitation has led to a relevantenvironmental impact, with vast surfaces coveredwithmining residuesand subjected to erosion (Sainz and Ruiz, 2006). The GoC is responsiblefor 510% of sh and shell-sh catches of Spain and Portugal (Beckerset al., 2007), holding important living resources of commercial andGulf of Cadiz plumes reach the Strait of Gibraltar, thus the three rivers constitute a source of pollutants into theSediment transportHeavy metals

    sediment and metal transpocontamination of sediments collected along the southern coast of Spain iswell reproduced by themodel. MetalEnvironmental modelling in the Gulf of Cand sediments

    R. Periez Dpto. Fsica Aplicada I, E.U. Ingeniera Tcnica Agrcola, Universidad de Sevilla. Ctra. Utrera

    a b s t r a c ta r t i c l e i n f o

    Article history:Received 3 September 2008Received in revised form 16 December 2008Accepted 9 January 2009Available online 25 February 2009

    Keywords:Numerical modellingHydrodynamics

    The Gulf of Cadiz (GoC) connGoC is carried out throughcirculation and a 2D barotrosediment transport model wGoC. Then heavymetal dispemetal interactions and usestransport model has been athree rivers draining the Ibe

    j ourna l homepage: www.end OdielTinto rivers, instronglyenhancedheavyian Pyrite Belt (Sainz andng areas in the south of

    ll rights reserved.iz: Heavy metal distributions in water

    1, 41013-Sevilla, Spain

    s the Atlantic Ocean and theMediterranean Sea. An environmental study of theerical modelling. First, a 3D baroclinic model is used to obtain the residual

    model is applied to calculate tides. The results of thesemodels are used by a 3Dh provides suspended matter concentrations and sedimentation rates in theon patterns are investigated using a 3Dmodel which includes watersedimente outputs of the hydrodynamic and sediment transport models. The metalied to simulate the dispersion of Zn, Cu and Ni introduced into the GoC fromPyrite Belt, in the southern Iberian Peninsula. Results from the hydrodynamic,

    al Environment

    v ie r.com/ locate /sc i totenvModels have been widely applied to simulate contaminant dispersionsince they may provide insights on the main environmental processesgoverning such dispersion and, consequently, may help to describeand characterize the environment (Scott, 2003). In particular, modelshave beenwidely applied to heavymetal (Tappin et al., 1997;Wu et al.,2005; Prandle et al., 1996; Liu et al., 1998) and radioactive element(Harms, 1997; Cetina et al., 2000; Periez, 2003; Monte et al., 2006)dispersion in coastal waters.

  • Fig. 1. General localization of the study area and topography of the GoC (depths in m). The localization of Guadiana, OdielTinto and Guadalquivir rivers is also shown.

    3393R. Periez / Science of the Total Environment 407 (2009) 33923406

  • 3394 R. Periez / Science of the Total Environment 407 (2009) 33923406Although some interesting modelling works describing watercirculation off Iberia and Morocco coasts have been published(Johnson and Stevens, 2000; Batteen et al., 2000), these models havea relatively low resolution, not providing detailed information aboutthe GoC basin circulation features. In the rst case, spatial resolution is10 min in both longitude and latitude (5 times the grid cell in thepresent model). In the second reference it is 10 km in longitude andlatitude, about three times larger than in this work. Other modellingworks are specically devoted to the study of Mediterranean waterspreading (Jungclaus and Mellor, 2000; Johnson et al., 2002; Serra etal., 2005). The excellent paper by Peliz et al. (2007) presents threenested model domains aimed at reproducing known features of theAzores current and of circulation inside the GoC.

