Particulate metal distribution in Guadiana estuary punctuated by flood episodes

Estuarine, Coastal and Shelf Science 70 (2006) 109e116www.elsevier.com/locate/ecss

Particulate metal distribution in Guadiana estuarypunctuated by flood episodes

Miguel Caetano*, Carlos Vale, Manuela Falc~ao

National Institute for Agronomy and Fisheries Research e IPIMAR, Av. Brasılia, 1449-006 Lisbon, Portugal

Received 3 September 2004; accepted 28 September 2005

Available online 8 August 2006

Abstract

The distribution of major (Al, Ca, Mg, Fe and Mn) and trace metals (Zn, Cu, Pb and Cd) in particulate material was determined in differentflow and tidal conditions along the Guadiana estuary. Under moderate river flows, concentrations of Al in SPM decreased seawards, while Caand Mg showed an opposite trend reflecting the physical mixing of fluvial and marine particles along the salinity gradient. Ratios of Cu/Al,Zn/Al, Pb/Al and Cd/Al increased sharply in the estuary mouth as a result of local inputs of urban sewage from the two major cities locatedat the estuary mouth. Under these flow conditions Cu/Al and Cd/Al ratios were higher in spring, indicating that retention by phytoplanktonwas the major factor influencing these metal gradients in SPM. Sediments transported during an exceptional flood contained higher inorganicfraction and lower metal content (two orders of magnitude) than particles transported under moderate river flows. The material exported duringflood peaks was recorded in the Guadiana shelf as coarser sediment layer with diminished levels of Fe, Cu and Pb. In spite of metal mobility inupper sediment layers of the coastal zone, these metals reflect the export episodes in Guadiana. The construction of Alqueva dam will attenuatein the future these flood events decreasing the amount of sediment supplied by the river to the coastal zone.� 2006 Elsevier Ltd. All rights reserved.

Keywords: suspended matter; trace metals; floods; seasonal variability; Guadiana estuary

1. Introduction

Most estuaries trap suspended matter transported by riversto the adjacent coastal waters (Turner and Millward, 2002).Chemical composition of suspended particulate material in es-tuaries is influenced by physical mixing, coupled with biogeo-chemical processes existing in transitional waters: mixing ofriverine suspended matter and particles of marine origin (Nolt-ing et al., 1990); resuspension of sediments (Turner et al.,1991); mobilization of Fe and Mn in reducing sediments(Feely et al., 1986); flocculation of colloidal material (Turnerand Millward, 2002); sorption in low salinity and high turbid-ity zones (Gobeil et al., 1981); production of organic matter(Collier and Edmond, 1984) and industrial and urban waste

* Corresponding author.

E-mail address: [email protected] (M. Caetano).

0272-7714/$ - see front matter � 2006 Elsevier Ltd. All rights reserved.

doi:10.1016/j.ecss.2006.05.030

water discharges (Nolting et al., 1999). However, during epi-sodic events of pronounced freshwater discharges, large quan-tities of water and of freshly eroded material, eventuallycontaining mobilised contaminants retained in soil, may enterthe estuary (Makepeace et al., 1995). Abrupt changes on avail-ability of contaminants to filter feeders during these episodeshave been reported (Castro et al., 1990; Vale et al., 1993).

The Guadiana River estuary is a single-channel meso-tidalestuary of 76 km long, 70 to 800 m wide and 5 to 15 m depth.The bottom consists of coarse material in the middle of thechannel and finer particles near the margins (Fortunatoet al., 2002). The Guadiana River is the main freshwater inputwith an irregular flow on both seasonal and inter-annual timescales (Morales, 1997; Gonzalez et al., 2004). In general, flowsare higher between December and March (200e600 m3 s�1)and the period of low flows (0.1e20 m3 s�1) is prolongedfor several months during the year (Cravo et al., 2003; Dom-ingues et al., 2005). The river crosses extensive rural area of

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110 M. Caetano et al. / Estuarine, Coastal and Shelf Science 70 (2006) 109e116

massive sulphide deposits, the Iberian Pyrite Belt, with intensemining extraction since Roman Age (Palanques et al., 1995;Leistel et al., 1998). Pyrite extraction has ceased in the last de-cade. Besides this influence the estuary receives the domesticsewages of two cities located near the mouth (Chicharo et al.,2001; Domingues et al., 2005). Since 2002, more than 80% ofthe freshwater flow is regulated by the Alqueva dam con-structed 140 km upstream the estuarine mouth, minimisingthe abrupt river discharges to the estuary. A considerable riverflood occurred in January 2001, providing the last opportunityto examine the impact of flood events on the Guadiana system.This paper describes the elemental composition of suspendedparticulate matter transported under contrasting river flows, in-cluding storm water runoff periods, as well as the historical re-cord in deposited sediments of the coastal area.

