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Fifty-year sedimentary record of heavy metal pollution in the lagoon of Oualidia (Moroccan Atlantic coast)
Estuarine, Coastal and Shelf Science 72 (2007) 359e369 www.elsevier.com/locate/ecss
Fifty-year sedimentary record of heavy metal pollution in the lagoon of Oualidia (Moroccan Atlantic coast) B. Zourarah a,*, M. Maanan b, C. Carruesco c, A. Aajjane a, K. Mehdi a, M. Conceic¸~ao Freitas d a
Department of Geology, Faculty of Sciences, El Jadida, Morocco b UMR 6554 LETGeGe´olittomer, University of Nantes, France c Department of Geology and Oceanography, Bordeaux I University, Talence, France d Centro Departamento de Geologia da Universidade de Lisboa, Portugal Received 10 April 2006; accepted 20 November 2006 Available online 8 January 2007
Abstract The Oualidia lagoon is known for its heavy metal pollution resulting from mining and smelting activities since the late 19th century. Here, we report 137Cs and 210Pb activities and heavy metal concentration depth profiles from sediment cores retrieved in 1997. High mean sedimentation rates of 0.6e1 cm/y are indicated by 210Pb and 137Cs dating. The lagoon sediments have recorded heavy metal deposition and thus allow establishment of a connection between the temporal evolution of the heavy metal pollution and historical changes in smelting and waste-treatment proceedings. Through a study of the evolution of heavy metal contents, we can distinguish between two categories of metals: 1- Al, Fe and Cr contents have varied around a mean value over the last seven decades and could have natural origins. 2- Pb, Zn, Cu and Hg have relatively more elevated contents than those of the natural geochemical background, especially in the station in front of illegal sewerage discharges. These metals are enriched at depths between 20 and 30 cm; this corresponds to the period between 1960 and 1975, which was characterized by the setting up of the main suburbs and the first aquaculture farms that surround the lagoon. The pollution intensity of the lagoon is determined by enrichment factors and the geo-accumulation index, which show that the lagoon of Oualidia is unpolluted to moderately polluted on the Geo-I scale of Mu¨ler (1979. Schwermetalle in den Sedimenten, des Rheins-Vera¨nderungen seit 1971. Umschau 79 (24), 778e783.). Ó 2006 Elsevier Ltd. All rights reserved. Keywords: Morocco; lagoon; heavy metals; pollution; sediment; cesium
1. Introduction Coastal environments are under increasing pressure as a result of increasing human population. In fact coastal areas, including estuaries, bays, shorelines, lagoons and continental shelves, are used intensively and receive the by-products of inland human activities, mainly via rivers. The human impact on these productive and economically important environments has become a major concern (Cearetta et al., 2000).
* Corresponding author. E-mail address: [email protected] (B. Zourarah). 0272-7714/$ - see front matter Ó 2006 Elsevier Ltd. All rights reserved. doi:10.1016/j.ecss.2006.11.007
Over the past century, heavy metals have been discharged into the world’s coastal environments as a result of the rapid industrial development (Lee and Cundy, 2001; Bellucci et al., 2002; Munksgaard et al., 2003; Spencer et al., 2003; Maanan et al., 2004). The delivery processes are closely linked to those of fine-grained suspended sediments, acting as effective carriers, onto which heavy metals are bound. 210 Pb (t1/2 ¼ 22.3 years) provides a reliable method of dating sediments deposited over the last 100e150 years. 210Pb is found in sediment minerals first in a supported form by in situ production from 238U or 226Ra decay, at equal activity to its parent nuclides, which is normally assumed to be constant down a core profile. The second fraction, the unsupported or
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excess 210Pb (210Pbxs) has an atmospheric origin, being produced by decay of 222Rn which emanates from soils after the decay of 226Ra. Atmospheric 210Pb enters the aquatic environments by direct deposition in rain and also by runoff and erosion from catchment areas. Geochronology with 210Pb is based on the pattern of radioactive decay of 210Pbxs with depth in the sedimentary column, and the exponential law describes this decay through time as follows: 210
PbðzÞ ¼ 210 Pbð0Þ elt
ð1Þ
pollution studies in the coastal lagoon, where these techniques have been scarcely used. It summarizes the investigations on the origin and temporal changes in trace metal inputs reflected in the sedimentary record of a coastal lagoon. 2. Methods 2.1. Study site Oualidia lagoon (32 400 4200 Ne32 470 0700 N and 8 520 3000 We 9 020 5000 W) is located on the Atlantic Ocean (Fig. 1). This lagoon is 7 km long, on average 0.5 km wide, and exchanges water with the ocean through a major inlet about 150 m wide and 2 m deep. During spring tides there is also a secondary, shallower inlet about 50 m wide. An internal delta with a surface area of about 0.2 km2 is normally found close to the inlet. The lagoon morphology is characterized by side channels, connected to a meandering main channel, in which the mean depth is 2 m and the maximum depth during flood tides does not exceed 5 m (Bidet and Carruesco, 1982). Intertidal areas on both sides of the channels occupy about 53% (1.6 km2) of the 3-km2 surface area of the lagoon at low tide. Flood tides cover more than 75% (2.25 km2) of the lagoon surface, bringing salt water up to the upstream reaches of the lagoon and into a saline marsh beyond the second dam. The dam or causeway, built fairly recently to facilitate the crossing of the lagoon, includes a breach of about 10 m wide and 2 m deep, which allows some water exchange at high water levels. In geomorphologic terms, it belongs to an occidental coastal basin called the Sahel. This basin is made of the morphology of a depression limited by a Plio-Quaternary continental dead cliff and a coastal consolidated dune ridge in a southwestenortheast direction. It is constituted mainly of yellow detritus limestone, formed of shelly sands. This morphology is the result of the post-Ouljienne regression (Bidet and Carruesco, 1982). Rainfall over the region accounts for only 1% of the fresh water entering the lagoon. The annual average rainfall, estimated from 1977 to 1998, is about 390 mm, with a maximum in December and no rain during the dry period. The annual estimate of evaporation minus precipitation is 650 mm. The predominant wind directions are WSW to NW during the wet season and NNE to NE during the dry season. Sporadic violent winds (Chergui) occasionally blow from the ENE during the dry period and may contribute to high evaporation rates over the lagoon, as well as to extreme air temperatures, which can reach up to 40 C. More generally, winds blowing from the northern sector will produce southerly geotropic currents along the coast, offshore transport and coastal upwelling of nutrient rich deep waters close to the coast. Upwelled waters with high nutrient content can be advected by flood tides into the lagoon, supporting biological production and enhancing aquaculture yields. The major aquaculture activity in the lagoon is oyster culture. To date, five oyster farms occupy one-sixth of the lagoon surface. In addition, during summer Oualidia lagoon is the site of intense tourism activities. Discharging farm and domestic sewage water without treatment can cause increased heavy metal
where 210Pb(0) is the surface activity, 210Pb(z) is the activity at depth z, and l is the decay constant (0.03114 y1). This equation assumes a constant supply of 210Pb to the sediments, as well as a negligible post-depositional migration of 210Pb within the column. However, in areas where the 210Pb distribution is open to alternate interpretations, the use of two or more radionuclides, with different half-lives and different input histories, allows a better characterization of sediment reworking than it is possible with just one tracer (Legeleux et al., 1994). In the C.F.C.S.R. model (Constant Flux and Constant Sedimentation Rate), the sedimentation rate (R) can be expressed as: R ¼ z=t
ð2Þ
Then, Eq. (1) becomes, 210
PbxsðzÞ ¼ 210 Pbxsð0Þ eðlz=RÞ
Consequently, ln 210 PbxsðzÞ ¼ ln 210 Pbxsð0Þ ðlz=RÞ
ð3Þ
ð4Þ
Hence, a plot of ln 210Pbxs as a function of depth should be linear with a gradient of l/R. The 210Pbxs activity is always constant in the mixed layer, which has a thickness of several centimetres, and the decrease only starts below the core. The combination of 210Pb with 137Cs is often used to reduce the uncertainty in dating (Benninger et al., 1979). Typically, 210 Pb dating of river sediment has shown good results for rivers draining a small watershed (Farmer et al., 1997). 137 Cesium (t1/2 ¼ 30.2 years) found in marine and fluvial sediments is an anthropogenic radionuclide originated from above-ground nuclear testing carried out in 1945e1972, but 90% of the radionuclide releases occurred between 1963 and 1964 (Callender and Robbins, 1993). 137Cesium has been used as an indicator for the amount of global fallout deposited in soil and reservoir sediments. The use of 137Cs as an indicator of sedimentary processes is consistent as it binds almost irreversibly to clay and silt particles (Poinssot et al., 1999) and because of its long half-life (30.2 years). Moreover, the Chernobyl nuclear accident of 1986 has been recorded in European sediments. Thus, 137Cs activity depth profiles are often used for sediment core dating (Grousset et al., 1999). This paper reports on trace metal pollution based on preliminary investigation. The aim of this paper is to demonstrate the usefulness of radioactive and stable isotopic tracers for
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Fig. 1. Schematic map showing the superficial sample and core locations in the Oualidia lagoon.