    Published models describing trace metal dispersion in the GoCconsider metals as conservative tracers, no interacting with sedimentsand without any other sources and sinks (Elbaz-Poulichet et al., 2001;Beckers et al., 2007). The rst authors use a model to estimate thedilution of a conservative tracer released by the OdielTinto rivers.Beckers et al. (2007) apply a numerical model to reproduce observedmetal (again considering metals as conservative tracers) concentra-tions in surface waters of the northern GoC. Analysis of model resultsshowed that sources/sinks of metals due to interactions with sedi-ments (adsorption/desorption reactions as well as erosion and depo-sition processes) were apparent. Models which try to reproducemeasured levels of metals in bed sediments of the GoC have not beenpublished yet, to the author's knowledge.

    The model described in this paper consists of three sub-models:rstly, a hydrodynamic module which provides currents over thedomain. Two hydrodynamic models are used. A 2D barotropic modelis applied to calculate tides and 3D baroclinic model is used to obtainthe residual (mean) circulation. Tidal currents must be calculatedsince they may increase the bed stress and hence enhance sedimentresuspension and affect deposition of particles as well. Indeed, thesecond sub-model is a sediment transport model which providessuspended matter concentrations and sedimentation rates over thedomain. The third sub-model is the metal transport module, whichincludes advection/diffusion plus uptake/release reactions of metalsbetween the dissolved and solid (suspended matter in the watercolumn and bed sediments) phases.

    The threemodules, aswell as the numerical techniques used to solvethe involved equations are described in Section 2. Next, model resultsare discussed separately for water circulation, sediment transport andmetal distributions. Some sensitivity analysis are nally described.

    2. Model description

    2.1. Hydrodynamic models

    As commented above, tides are required to calculate bed stress overthe domain, since it will affect sedimentation rates in the shallowerareas and/or where stronger tidal currents exist. A 2D depth-averagedmodel has been applied to calculate surface tides. This is a reasonableapproach that has already been successfully used in the Strait ofGibraltar (Tejedor et al.,1999; Periez and Pascual-Granged, 2008), theAlborn Sea (Periez, 2008) and even in the complete MediterraneanSea (Tsimplis et al.,1995). As has been shown, it is safe to neglect densitydifferences in tidal computations (Dyke, 2001; Yanagi, 1999).

    Equations are standard and may be seen for instance in Periez(2005a). The solution of these equations provides the water currentsat each point in themodel domain and for each time step. Currents aretreated through standard tidal analysis (Pugh, 1987) and tidalconstants are stored in les that will be read by the sedimenttransport model. The barotropic model includes the two main tidalconstituents,M2 and S2. Thus, the hydrodynamic equations are solvedfor each constituent and tidal analysis is also carried out for each

    constituent separately.The long-term circulation is obtained from a full 3D, primitiveequation, baroclinic hydrodynamic model. It is based upon thehydrostatic and Boussinesq approximations on a plane. The modelincludes equations for salinity and temperature evolution and waterdensity is calculated from them using a standard state equation(Batteen et al., 2000). The one-equation turbulencemodel described inPeriez (2005b) has been used to calculate the vertical eddy viscosity.Details on the 3D equationsmay be seen, for instance, in Kowalick andMurty (1993). A summary of the main equations involved in bothhydrodynamic models is presented in Appendix A.

    2.2. Sediment transport

    The transport of sediments is described by a 3D advectiondiffusionequation to which some terms are added. These are external sources ofparticles (river supply), terms describing particle deposition on theseabed and erosion from the bed to the water column, and verticalsettling. The formulation of these processes is based upon standardformulae. Thus, the erodability constant is used for the erosion term.Particle settling and deposition are described using the settling velocity,which isobtained fromStoke's law.Critical erosionanddepositionstressesare used as usual. Details on the mathematical formulation may be seenelsewhere (Periez, 2005b; Liu et al., 2002a; Lumborg and Windelin,2003; Cancino and Neves,1999). Finally, it is also possible to calculate netsedimentation rates (SR) as the balance between the deposition anderosion terms. A summary of the equations may be seen in Appendix B.