2. Materials and methods

2.1. Sampling

Three campaigns were carried out during 2001 in theGuadiana estuary (Fig. 1): during an exceptional river flood

A

B

C

D

E

F

G

H

Po

rtu

gal

Sp

ain

AtlanticOcean

Portugal

Spain

S

Fig. 1. Location of the sampling stations in the Guadiana estuary and adjacent

coastal area.

in January (max. flow of 1200 m3 s�1); under moderate riverflow (October, 20e248 m3 s�1); and under low flows (May,42e72 m3 s�1). Coarser material transported by the floodand settled in the upper estuary (site A), fine particles depos-ited on the margins during the decline phase of the flood (sitesA to G) were collected with a plastic spatula. Surface waterwas also sampled during this flood period. Surface and bottomwater was collected at low and high tide of two semidiurnaltidal cycles in May and October at eight sites (A to H). InMay, tidal amplitude ranges from 2.1 to 3.1 m and in Octoberfrom 1.1 m to 3.3 m. Water samples were collected with de-contaminated Niskin bottles, 0.5 m below the surface and0.5 m above the bottom. Suspended particulate matter was ob-tained by filtration of the collected water through 0.45 mm poly-carbonate membranes and retained particles dried in a cleanroom. Four sediment cores were collected at site S in the frontof the estuary mouth (20-m depth) using a multi-corer Midi-corer Mark II-400 equipped with transparent tubes. Visual ob-servation of the cores evidences the presence of a thicker sandlayer near the surface. One of the cores was selected for analy-sis and sliced onboard in layers of 2e3 cm thickness. Sam-ples were freeze-dried, ground and homogenised with anagate mortar.

2.2. Analytical methods

Sediment samples were weighted and transferred into Teflonbombs, followed by fluoridic acid, Aqua Regia (HCl:HNO3,3:1) and digested at 100 �C in a oven for 1 h. Subsequently,the bombs content was poured into volumetric flasks containingboric acid and filled up with Milli-Q water (Rantala and Loring,1975, 1977). Aluminium, Ca and Mg were analysed by flameatomic absorption spectrometry (Perkin Elmer AA100) witha nitrous oxide-acetylene flame and Fe, Mn, and Zn with anair-acetylene flame. Metal concentrations were determinedwith the method of standard additions. Copper, Pb and Cdwere analysed with a transversely heated graphite furnaceatomic absorption spectrometer (Perkin Elmer 4110 ZL) withZeeman background correction and concentrations determinedwith the method of standard additions. The detection limits ofAl, Ca, Mg, Fe, Mn, Zn, Cu, Pb and Cd were 0.11, 0.14, 0.17,0.14%, 12, 14, 3.7, 1.3 and 0.012 mg g�1, respectively. The pre-cision expressed as relative standard deviation was 2% for Al,Ca, Mg, Fe, Mn and Zn and 3% for Cu, Pb and Cd. Internationalcertified standards were used to ensure the accuracy and preci-sion was determined by analysing replicate samples. Resultsshowed that obtained and certified values (Table 1) were not sta-tistically different ( p < 0.01).

Salinity, dissolved oxygen and pH were measured in situwith a multiparametric Yellow Spring probe and suspendedparticulate matter concentrations were determined gravimetri-cally. Organic carbon (Corg) in SPM and sediments was ana-lysed in a Carlo Erba Elemental Analyser, before and afterheating the samples at 450 �C (Byers et al., 1978). Chlorophylla was extracted with acetone (90%) and concentrations weredetermined by fluorimetry according to the method describedby Lorenzen (1967).