Table 1 Limit of detection, uncertainties and contents of heavy metals certified and that detected in reference materials (n ¼ 5) (mean standard deviation). PACS-1 and BCSS-1 are Certified Reference Material issued by the National Research Council of Canada
Al (%) Fe (%) Pb (mg/kg) Zn (mg/kg) Cu (mg/kg) Cr (mg/kg) Hg (mg/kg)
Limit of detection
Uncertainties 95% CI
0.005 0.03 8 0.5 0.5 8 0.05
0.001 0.03 1 0.5 0.5 1 0.01
PACS-1
BCSS-1
Certified value
Observed value
Certified value
Observed value
e e 183 8 364 23 310 12 310 12 3.04 0.2
e e 181.2 4 352 12 302 18 295.45 15 2.95 0.4
0.62 0.025 0.32 0.009 22.7 3.04 119 11.09 18.5 2.59 123 13.53 0.129 0.01
0.63 0.02 0.31 0.01 21.07 2.7 110.08 9.1 20.56 3.6 118.5 11.3 0.132 0.01
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362 40
% Organic carbon
% Dry weight
35
% Clays
30 25 20 15 10 5 0 S1
S3
S5
S7
S8
SW
S12
S14
S16
S18
S20
Stations
NE
Fig. 2. Variations of the contents of clays and organic carbon in the sampling stations in the Oualidia lagoon.
contamination. However, the evolution of contaminant distribution in the Oualidia lagoon sediments due to recent human activities is little understood. 2.2. Sampling and analytical methods Superficial sediment samples were collected from 10 stations (S-3 and S-14 are in front of the oyster farm sewage discharges) in the lagoon in December 2000 using Van Venn skip. At the same time, we retrieved two cores (Old-97-1 and Old97-2), using PVC tubes (Fig. 1). The samples were dried in a steamer between 35 and 40 C. Grain size measurements were performed with a laser granulometer (Malvern, Mastersizer type). The organic carbon content was determined by combustion in an LECO CS 125 analyser. Dissolved and particulate heavy metal concentrations were measured using ICPeMS (Elan 5000, PerkineElmer). Mineralization of SPM samples was undertaken using a modified method of Loring and Rantala (1992). Representative sub-samples (i.e. 30 mg of dry, powdered and homogenized material) were digested in closed Teflon bombs (SavillexÒ) on a heating plate (2 h at 110 C) using 750 ml HCl (12 N, suprapur) and 250 ml HNO3 (14 N, suprapur) with 2 ml HF (26 N, suprapur). After complete cooling, the digested solution was evaporated to dryness. Each sample was brought to 5 ml using 150 ml HNO3 and double-deionised water (MilliQÒ). Each batch of samples included blanks and digestion of certified international marine sediment reference materials (BCSS-1 and PACS-1) (Table 1). Al, Fe, Pb, Zn, Cu and Cr contents were analyzed by atomic absorption spectrometry. The same metals, along with Hg, were determined on samples with H2O2 and leaching with conc. HNO3 under reflux. 137 Cesium and 210Pb activities were only measured on the sediment Old-97-2 core. These measurements were done on 6e10 g dry samples with a low background g-ray spectrometer (Intertechnique EGSP 2200-25 from EURYSIS Measures), using a semi-planar germanium detector coupled to a multichannel (8000 channels) analyser. 210Pb activity was determined by direct measurement of its gamma decay energy at 46.5 keV. 210 Pbxs was calculated from 210Pb activity from which is subtracted 226Ra activity, the supported 210Pb. The use of the natural radionuclide 210Pb (t1/2 ¼ 22.3 years) is a well-established technique for the determination of marine sediment accumulation
rates during the past century or so (Nittrouer et al., 1979). Pbxs apparent sedimentation rates were estimated based on a one-dimensional and two-layered model. The precision of this method is typically better than 10% (Jouanneau et al., 1999). Trace element data were normalized to Al to compensate for mineralogical and grain size variations in the sediments. Aluminium is a suitable geochemical normaliser for these sediments, showing strong correlations with grain size which is one of the most important constituents of the aluminosilicate mineral fraction, often used as a conservative element. Covelli and Fontolan (1997) consider that, for estimating anthropogenic inputs, it is more useful to calculate the nondimensional enrichment factor (Balachandran et al., 2005). Using EF to detect or ‘prove’ human influence on element cycles in remote areas should be avoided because, in most cases, high EF cannot conclusively demonstrate, nor even suggest, such an influence (Reimann and de Caritat, 2005). We used local background (in the base of Old-97-2 core) to calculate EF. In this study, heavy metal concentrations in sediments from Oualidia lagoon were examined and normalized to Al (taken as the conservative element). Additionally, the enrichment factor (EF) and geo-accumulation index (Geo-I, Mu¨ler, 1979) were also estimated for the elements analyzed.