    2.3. Metal transport

    Non conservative pollutants are those which do not remaindissolved in the water column, but have a certain afnity to be xedto particles (suspendedmatter and bed sediments). Thus, three phasesare included in the model: dissolved, suspended particles and bedsediments. Only ne sediments are considered (particles withdiameter b62.5 m) since metals are predominantly xed to these(Eisma, 1993). The exchanges between the dissolved and solid phasesmay be described in terms of kinetic transfer coefcients. Thus,assuming that adsorption/release reactions are governed by a singlereversible reaction, a coefcient k1 governs the transfer from the liquidto the solid phase and a coefcient k2 governs the inverse process. Also,the migration of metals to the deep sediment is included. Thus, metalsdeposited on the sediment surfacewill be buried byparticle depositionand will migrate below the mixed sediment layer which directlyinteractswith the dissolvedphase. This effectmaybe easily treated as adecay process with constant burial given by (Periez, 2008):

    burial =SRsL

    1

    where L is the sediment mixing depth (the distance to which thedissolved phase penetrates the sediment), s is the sediment bulkdensity (dry mass divided by wet volume) and SR is the sedimenta-tion rate provided by the sediment transport sub-model.

    It is known that adsorption depends on the surface of particles perwater volume unit at each point and time. This quantity has beendenoted as the exchange surface (Periez, 1999, 2000, 2002). Thus,the kinetic coefcient k1 is written as:

    k1 = Sm + Ssb = km1 + ks1 2

    where Sm and Ss are the exchange surfaces for suspended matter andbottom sediments respectively (dimensions [L]1) and is a parameterwith the dimensions of a velocity denoted as the exchange velocity(Periez, 1999, 2000, 2002). The delta function is introduced to takeinto account that only the deepest water layer interacts with the bed

    sediment. Thus b=1 for the deepest layer and b=0 elsewhere.

  • 3395R. Periez / Science of the Total Environment 407 (2009) 33923406Assuming spherical particles, the exchange surfaces are written as(see references cited above):

    Sm =3mR

    3

    and

    Ss =3Lf 1 p

    R4

    where R and are particle radius and density respectively, m is thesuspended matter concentration, f is the fraction of ne particles inthe sediment, p is sediment porosity and is a correction factor thattakes into account that part of the sediment particle surface may behidden by other sediment particles. Finally, is the thickness of thedeepest water layer. This formulation has been successfully used in allmodelling works cited above. Real particles are not spheres, but withthis approach it is possible to obtain an analytical expression for theexchange surface (Duursma and Carroll, 1996). The kinetic coefcientk2 is considered to be constant.

    The equation that gives the time evolution of metal concentrationin the dissolved phase, Cd, is:

    @Cd@t

    + u@Cd@x

    + v@Cd@y

    + w@Cd@z

    = A@2Cd@x2

    +@2Cd@y2

    !+

    @

    @zK@Cd@z

    k1Cd

    + k2mCs + bk2LsfAs

    5

    where Cs and As are, respectively, the concentrations of metals insuspendedmatter and bottom sediments. u, v andw arewater velocitiesalong the x, y and z axis and A and K are, respectively, the horizontal andvertical diffusion coefcients.

    The equation that gives the time evolution of metal concentrationin suspended matter is:

    @ mCs @t

    + u@ mCs

    @x+ v

    @ mCs @y

    + w ws @ mCs

    @z= A

    @2 mCs @x2

    +@2 mCs @y2

    !

    +@

    @zK@ mCs

    @z

    + km1 Cd k2mCs bSED 6

    where ws is the particle settling velocity and SED is the deposition ofmetals from the deepest water layer to the sediment evaluatedaccording to:

    SED = SRCs b

    : 7

    Note that (b) means that the corresponding magnitude isevaluated at the deepest water layer.

    The equation for the temporal evolution of metal concentration inthe bottom sediment mixed layer is:

    @As@t

    = ks1Cd b Lsf

    k2As burialAs + SED 8

    where the deposition is now calculated as:

    SED = SRCs...

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