111M. Caetano et al. / Estuarine, Coastal and Shelf Science 70 (2006) 109e116

3. Results and discussion

3.1. Salinity

The exceptional flood resulted in low salinity (3) all overthe estuarine zone. This contrasts with conditions observed un-der moderate river flows where the upper limit of seawater in-trusion was at site C (salinity <0.3), 30 km upstream of theestuarine mouth (Fig. 2), and the physical tidal influence ex-tends to 60 km. Salinity gradients were similar in the springtides of May and October and showed pronounced differencesbetween low and high tide, isohalines migrating around 10 kmup and downstream on a semi-diurnal time scale. The longitu-dinal fluctuation was smaller during the neap tides. The salin-ity fluctuation extends the flocculation, precipitation andcomplexation mechanisms, and consequently the equilibriumof trace metals between the dissolved and particulate fraction,to broader area in the upper estuary. Moreover, plankton diver-sity and abundance are sensitive to salinity changes advectingpopulations up- and downriver (Chicharo et al., 1999, 2001).

3.2. Suspended particulate matter concentration

Under moderate river flow conditions, the tidal freshwaterzone accounts to 50% of the estuarine length. The suspended

Table 1

Certified and measured average concentrations of Al, Ca, Mg, Fe, Mn, Zn, Cu,

Cd and Pb and standard deviations in the international reference materials

(BCSS-1 and MESS-1)

MESS-1 BCSS-1

Certified Obtained Certified Obtained

Al (%) 5.84 � 0.20 5.88 � 0.04 6.26 � 0.22 6.38 � 0.10

Ca (%) 0.48 � 0.05 0.59 � 0.18 0.54 � 0.05 0.50 � 0.01

Mg (%) 0.87 � 0.05 0.87 � 0.02 1.47 � 0.14 1.45 � 0.03

Fe (%) 3.05 � 0.17 3.07 � 0.02 3.29 � 0.10 3.35 � 0.05

Mn (mg g�1) 513 � 25 517 � 4 229 � 15 233 � 4

Zn (mg g�1) 191 � 17 182 � 9 119 � 12 114 � 10

Cu (mg g�1) 25.1 � 3.8 22 � 3.6 18.5 � 2.7 19 � 3.3

Cd (mg g�1) 0.59 � 0.10 0.66 � 0.16 0.25 � 0.04 0.36 � 0.10

Pb (mg g�1) 34 � 6.1 32 � 5.4 22.7 � 3.4 22 � 3.3

0

10

20

30

40

0 10 20 30 40 50 60 70Distance (km)

Salin

ity

EHF

LTs

LTb

HTs

HTb

SeaRiver

Fig. 2. Salinity distribution along the Guadiana estuary between site A (river

end-member) and H (estuary mouth) in January and May. HT-high tide; LT-

low tide; s-surface; b-bottom; EHF-extremely high flows.

particulate matter (SPM) in fluvial waters varied two orders ofmagnitude (7.6e166 mg L�1), while in the lower estuary a nar-rower range was recorded (1.2e22 mg L�1). In order to betterrepresent the longitudinal distribution of SPM concentration inthose two estuarine zones (Fig. 3), concentrations are depictedagainst distance (km) in the upstream tidal-influenced riverand versus salinity downstream of site C (that showedsalinity below 0.3 at all the observed tidal conditions). Nosignificant differences ( p < 0.05) were observed in SPM con-centrations between surface and bottom waters. The highervariability in fluvial waters may be attributed to changes incurrent velocities (Nolting et al., 1999). Suspended particlesretained in the upper estuary resulted in a turbidity maximumzone whose intensity increased with the tidal range, as ob-served in other estuaries (Vale and Sundby, 1987). HigherSPM levels in October when the river flow increased due toshort periods of heavy rain may results from erosion of deposi-ted material. During the elevated discharges in January SPMconcentrations in surface waters along the estuary ranged be-tween 53 and 167 mg L�1 (not shown in Fig. 3).

3.3. Oxygen, pH and chlorophyll a

The dissolved oxygen concentration varied randomly be-tween 5.6 and 10.7 mg L�1, which is indicative of relativelywell oxygenated estuarine waters (Fig. 3). Values of pH werelower in fluvial waters (7.3e7.5; site A and B) and increased ex-ponentially ( y ¼ 0.23Ln(x) þ 7.7; r2 ¼ 0.96; p < 0.001) to-wards the estuary mouth (Fig. 3) as result of seawater mixingwith more acidic fluvial waters. Suspended particulate mattercontained higher chlorophyll a in May (Fig. 3) suggesting thepresence of spring blooms. Previous works showed the occur-rence of diatom spring blooms followed by increased abundanceof green algae in Guadiana upper estuary (Chicharo et al., 2001;Rocha et al., 2002; Domingues et al., 2005). Chlorophyll a con-centrations were higher in fluvial waters decreasing sharply to-ward the estuary mouth, as freshwater phytoplankton declineswith the increase of salinity (Miguel et al., 1998). Consequently,particles in the river and upper estuary were up to 2 times en-riched in organic carbon.