210
EF ¼
ðMetal content=Al contentÞsediment ðMetal content=Al contentÞgeochemical background
Geo-I ¼
logðMetal contentÞsediment 1:5ðMetal contentÞgeochemical background
Table 2 Mean, minimum, maximum, standard deviation values of the heavy metals in the fine fraction (below 63 mm) of Oualidia sediments (n ¼ 10)
Al (%) Fe (%) Pb (mg/kg) Zn (mg/kg) Cu (mg/kg) Cr (mg/kg) Hg (mg/kg) Organic carbon (%) Clay content (%)
Mean
Minimum
Maximum
Standard deviation
10.86 6.91 54.59 227.86 36.46 52.48 0.66 11.03 16.81
5.26 4.62 30.56 202.56 20.00 46.00 0.06 7.56 6.00
14.36 7.90 88.00 260.00 90.00 62.00 1.23 15.23 38.00
2.54 0.89 16.62 16.31 22.86 6.14 0.31 2.63 9.95
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3. Results 3.1. Sedimentological and geochemical characterization of superficial sediments The sediments of the North Eastern sector are richer in clays than those of the South Western sector (Fig. 2). The content in clays varies between 6 and 15.56% in the South Western sector (S-1, S-3, S-5 and S-8) and between 18.89 and 38% (S-12, S-14, S-16 and S-20) in the North Eastern sector. However, station S-1 presents the lower rates. The percentages of organic carbon range between 7.56 and 15.23% with an
A
363
average of 11.03. The most important contents are localized at stations S-3, S-14 and S-16 close to the main areas of oyster farms. Table 2 shows the minimum, maximum and average concentrations of heavy metals at the level of the intertidal zone of the Oualidia lagoon. Elements can be grouped as follows: - Al, Fe and Cr contents vary between 5.26 and 14.36%, 4.62 and 7.89% and 46 and 62 mg/kg; the average being 10.86%, 1.09% and 52.68 mg/kg, respectively. The contents of these three metals increase towards the northern lagoon (Fig. 3A and B).
Al
16
Fe 14
% Dry weight
12 10 8 6 4 2 0 S1
S3
S5
S7
S8
SW
B
S12
S14
S16
S18
S20
Stations
NE Pb
300
Cu 250
Zn Cr
mg/kg
200 150 100 50 0 S1
S3
S5
S7
S12
S14
S16
S18
S20
Stations
SW
C
S8
NE Cd
2,00
Hg
mg/kg
1,50
1,00
0,50
0,00 S1
SW
S3
S5
S7
S8
S12
S14
S16
S18
S20
Stations
Fig. 3. Variations of the contents of metal elements in different stations in Oualidia lagoon.
NE
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364
Fe are important, and another interval, between 80 cm and the surface, where the contents of Al and Fe increase (Fig. 4). The Cr profile does not present any variation. The Cr content is similar to continental environment and the geochemical background (Fig. 5). The contents of Pb, Zn, Cu and Hg show a recent enrichment towards the top. The average contents of Pb, Zn, Cu and Hg in the two cores increased from 24.03, 141, 26.60 and 0.06 mg/kg to 42.40, 168.17, 59.93 and 0.22 mg/kg, respectively, with more important concentrations towards the upper 20e30 cm. These contents are higher compared to those of the immediate continental environment and to those of the geochemical background, calculated from the value at the base of the core. 210 Pb activity found in the core on the Oualidia lagoon is comparable with those found in other studies of coastal zones such as along Iceland (Benninger et al., 1979), the coast of San Francisco (Fuller et al., 1999), the estuary of Culiacan (RuizFernandez et al., 2003) and the estuary of Coatzacoalcos in Mexico (Rosales-Hoz et al., 2003). The vertical profile of 210Pb decreases in activity approximately exponentially (Table 3; Fig. 6). The maximum activity (1308 Bq/kg) occurs 1 cm below the wateresediment interface. In the same way, one notes a certain irregularity of the
- Pb, Zn and Cu contents vary from 20 to 50, 170 to 200 and 20 to 30 mg/kg , respectively. However, Fig. 3C shows two maxima at stations S-3 and S-14 (60e78 mg/kg for Pb, 230e250 mg/kg for Zn and 85e90 mg/kg for Cu). The average contents are 44.59, 202.89 and 37.46 mg/kg, respectively. The Hg content (Fig. 3C) shows enrichment at stations S-3 and S-14. The elevated concentrations of those metals are because of sewerage discharges from the nearby oyster farms.
3.2. Metal contents of the cored deposits The distribution of facies in the Old-97-1 core shows a variation of the depositional environment. The base of this core is characterized by muddy sediment associated with bioclast remains (120e80 cm). It becomes higher in silt in its central part (80e20 cm), then is enriched in vegetable remains and shells of lamellibranches in its upper part. The average content of organic carbon varies from the base to the top of the core (6.01e6.14%, respectively) (Fig. 4). Al and Fe show some variations (5e10%) in two phases, one between 80 and 120 cm in which the contents of Al and
Median grain size (%) 0
20
0 20
Clays content (%) 0
40 1998
0
1973
20
80
1873 1848
100
40
depth (cm)
depth (cm)
1898
1923 1898
80
1873 1848
100 1823
1798 1773
140
1973
60
1823 120
1998
Time in years
60
Time in years
1923
40
1948
1948 40
20
120
1798
140
1773
Old-97-1 Old-97-2
Al (%) 0 20
5
10
Fe (%) 15
0 1998
0
1973
20
1948
80
1873
100
1848
40
depth (cm)
depth (cm)
1898
1973
20
1923
60
1898
80
1873
100
1823
1848 1823
120
1798
120
140
1773
140
1973 1948
40
1923
60
1898
80
1873
100
1848
Time in years
60
5,5 6,0 6,5 7,0 7,5 1998 0
Time in years
1923
Organic Carbon (%) 10 1998
1948
Time in years
40
5
depth (cm)
0
1823
1798
120
1798
1773
140
1773
Fig. 4. Profile of lithology of Old-97-1, median grain size, clay, organic carbon, Al and Fe contents in Old-97-1 and Old-97-2 cores.
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Pb (mg/kg) 50
Zn (mg/kg) 0
100 1998
0
Depth (cm)
Depth (cm)
1848
100
40
1923
60
1898
80
1873 1848
100
1798
140
1773
1923
60
1898
80
1873 1848
100
1823
1823 120
1948 40
Time in years
80
1873
1973
20
Time in years
1898
100 1998
1948
Time in years
1923
60
50
0
1973
20
1948 40
Cu (mg/kg) 0
200 1998
0
1973
20
100
Depth (cm)
0
365
1823
120
1798
120
1798
140
1773
140
1773
Old-97-1 Old-97-2
Cd (mg/kg)
20
0,20
1973
1848 100 1823
Depth (cm)
Depth (cm)
80
1873
40
1923
60
1898
80
1873 1848
100
1973 1948
40
1898
80
1873
100
1823
1923
60
1848
Time in years
1898
20
Time in years
60
25,00 50,00 1998
1948
Time in years
1923
0,00 0
1973
20
1948 40
Cr (mg/kg)
Hg (mg/kg) 0,00 0,50 1,00 1,50 1998 0
0,40 1998
Depth (cm)
0,00 0
1823
120
1798
120
1798
120
1798
140
1773
140
1773
140
1773
Fig. 5. Profile of contents of Pb, Zn, Cu, Cd, Hg and Cr in Old-97-1 and Old-97-2 cores. 210
Pb activity in the first 10 cm of the core. It changes in a temporal way and shows higher rates at the top of the core, contrary to the base. The profile of 137Cs in the lagoon makes it possible to determine the age of the material. Two phases can be distinguished:
An area with an essentially marine influence, made of a sandy facies which is poor in metallic elements and organic matter but rich in biogenic carbonates. An area under a typically lagoonal influence characterized by silty to siltyemuddy facies with high concentrations of trace metals and organic matter, but poor in carbonates.
- a phase between 40 and 15 cm where the contents vary between 7.8 and 3.4 Bq/kg with a peak of 8 Bq/kg located at a depth of 19 cm in the core, - a phase located between 15 and 5 cm where the contents of 137Cs increase and vary from 3.54 to 8.9 Bq/kg with a peak of 10.6 Bq/kg around 5 cm.
The accumulation of organic carbon is native and linked to the development of an abundance of phanerogam in the upstream zone, mostly from the decomposition of macro-algae. It also reflects low hydrodynamics which favours accumulation
Based on both sedimentation accumulation rate and date of recovery of the core (1997), the two peaks of the two phases would correspond, respectively, to the years 1975 and 1989. 4. Discussion The distribution of the sedimentary facies within the lagoon is mainly governed by the marine dynamics. A granulometric study, carried out by the authors, indicates dynamic environment with coarse facies (sands) close to inlets and channels, and finer sediments towards the lagoon floor. This study distinguished two areas:
Table 3 Activity values of
210
Pb (Bq/kg) and the error rate for each level
Depth (cm)
Activity values of
0 1 3 5 7 9 11 13 15 17 19 35 39
1281 1308 1223 1095 1082 977 969 920 841 672 662 264 214
210
Pb
Error rate (þ/) 28 28 26 23 27 20 17 21 15 17 17 10 8
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500
0
1000 1500
5
10
15
0
0
0
5
5
5
10
10
10
15
15
15
20 25
Depth (cm)
0
Depth (cm)
Depth (cm)
0
Sedimentation rate (cm/y)
137Cs (Bq/Kg)
210Pbxs (Bq/Kg)
20 25
25
30
30
35
35
35
40
40
40
45
45
45
210
Pb,
10
20
30
Fig. 6. Excess
5
137
Cs and sedimentation rate in Oualidia lagoon.
of organic carbon mainly due to settlement of organic-rich fine sediments. In addition, the low energy of the tidal surge allows the formation of fine and stable sediments which permits the settlement of an abundant fauna. Organic carbon concentrations are generally dependent on sediment grain size distribution, and the correlation is better with clays than with the total fines. The high content of organic carbon results from the faecal debris of the oyster farms. The sediments of the lagoon of Oualidia are richer in organic carbon than those of other similar ecosystems such as the Sidi Moussa lagoon (Maanan et al., 2004) and the Nador lagoon (El Alami et al., 1998). The spatial distribution of the metallic elements is variable and their content increases from the upstream to the lagoon downstream area. The enrichment of surface sediments of NW Oualidia lagoon in Fe, organic carbon, and trace elements most likely results from the high surface area of fine-grained material. In order to define the possible affinities among the analyzed metals with the mineral and/or organic constituents, we have adopted a linear correlation approach. The correlation between the median grain size, metallic elements (Al, Fe, and Cr) and organic carbon and clays (Table 4) is statistically significant ( p < 0.05). These correlations
probably indicate an association between clays, Al and Cr, in the form of metaleclay complexes of continental origin (Maanan et al., 2004). Furthermore, these metals are influenced by the chemical composition and the grain size. Pb, Zn, Cu and Hg show statistically significant ( p < 0.05) correlation between themselves. They probably have the same origin and are influenced by the same factors and processes. The strong correlation between these elements and organic carbon underlines an association in the form of organometallic complexes (Baptista Neto et al., 2000). Hierarchical R-mode clustering (Fig. 7) makes it possible to identify these two major groups of elements with a group made of Al, Fe, Cr and clays representing an aluminosilicate supply of continental origin, and a second group made of Pb, Zn, Cu and Hg primarily related to the organic matter coming from the oyster farm discharges and the illegal discharges. The main sources of heavy metals in soils from anthropogenic activities are atmospheric deposition, the corrosion of building materials and traffic-related emissions in urban centres (Maanan et al., 2004). For agricultural soils, the most important concentration comes from use of agrochemicals.
Table 4 Pearson correlation coefficients between metal elements, clays and organic carbon [(n ¼ 10) significant correlation marked (*p < 0.05)]
Al Fe Pb Zn Cu Cd Cr Hg Organic carbon Clays
Al
Fe
Pb
Zn
Cu
Cr
Hg
Organic carbon
Clays
1 0.89* 0.26 0.24 0.4 0.02 0.85* 0.15 0.01 0.85*
1 0.05 0.03 0.36 0.25 0.6 0.25 0.06 0.65*
1 0.96* 0.87* 0.03 0.36 0.44 0.9* 0.49
1 0.83* 0.06 0.27 0.48 0.83* 0.42
1 0.21 0.4 0.68* 0.82* 0.55
1 0.16 0.1 0.93*
1 0.57 0.28
1 0.25
1
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367
Fig. 7. Hierarchical R-mode clustering.
et al., 1996). In addition, 210Pb data were characterized by a great variability with depth. Radioisotope dating with the 210Pb made it possible to determine a sediment accumulation rate similar to that found in other ecosystems such as the basin of Arcachon in France (Carruesco and Lapaquellerie, 1985). This sedimentation rate is small at the base of the core (0.6 cm/y), and becomes more important towards the top (1 cm/y). These results confirm the findings from sedimentological studies of cores in the Oualidia lagoon (Bidet and Carruesco, 1982) characterized by the presence of coarse marine sands at the base and a finer lagoonal mud towards the top part. This model for evolution of the lagoon is similar to that of Bird (1994). The sedimentation rates ranged from 0.6 to 1 cm/y corresponding to different sedimentary sequences. So calculation of a mean sedimentation rate for the whole profile gives only a coarse estimate. Episodes with different rates of sedimentation have occurred at the same place, separated by erosion events. The plot of 137Cs activity against depth for the Oualidia sediment core is displayed in Fig. 6. Two peaks of 137Cs activity are identified at 7 and 17 cm depth. Uncertainties in the interpretation of 137Cs depth profile may arise from possible post-depositional processes related to 137Cs mobility in the sediment. Although 137Cs is known to strongly bind mainly to the clay lattice, it can also be sorbed exchangeably to the
The heavy traffic along the coastal road between El Jadida and Safi, close to the Oualidia lagoon, can also likely contribute metals to the sediments via atmospheric deposition and runoff. Cochran et al. (1998) showed that the history of atmospheric fluxes confirms the trends in Venice lagoon sediments. Furthermore, Green-Ruiz and Pa`ez-Osuna (2001) showed that the increase of heavy metal levels in coastal systems in the SE Gulf of California is associated with intensive agriculture under the influence of urbanisation. The distribution of the factor of enrichment (EF) (Fig. 8) shows relatively high values (higher than 2.5) for Pb, Zn and Cu at stations S-1, S-3, S-12 and S-18. The standardization of the contents compared to Al confirms the anthropogenic components of these elements as stated before. The <1 values of the geo-accumulation index for Pb, Zn, Cu and Hg (Fig. 9) in the superficial sediments of the lagoon show that the lagoon is unpolluted to moderately polluted at maximum. Comparison of these values with the standard values suggested by the US Environmental Protection Agency (in Hamdy and Post, 1985), suggests that the sediments in this scale would rank as only ‘slightly polluted’. Measurements of 210Pb activities in the Oualidia lagoon sediment were performed in the bulk sediment and appeared to be influenced by grain size. This result is not surprising since adsorption of 210Pb is known to be greater for fine particles (Monna
Pb
5
Zn Cu
4
Hg
EFs
3
2
1
0 S1
S3
S5
S7
S8
S12
S14
S16
S18
S20
Stations Fig. 8. Enrichment factors for heavy metals in superficial sediment of Oualidia lagoon.