3.4. Moderate flows: elemental composition of SPM

The concentrations of Al, Ca, Mg, Fe and Mn in SPM col-lected in May and October surveys were also presented againstdistance to site C and versus salinity in brackish waters(Fig. 4). Surface and bottom SPM showed no significant dif-ferences of major elemental composition. Broad range of Alconcentrations (May: 3.2e7.6%; October: 1.1e11.3%) indi-cates particles with different organic and lithogenic fractions.Aluminium decreased seaward, reaching levels similar tothose observed in the adjacent coastal zone (Caetano andVale, 2003; Ferreira, 2002). Similar decreases in the St. Law-rence estuary (Gobeil et al., 1981) and the Scheldt estuary(Zwolsman and van Eck, 1999) were interpreted as settlingof coarser particles as intensity of tidal currents decreases.The lithogenic fraction of the suspended load was lower in


0

30

60

90

120

150

180

-30 -20 -10 0 10 20 30 40

-30 -20 -10 0 10 20 30 40

-30 -20 -10 0 10 20 30 40

-30 -20 -10 0 10 20 30 40

-30 -20 -10 0 10 20 30 40

SP

M (m

g L

-1)

May 2001Oct. 2001

May 2001Oct. 2001

7.2

7.4

7.6

7.8

8

8.2

8.4

8.6

pH

0

5

10

15

20

25

Co

rg (%

)

0

1

2

3

4

5

Ch

lo

a

(µ

g L

-1)

Distance (km) Salinity


4

6

8

10

12

DO

(m

g L

-1)

Fig. 3. Longitudinal distributions of suspended matter (SPM) concentration, dissolved oxygen (DO), pH, organic carbon (Corg) and chlorophyll a against distance

(km) in the upstream tidal-influenced river and versus salinity downstream of site C (salinity <0.3); surface and bottom at low and high tide of neap and spring

tides, in May (-) and October (6).

May due to the abundant phytoplankton, while in October theinput of fine-grained material associated with the rainenhanced the lithogenic fraction in the turbidity zone.

The Ca concentrations in fluvial particles varied withina narrow range (0.6e1.8%) and were higher in the upper limitof salinity. This enhancement is in line with high quantities ofzooplankton (foraminifers and ostracods) that have been ob-served in the upper estuary (Amarillo et al., 1997). Broaderrange of Ca levels in the estuarine mouth may result fromthe mixing of marine particles as reported in several studies(e.g. Nolting et al., 1990). Both the seaward increase of Mgand decrease of Mn also appear to reflect the physical mixing.Particles showed higher Fe content in October (2.3e13%) thanin May (2.2e5.5%). The enhancement, which is not correlatedto Al, may result from the presence of particles derived fromeroded Fe-rich soil (Leistel et al., 1998) during the shortperiod of rain.

3.5. Moderate flows: trace elements in SPM

Trace element concentrations were also normalised to Alcontent in order to minimise differences associated with

particle nature (Windom et al., 1989). The longitudinal distri-butions of element/Al ratios were presented against distanceto site C and versus salinity (Fig. 5). The Zn/Al and Cu/Al ra-tios were relatively uniform until salinity 27 in comparison tothe broader ranges found in the estuary mouth (max. of 1337for Zn/Al and 305 for Cu/Al). The ratios of Pb/Al showedalso peak values in this zone but less pronounced. The enrich-ment of particulate metals in the estuary mouth is probably re-lated to the input domestic sewage from two cities located inthis part of the estuary. This anthropogenic signal was betterobserved in October (Zn and Cd). Ratios of Cu/Al and Cd/Al were one or two orders of magnitude higher than inSPM from adjacent waters (Caetano and Vale, 2003). Thecontribution of metal enriched particles to the adjacent Guadi-ana shelf appears thus to be small under these river flowconditions.