B. Zourarah et al. / Estuarine, Coastal and Shelf Science 72 (2007) 359e369
368 2
Pb Zn
Geo-index
1,5
Cu Hg
1
0,5
0 S1
S3
S5
S7
S8
S12
S14
S16
S18
S20
Station Fig. 9. Geo-accumulation index values for heavy metals in superficial sediment of Oualidia lagoon (0, not polluted; >0e0.9, slightly polluted; 1e1.9, moderately polluted; Mu¨ler, 1979).
sediment (Smith et al., 2000). However, this result must be treated with care because the propagation of the Chernobyl radioactive cloud was very random on the Maghreb. 5. Conclusion The present study of trace metal abundances and distributions in 137Cs dated sediment cores from the Oualidia lagoon confirms the historic (<50 years) metal pollution due to former anthropogenic activities in the lagoon. The sediments showed well-preserved 137Cs peaks estimated to have occurred in 1975 and 1989 as a result of atmospheric deposition. High mean sedimentation rates of 0.6e1 cm/y were calculated for the sediments in the core. These sediments clearly record the temporal evolution of the heavy metal pollution. The study of the evolution of the contents of metals through time allowed the description of two groups of metals: - a first group of metals consisting of Al, Fe and Cr, contents of which varied a little during the last seven decades and origins are exclusively natural. These metals form the aluminosilicate group. - a second group is made of Pb, Zn, Cu and Hg, which presents an enrichment from 20 to 30 cm of depth; what would correspond to the period between 1960 and 1975 which is characterized by the installation of the first suburbs and the oyster farms in the lagoon. The increase in these pollutant metals could thus come from anthropogenic activities. The enrichment factor and the geo-accumulation index show that currently the lagoon of Oualidia is, at most, only moderately polluted. Table 5 Comparison of contents of Pb, Zn and Cu (mg/kg) in the Oualidia lagoon
Bidet and Carruesco (1982) (n ¼ 48 samples) Present study (n ¼ 10 samples)
Pb
Zn
Cu
8 (4e20) 54 (30.56e88)
63 (18e107) 227 (260e202.56)
23 (6e37) 36 (20e90)
The comparison of the sediment metal content with former work on the same lagoon (Bidet and Carruesco, 1982) (Table 5) shows that the content of Pb, Zn and Cu increased in relation to the socio-economic development of the area. If the current state of contamination is not worrying, the touristy and industrial development in this littoral zone require a thorough study and a regular follow-up of the sedimentological and geochemical conditions. It would, thus, be necessary to be attentive to the threshold beyond which the disturbances undergone by the lagoon ecosystem are likely to become irreversible. Acknowledgements The authors gratefully acknowledge reviewers for their critical scientific suggestions and comments; we appreciate their effort in bringing the text to a publishable state. This study was carried out in the framework of a LagMar/ REMER program financed by the Ministry for National Education, the Higher Education and Scientific Research (Morocco) and the Service of Cooperation and Cultural Action (SCAC) of the Embassy of France in Rabat. This study was also supported by grants from integrated action MoroccanFrench ‘‘MA/04/101’’. References Balachandran, K.K., Lalu Raj, C.M., Nair, M., Joseph, T., Sheeba, P., Venugopal, P., 2005. Heavy metal accumulation in a flow restricted, tropical estuary. Estuarine, Coastal and Shelf Science 65 (1e2), 361e370. Baptista Neto, J.A., Smith, B.J., McAllister, J.J., 2000. Heavy metal concentrations in surface sediments in a near shore environment, Jurujuba Sound, Southeast Brazil. Environmental Pollution 109, 1e9. Bellucci, L.G., Frignani, M., Paolucci, D., Ravanelli, M., 2002. Distribution of heavy metals in sediments of the Venice lagoon: the role of the industrial area. Science of the Total Environment 295, 35e49. Benninger, L.K., Aller, R.C., Cochran, J.K., Turekian, K.K., 1979. Effects of biological sediment mixing on the 210Pb chronology and trace metal distribution in a Long Island sound sediment core. Earth and Planetary Science Letters 43, 241e259. Bidet, J.C., Carruesco, C., 1982. Etude se´dimentologique de la lagune de Oualidia (Maroc). Oceanologica Acta N Sp., 29e37.
B. Zourarah et al. / Estuarine, Coastal and Shelf Science 72 (2007) 359e369 Bird, E.C.F., 1994. Physical setting and geomorphology of coastal lagoons. In: Kjerfve, B. (Ed.), Coastal Lagoon Processes. Elsevier Science Publisher, Amsterdam, pp. 9e39. Callender, E., Robbins, J.A., 1993. Transport and accumulation of radionuclides and stable elements in a Missouri River reservoir. Water Resources Research 29, 1787e1804. Cearetta, A., Irabien, M.J., Leorri, E., Yusta, I., Croudace, I.W., Cundy, A.B., 2000. Recent anthropogenic impacts on the Bilbao estuary, northern Spain: geochemical and microfaunal evidence. Estuarine, Coastal and Shelf Science 50, 571e592. Carruesco, C., Lapaquellerie, Y., 1985. Heavy metal pollution in the Arcachon Basin (France): bonding states. Marine Pollution Bulletin 16 (12), 493e497. Cochran, J.K., Frignani, M., Salamanca, M., Bellucci, L.G., Guerzoni, S., 1998. Atmospheric inputs of heavy metals to the Venice Lagoon. Marine Chemistry 62, 15e29. Covelli, S., Fontolan, G., 1997. Application of a normalization procedure in determining regional geochemical baselines. Environmental Geology 30 (1/2), 34e45. El Alami, M., Mahjoubi, R., Damnati, B., Kamel, S., Icxole, M., Maurice, T., 1998. Se´dimentologie et ge´ochimie organique des se´diments superficiels de la lagune de Nador (Maroc Nord Oriental). Journal of African Earth Sciences 26 (2), 249e259. Farmer, J.G., MacKenzie, A.B., Sugden, C.L., Edgar, P.J., Eades, L.J., 1997. A comparison of the historical lead pollution records in peat and freshwater lake sediments from central Scotland. Water, Air and Soil Pollution 100, 253e270. Fuller, C.C., Van Green, A., Baskaran, M., Amina, R., 1999. Sediments chronology in San Francisco Bay, California, defined by 210Pb, 234Th, 137Cs and 240Pu. Marine Chemistry 64, 7e27. Green-Ruiz, C., Pa`ez-Osuna, F., 2001. Heavy metal anomalies in lagoon sediments related to intensive agriculture in Altata-Ensenada del Pabellon coastal system (SE Gulf of California). Environment International, 265e273. Grousset, F., Jouanneau, J.M., Castaing, P., Lavaux, G., Latouche, C., 1999. A 70 year record of contamination from industrial activity along the Garonne River and its tributaries (SW France). Estuarine, Coastal and Shelf Science 48, 401e414. Hamdy, Y., Post, L., 1985. Distribution of mercury, trace organics and other heavy metals in Detroit River sediments. Journal of Great Lakes Research 11, 353e365. Jouanneau, J.M., Castaing, P., Grousset, F., Buat Me´nard, P., Pedemay, P., 1999. Enregistrement se´dimentaire et chronologie d’une contamination en cadmium dans l’estuaire de la Gironde (France). Comptes Rendus de l’Acade´mie des Sciences 239, 265e270.
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Lee, S.V., Cundy, A.B., 2001. Heavy metal contamination and mixing processes in sediments from the Humber Estuary, Eastern England. Estuarine, Coastal and Shelf Science 53, 619e636. Legeleux, F., Reyss, J.L., Schmidt, S., 1994. Particle mixing rates in sediments of the northeast tropical Atlantic: evidence from 210Pbxs, 137Cs, 228Thxs and 234 Thxs. Earth and Planetary Science Letters 128, 545e562. Loring, D.H., Rantala, R.T.T., 1992. Manual for the geochemical analyses of marine sediments and suspended particulate matter. Earth Science Reviews 32, 235e283. Maanan, M., Zourarah, B., Carruesco, C., Aajjane, A., Naud, J., 2004. Distribution of heavy metals in Sidi Moussa lagoon sediments (Atlantic Moroccan Coast). Journal of African Earth Sciences 39 (3e5), 473e484. Monna, F., Lancelot, J., Bernat, M., Mercadier, H., 1996. Sedimentation rate in the Thau basin (France) according to geochronological, geochemical and stratigraphical data. Oceanologica Acta 20, 627e638. Mu¨ler, G., 1979. Schwermetalle in den Sedimenten, des Rheins-Vera¨nderungen seit 1971. Umschau 79 (24), 778e783. Munksgaard, N.C., Lim, K., Parry, D.L., 2003. Rare earth elements as provenance indicators in North Australian estuarine and coastal marine sediments. Estuarine, Coastal and Shelf Science 57, 399e409. Nittrouer, C.A., Sternberg, R.W., Carpenter, R., Bennett, J.T., 1979. The use of Pb-210 geochronology as a sedimentological tool: application to the Washington continental shelf. Marine Geology 31, 297e316. Poinssot, C., Baeyens, B., Bradbury, M., 1999. Experimental and modelling studies on caesium sorption on illite. Geochimica et Cosmochimica Acta 63, 3217e3227. Reimann, C., de Caritat, P., 2005. Distinguishing between natural and anthropogenic sources for elements in the environment: regional geochemical surveys versus enrichment factors. Science of the Total Environment 337, 91e107. Rosales-Hoz, L., Cundy, A.B., Bahena-Manjarrez, J.L., 2003. Heavy metals in sediment cores from a tropical estuary affected by anthropogenic discharges: Coatzacoalcos estuary, Mexico. Estuarine, Coastal and Shelf Science 58, 117e126. Ruiz-Fernandez, A.C., Hillaire-Marcel, C., Pa`ez-Osuna, F., Ghaleb, B., SotoJime´nez, M., 2003. Historical trends of metal pollution recorded in the sediments of the Culiacan River Estuary, Northwestern Mexico. Applied Geochemistry 18, 577e588. Smith, J.T., Comans, R.N.J., Ireland, D.G., Nolan, L., Hilton, J., 2000. Experimental and in situ study of radiocaesium transfer across the sedimentewater interface and mobility in lake sediments. Applied Geochemistry 15, 833e848. Spencer, K.L., Cundy, A.B., Croudace, I.W., 2003. Heavy metal distribution and early-diagenesis in salt marsh sediments from the Medway Estuary, Kent, UK. Estuarine, Coastal and Shelf Science 57, 43e54.
Fifty-year sedimentary record of heavy metal pollution in the lagoon of Oualidia (Moroccan Atlantic coast) B. Zourarah a,*, M. Maanan b, C. Carruesco c, A. Aajjane a, K. Mehdi a, M. Conceic¸~ao Freitas d a
Department of Geology, Faculty of Sciences, El Jadida, Morocco b UMR 6554 LETGeGe´olittomer, University of Nantes, France c Department of Geology and Oceanography, Bordeaux I University, Talence, France d Centro Departamento de Geologia da Universidade de Lisboa, Portugal Received 10 April 2006; accepted 20 November 2006 Available online 8 January 2007
Abstract The Oualidia lagoon is known for its heavy metal pollution resulting from mining and smelting activities since the late 19th century. Here, we report 137Cs and 210Pb activities and heavy metal concentration depth profiles from sediment cores retrieved in 1997. High mean sedimentation rates of 0.6e1 cm/y are indicated by 210Pb and 137Cs dating. The lagoon sediments have recorded heavy metal deposition and thus allow establishment of a connection between the temporal evolution of the heavy metal pollution and historical changes in smelting and waste-treatment proceedings. Through a study of the evolution of heavy metal contents, we can distinguish between two categories of metals: 1- Al, Fe and Cr contents have varied around a mean value over the last seven decades and could have natural origins. 2- Pb, Zn, Cu and Hg have relatively more elevated contents than those of the natural geochemical background, especially in the station in front of illegal sewerage discharges. These metals are enriched at depths between 20 and 30 cm; this corresponds to the period between 1960 and 1975, which was characterized by the setting up of the main suburbs and the first aquaculture farms that surround the lagoon. The pollution intensity of the lagoon is determined by enrichment factors and the geo-accumulation index, which show that the lagoon of Oualidia is unpolluted to moderately polluted on the Geo-I scale of Mu¨ler (1979. Schwermetalle in den Sedimenten, des Rheins-Vera¨nderungen seit 1971. Umschau 79 (24), 778e783.). Ó 2006 Elsevier Ltd. All rights reserved. Keywords: Morocco; lagoon; heavy metals; pollution; sediment; cesium
1. Introduction Coastal environments are under increasing pressure as a result of increasing human population. In fact coastal areas, including estuaries, bays, shorelines, lagoons and continental shelves, are used intensively and receive the by-products of inland human activities, mainly via rivers. The human impact on these productive and economically important environments has become a major concern (Cearetta et al., 2000).