The Cu/Al and Cd/Al ratios were higher in May than inOctober indicating that biogenic material, which was moreabundant in spring, is an important factor influencing Cuand Cd concentrations in Guadiana estuary. These resultsare in line with findings in other environments (e.g. Collierand Edmond, 1984; Monteny et al., 1993; Gonzalez-Davila,


0

4

8

12

-30 -20 -10 0 10 20 30 40 -30 -20 -10 0 10 20 30 40

-30 -20 -10 0 10 20 30 40

-30 -20 -10 0 10 20 30 40

-30 -20 -10 0 10 20 30 40

Al (%

)

May 2001Oct. 2001

May 2001Oct. 2001

0

2

4

6

8

Ca

(%

)

0

0.5

1

1.5

2

Mg

(%

)

0

3

6

9

12

15

Fe (

%)



0

1000

2000

3000

4000

5000

Mn

(µ

g g

-1)

Fig. 4. Concentrations of Al, Ca, Mg, Fe and Mn in suspended particulate matter against distance (km) in the upstream tidal-influenced river and versus salinity

downstream of site C (salinity <0.3); surface and bottom at low and high tide of neap and spring tides, in May (-) and October (6).

0

500

1000

1500

-30 -20 -10 10 20 30 40

Zn

.10

-4 /A

l

0

100

200

300

400

-30 -20 -10 10 20 30 40

Cu

.10

-4 /A

l

May 2001Oct. 2001

May 2001Oct. 2001

0

30

60

90

-30 -20 -10 10 20 30 40

Pb

.10

-4 /A

l


0

0.2

0.4

0.6

-30 -20 -10 10 20 30 40

Cd

.10

-4 /A

l


0 0

00

Fig. 5. Ratios of Zn/Al, Cu/Al, Pb/Al and Cd/Al in suspended particulate matter against distance (km) in the upstream tidal-influenced river and versus salinity

downstream of site C (salinity <0.3); surface and bottom at low and high tide of neap and spring tides, in May (-) and October (6).


1995). The phytoplankton effect did not appear to be so im-portant for Zn since Zn/Al ratios along the estuary were sim-ilar in the two surveys (Fig. 5). The relatively constancy ofparticulate Zn in salinity gradients has already been reportedin the Gironde, France (Kraepiel et al., 1997), Paraiba do Sul,Brasil (Carvalho et al., 1999) and Port Jackson, USA (Hatjeet al., 2001). The longitudinal distribution of Pb/Al ratios wasdifferent, fluvial particles containing lower Pb (Pb/Al be-tween 4.0 and 12 � 10�4) than particles of the transitionzone. Besides adsorption being facilitated by the pH increase,contamination may also contribute to this enhancement of Pb.

3.6. Extremely high flows: trace elements inparticulate material

According to Morales (1997), sands transported duringmaximum instantaneous river discharges tends to settle inthe upper estuary, around 60 km upstream the estuary mouth.After the flood of January 2001 it was also found large quan-tities of sand on the margins of this area. Elemental composi-tion and trace-element/Al ratios of this sand, of fine materialdeposited on the margins after the flood peak, and of sus-pended particulate matter (SPM) are presented in Table 2.Sands contained higher inorganic fraction and lower tracemetal content than fine deposits and SPM. During this flood,fresh water fills the entire estuarine zone (Ferreira et al.,2003) and resident time of water and SPM is very short. Theseconditions do not favour chemical alterations of the particlesin transit and explain the low spatial variation on the elementalcomposition of both, SPM and fine material deposited on themargins. Trace metal content in materials transported duringthe flood was up to one order of magnitude lower than valuesin SPM under moderate flows. This difference agrees with theabsence of industrial activities in the drainage basin and con-trasts with the findings in other urbanised river basins (Vale,1990; Pohl et al., 2002). The flood event masks the signal oflocal anthropogenic sources at the estuary mouth detected un-der moderate river flows in the SPM.

3.7. Chemical composition of coastal sediments

The vertical distributions of Al, Fe, Mn, Cu, Pb, Zn and Cdin a sediment core collected in the Guadiana shelf after theflood of January 2001 are presented in Fig. 6. Although the sed-iment core was not dated, the presence of a sharp minimum inthe upper layers and several depth irregularities on the elemen-tal composition suggest historical records of the river flood.

The two minima of Al concentrations at 4e8 and 12e13 cm in-dicate the presence of two sediment layers consisting of coarsermaterial. Morales (1997) proposed that most of eroded materialduring floods settle inside the fluvial-estuarine system, but thevertical profile of Al, as well as of Si/Al (Ferreira et al., 2003),point to the episodic export of coarser sediments and depositionin front of the estuary. Since the sediment core was collectedafter the river flood in January 2001, the 4e8 cm layer maybe related to this flood event. In spite of the eastward long-shore current (Morales, 1997; Gonzalez et al., 2004) fine mate-rial transported under moderate flow conditions also settles inthis site, as suggested by the higher metal/Al ratios observedin deeper sediment layers (Fig. 6).