* Corresponding author. E-mail address: [email protected] (B. Zourarah). 0272-7714/$ - see front matter Ó 2006 Elsevier Ltd. All rights reserved. doi:10.1016/j.ecss.2006.11.007
Over the past century, heavy metals have been discharged into the world’s coastal environments as a result of the rapid industrial development (Lee and Cundy, 2001; Bellucci et al., 2002; Munksgaard et al., 2003; Spencer et al., 2003; Maanan et al., 2004). The delivery processes are closely linked to those of fine-grained suspended sediments, acting as effective carriers, onto which heavy metals are bound. 210 Pb (t1/2 ¼ 22.3 years) provides a reliable method of dating sediments deposited over the last 100e150 years. 210Pb is found in sediment minerals first in a supported form by in situ production from 238U or 226Ra decay, at equal activity to its parent nuclides, which is normally assumed to be constant down a core profile. The second fraction, the unsupported or
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excess 210Pb (210Pbxs) has an atmospheric origin, being produced by decay of 222Rn which emanates from soils after the decay of 226Ra. Atmospheric 210Pb enters the aquatic environments by direct deposition in rain and also by runoff and erosion from catchment areas. Geochronology with 210Pb is based on the pattern of radioactive decay of 210Pbxs with depth in the sedimentary column, and the exponential law describes this decay through time as follows: 210
PbðzÞ ¼ 210 Pbð0Þ elt
ð1Þ
pollution studies in the coastal lagoon, where these techniques have been scarcely used. It summarizes the investigations on the origin and temporal changes in trace metal inputs reflected in the sedimentary record of a coastal lagoon. 2. Methods 2.1. Study site Oualidia lagoon (32 400 4200 Ne32 470 0700 N and 8 520 3000 We 9 020 5000 W) is located on the Atlantic Ocean (Fig. 1). This lagoon is 7 km long, on average 0.5 km wide, and exchanges water with the ocean through a major inlet about 150 m wide and 2 m deep. During spring tides there is also a secondary, shallower inlet about 50 m wide. An internal delta with a surface area of about 0.2 km2 is normally found close to the inlet. The lagoon morphology is characterized by side channels, connected to a meandering main channel, in which the mean depth is 2 m and the maximum depth during flood tides does not exceed 5 m (Bidet and Carruesco, 1982). Intertidal areas on both sides of the channels occupy about 53% (1.6 km2) of the 3-km2 surface area of the lagoon at low tide. Flood tides cover more than 75% (2.25 km2) of the lagoon surface, bringing salt water up to the upstream reaches of the lagoon and into a saline marsh beyond the second dam. The dam or causeway, built fairly recently to facilitate the crossing of the lagoon, includes a breach of about 10 m wide and 2 m deep, which allows some water exchange at high water levels. In geomorphologic terms, it belongs to an occidental coastal basin called the Sahel. This basin is made of the morphology of a depression limited by a Plio-Quaternary continental dead cliff and a coastal consolidated dune ridge in a southwestenortheast direction. It is constituted mainly of yellow detritus limestone, formed of shelly sands. This morphology is the result of the post-Ouljienne regression (Bidet and Carruesco, 1982). Rainfall over the region accounts for only 1% of the fresh water entering the lagoon. The annual average rainfall, estimated from 1977 to 1998, is about 390 mm, with a maximum in December and no rain during the dry period. The annual estimate of evaporation minus precipitation is 650 mm. The predominant wind directions are WSW to NW during the wet season and NNE to NE during the dry season. Sporadic violent winds (Chergui) occasionally blow from the ENE during the dry period and may contribute to high evaporation rates over the lagoon, as well as to extreme air temperatures, which can reach up to 40 C. More generally, winds blowing from the northern sector will produce southerly geotropic currents along the coast, offshore transport and coastal upwelling of nutrient rich deep waters close to the coast. Upwelled waters with high nutrient content can be advected by flood tides into the lagoon, supporting biological production and enhancing aquaculture yields. The major aquaculture activity in the lagoon is oyster culture. To date, five oyster farms occupy one-sixth of the lagoon surface. In addition, during summer Oualidia lagoon is the site of intense tourism activities. Discharging farm and domestic sewage water without treatment can cause increased heavy metal
where 210Pb(0) is the surface activity, 210Pb(z) is the activity at depth z, and l is the decay constant (0.03114 y1). This equation assumes a constant supply of 210Pb to the sediments, as well as a negligible post-depositional migration of 210Pb within the column. However, in areas where the 210Pb distribution is open to alternate interpretations, the use of two or more radionuclides, with different half-lives and different input histories, allows a better characterization of sediment reworking than it is possible with just one tracer (Legeleux et al., 1994). In the C.F.C.S.R. model (Constant Flux and Constant Sedimentation Rate), the sedimentation rate (R) can be expressed as: R ¼ z=t
ð2Þ
Then, Eq. (1) becomes, 210
PbxsðzÞ ¼ 210 Pbxsð0Þ eðlz=RÞ
Consequently, ln 210 PbxsðzÞ ¼ ln 210 Pbxsð0Þ ðlz=RÞ
ð3Þ
ð4Þ
Hence, a plot of ln 210Pbxs as a function of depth should be linear with a gradient of l/R. The 210Pbxs activity is always constant in the mixed layer, which has a thickness of several centimetres, and the decrease only starts below the core. The combination of 210Pb with 137Cs is often used to reduce the uncertainty in dating (Benninger et al., 1979). Typically, 210 Pb dating of river sediment has shown good results for rivers draining a small watershed (Farmer et al., 1997). 137 Cesium (t1/2 ¼ 30.2 years) found in marine and fluvial sediments is an anthropogenic radionuclide originated from above-ground nuclear testing carried out in 1945e1972, but 90% of the radionuclide releases occurred between 1963 and 1964 (Callender and Robbins, 1993). 137Cesium has been used as an indicator for the amount of global fallout deposited in soil and reservoir sediments. The use of 137Cs as an indicator of sedimentary processes is consistent as it binds almost irreversibly to clay and silt particles (Poinssot et al., 1999) and because of its long half-life (30.2 years). Moreover, the Chernobyl nuclear accident of 1986 has been recorded in European sediments. Thus, 137Cs activity depth profiles are often used for sediment core dating (Grousset et al., 1999). This paper reports on trace metal pollution based on preliminary investigation. The aim of this paper is to demonstrate the usefulness of radioactive and stable isotopic tracers for
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361
Fig. 1. Schematic map showing the superficial sample and core locations in the Oualidia lagoon.
Table 1 Limit of detection, uncertainties and contents of heavy metals certified and that detected in reference materials (n ¼ 5) (mean standard deviation). PACS-1 and BCSS-1 are Certified Reference Material issued by the National Research Council of Canada
Al (%) Fe (%) Pb (mg/kg) Zn (mg/kg) Cu (mg/kg) Cr (mg/kg) Hg (mg/kg)
Limit of detection
Uncertainties 95% CI
0.005 0.03 8 0.5 0.5 8 0.05
0.001 0.03 1 0.5 0.5 1 0.01
PACS-1
BCSS-1
Certified value
Observed value
Certified value
Observed value
e e 183 8 364 23 310 12 310 12 3.04 0.2
e e 181.2 4 352 12 302 18 295.45 15 2.95 0.4
0.62 0.025 0.32 0.009 22.7 3.04 119 11.09 18.5 2.59 123 13.53 0.129 0.01
0.63 0.02 0.31 0.01 21.07 2.7 110.08 9.1 20.56 3.6 118.5 11.3 0.132 0.01
B. Zourarah et al. / Estuarine, Coastal and Shelf Science 72 (2007) 359e369
362 40
% Organic carbon
% Dry weight
35
% Clays
30 25 20 15 10 5 0 S1
S3
S5
S7
S8
SW
S12
S14
S16
S18
S20
Stations
NE
Fig. 2. Variations of the contents of clays and organic carbon in the sampling stations in the Oualidia lagoon.