Classic works (Aller, 1977; Gobeil and Silverberg, 1989)have demonstrated that Fe, Cu and Pb are highly mobile in up-per sediments due to intense organic matter degradation andassociated reactions. Despite this mobility pronounced minimaof concentrations and ratios to Al were registered at 4e8 cmfor those elements. These changes corroborate the depositionof sediments with lower metal content exported during theflood (Fig. 6). Above this layer, vertical profiles of Cu/Aland Pb/Al ratios showed higher values suggesting that Cuand Pb associated with the recent settled particles havea slow remobilization from the solid fraction of the sediment(Shaw et al., 1990). Otherwise Zn/Al and Cd/Al ratios showedno enhancement at the upper sediment layer. Two possible ex-planations may be invoked: concentrations in the flood mate-rial were not such different from the levels existing incoastal sediments; and faster remobilization of these elementsnear the sediment-water interface than Cu and Pb. The broadmaximum of Cu, Pb, Zn and Cd ratios to Al in deeper sedi-ments may be interpreted as internal mobilization of the reac-tive fractions, diffusion and precipitation as metal sulphides(Morse, 1994). The increase of Fe and Mn concentrationsabove the coarser layer may reflect the rapid mobility of theirreduced forms and subsequent precipitation as oxides near thesediment surface (Burdige, 1993).

4. Conclusions

The results of this work point to the importance of flood onthe transport of material to the estuary and adjacent coastalzone with lower metal content. This regime will change, sincethe presence of the Alqueva dam in the river eliminates orminimises such drastic inputs of low contaminated material.Local sources of contamination will influence permanentlythe composition of suspended particulate matter and sedimentsin the Guadiana estuary. Chemical composition of adjacent

Table 2

Concentrations of Al, Ca, Mg, Fe, (%) and Mn (mg g�1) and ratios of Zn/Al, Cd/Al, Cu/Al and Pb/Al in coarser material, settling particles and SPM in the Guadi-

ana during the flood event

Corg Al Ca Mg Fe Mn Zn/Al Cu/Al Pb/Al Cd/Al

(%) (mg g�1)

Flood peak material 0.43 5.4 1.8 0.41 3.3 740 7.4 0.50 0.72 0.028

Deposited sediments 2.1 � 0.4 7.2 � 0.6 0.15 � 0.04 0.13 � 0.01 4.3 � 0.02 1213 � 23 14 � 1 1.2 � 0.1 1.2 � 0.2 0.059 � 0.009

SPM 2.1 � 0.4 12 � 1 0.31 � 0.06 1.0 � 0.04 5.7 � 0.03 895 � 29 15 � 5 3.7 � 0.4 3.5 � 0.3 0.028 � 0.008


0

10

20

30

40

10 20 30 40 50Zn.10

-4/Al

Dep

th

(cm

)

0

10

20

30

40

3 9 12Cu.10

-4/Al

Dep

th

(cm

)

0 0.05 0.1 0.15Cd.10

-4/Al

2 8 10Pb.10

-4/Al

1 5Fe (%)

0.0 0.2 0.4 0.6Fe/Al

0

10

20

30

40

4 8 10 12Al (%)

Dep

th

(cm

)

500 1000 1500 2000 2500Mn (µg g

-1)

Mn/Al

0 60 120 180

6 3 7

646

Fig. 6. Depth profiles Al, Fe (%), Fe/Al, Mn (mg g�1), Mn/Al, Zn/Al, Cd/Al, Cu/Al and Pb/Al ratios in the sediment core from the Guadiana shelf. Black the

coarser sediment layers.

coastal sediments will result from a combination of anthropo-genic sources, particle mixing driven by longshore currentsand diagenetic processes.

Acknowledgments

The authors wish to thank the colleagues Joana Raimundo,Nuno Fonseca, Miguel Nuno, Pedro Brito, Ana Ferreira, Jo~aoCanario, Marta Martins, Milu, Regina Granja, Cristina Martinsfor the field work and technical assistance.

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Particulate metal distribution in Guadiana estuary punctuated by flood episodes

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