contamination. However, the evolution of contaminant distribution in the Oualidia lagoon sediments due to recent human activities is little understood. 2.2. Sampling and analytical methods Superficial sediment samples were collected from 10 stations (S-3 and S-14 are in front of the oyster farm sewage discharges) in the lagoon in December 2000 using Van Venn skip. At the same time, we retrieved two cores (Old-97-1 and Old97-2), using PVC tubes (Fig. 1). The samples were dried in a steamer between 35 and 40 C. Grain size measurements were performed with a laser granulometer (Malvern, Mastersizer type). The organic carbon content was determined by combustion in an LECO CS 125 analyser. Dissolved and particulate heavy metal concentrations were measured using ICPeMS (Elan 5000, PerkineElmer). Mineralization of SPM samples was undertaken using a modified method of Loring and Rantala (1992). Representative sub-samples (i.e. 30 mg of dry, powdered and homogenized material) were digested in closed Teflon bombs (SavillexÒ) on a heating plate (2 h at 110 C) using 750 ml HCl (12 N, suprapur) and 250 ml HNO3 (14 N, suprapur) with 2 ml HF (26 N, suprapur). After complete cooling, the digested solution was evaporated to dryness. Each sample was brought to 5 ml using 150 ml HNO3 and double-deionised water (MilliQÒ). Each batch of samples included blanks and digestion of certified international marine sediment reference materials (BCSS-1 and PACS-1) (Table 1). Al, Fe, Pb, Zn, Cu and Cr contents were analyzed by atomic absorption spectrometry. The same metals, along with Hg, were determined on samples with H2O2 and leaching with conc. HNO3 under reflux. 137 Cesium and 210Pb activities were only measured on the sediment Old-97-2 core. These measurements were done on 6e10 g dry samples with a low background g-ray spectrometer (Intertechnique EGSP 2200-25 from EURYSIS Measures), using a semi-planar germanium detector coupled to a multichannel (8000 channels) analyser. 210Pb activity was determined by direct measurement of its gamma decay energy at 46.5 keV. 210 Pbxs was calculated from 210Pb activity from which is subtracted 226Ra activity, the supported 210Pb. The use of the natural radionuclide 210Pb (t1/2 ¼ 22.3 years) is a well-established technique for the determination of marine sediment accumulation
rates during the past century or so (Nittrouer et al., 1979). Pbxs apparent sedimentation rates were estimated based on a one-dimensional and two-layered model. The precision of this method is typically better than 10% (Jouanneau et al., 1999). Trace element data were normalized to Al to compensate for mineralogical and grain size variations in the sediments. Aluminium is a suitable geochemical normaliser for these sediments, showing strong correlations with grain size which is one of the most important constituents of the aluminosilicate mineral fraction, often used as a conservative element. Covelli and Fontolan (1997) consider that, for estimating anthropogenic inputs, it is more useful to calculate the nondimensional enrichment factor (Balachandran et al., 2005). Using EF to detect or ‘prove’ human influence on element cycles in remote areas should be avoided because, in most cases, high EF cannot conclusively demonstrate, nor even suggest, such an influence (Reimann and de Caritat, 2005). We used local background (in the base of Old-97-2 core) to calculate EF. In this study, heavy metal concentrations in sediments from Oualidia lagoon were examined and normalized to Al (taken as the conservative element). Additionally, the enrichment factor (EF) and geo-accumulation index (Geo-I, Mu¨ler, 1979) were also estimated for the elements analyzed.
210
EF ¼
ðMetal content=Al contentÞsediment ðMetal content=Al contentÞgeochemical background
Geo-I ¼
logðMetal contentÞsediment 1:5ðMetal contentÞgeochemical background
Table 2 Mean, minimum, maximum, standard deviation values of the heavy metals in the fine fraction (below 63 mm) of Oualidia sediments (n ¼ 10)
Al (%) Fe (%) Pb (mg/kg) Zn (mg/kg) Cu (mg/kg) Cr (mg/kg) Hg (mg/kg) Organic carbon (%) Clay content (%)
Mean
Minimum
Maximum
Standard deviation
10.86 6.91 54.59 227.86 36.46 52.48 0.66 11.03 16.81
5.26 4.62 30.56 202.56 20.00 46.00 0.06 7.56 6.00
14.36 7.90 88.00 260.00 90.00 62.00 1.23 15.23 38.00
2.54 0.89 16.62 16.31 22.86 6.14 0.31 2.63 9.95
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3. Results 3.1. Sedimentological and geochemical characterization of superficial sediments The sediments of the North Eastern sector are richer in clays than those of the South Western sector (Fig. 2). The content in clays varies between 6 and 15.56% in the South Western sector (S-1, S-3, S-5 and S-8) and between 18.89 and 38% (S-12, S-14, S-16 and S-20) in the North Eastern sector. However, station S-1 presents the lower rates. The percentages of organic carbon range between 7.56 and 15.23% with an
A
363
average of 11.03. The most important contents are localized at stations S-3, S-14 and S-16 close to the main areas of oyster farms. Table 2 shows the minimum, maximum and average concentrations of heavy metals at the level of the intertidal zone of the Oualidia lagoon. Elements can be grouped as follows: - Al, Fe and Cr contents vary between 5.26 and 14.36%, 4.62 and 7.89% and 46 and 62 mg/kg; the average being 10.86%, 1.09% and 52.68 mg/kg, respectively. The contents of these three metals increase towards the northern lagoon (Fig. 3A and B).
Al
16
Fe 14
% Dry weight
12 10 8 6 4 2 0 S1
S3
S5
S7
S8
SW
B
S12
S14
S16
S18
S20
Stations
NE Pb
300
Cu 250
Zn Cr
mg/kg
200 150 100 50 0 S1
S3
S5
S7
S12
S14
S16
S18
S20
Stations
SW
C
S8
NE Cd
2,00
Hg
mg/kg
1,50
1,00
0,50
0,00 S1
SW
S3
S5
S7
S8
S12
S14
S16
S18
S20
Stations
Fig. 3. Variations of the contents of metal elements in different stations in Oualidia lagoon.
NE
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364
Fe are important, and another interval, between 80 cm and the surface, where the contents of Al and Fe increase (Fig. 4). The Cr profile does not present any variation. The Cr content is similar to continental environment and the geochemical background (Fig. 5). The contents of Pb, Zn, Cu and Hg show a recent enrichment towards the top. The average contents of Pb, Zn, Cu and Hg in the two cores increased from 24.03, 141, 26.60 and 0.06 mg/kg to 42.40, 168.17, 59.93 and 0.22 mg/kg, respectively, with more important concentrations towards the upper 20e30 cm. These contents are higher compared to those of the immediate continental environment and to those of the geochemical background, calculated from the value at the base of the core. 210 Pb activity found in the core on the Oualidia lagoon is comparable with those found in other studies of coastal zones such as along Iceland (Benninger et al., 1979), the coast of San Francisco (Fuller et al., 1999), the estuary of Culiacan (RuizFernandez et al., 2003) and the estuary of Coatzacoalcos in Mexico (Rosales-Hoz et al., 2003). The vertical profile of 210Pb decreases in activity approximately exponentially (Table 3; Fig. 6). The maximum activity (1308 Bq/kg) occurs 1 cm below the wateresediment interface. In the same way, one notes a certain irregularity of the
- Pb, Zn and Cu contents vary from 20 to 50, 170 to 200 and 20 to 30 mg/kg , respectively. However, Fig. 3C shows two maxima at stations S-3 and S-14 (60e78 mg/kg for Pb, 230e250 mg/kg for Zn and 85e90 mg/kg for Cu). The average contents are 44.59, 202.89 and 37.46 mg/kg, respectively. The Hg content (Fig. 3C) shows enrichment at stations S-3 and S-14. The elevated concentrations of those metals are because of sewerage discharges from the nearby oyster farms.
3.2. Metal contents of the cored deposits The distribution of facies in the Old-97-1 core shows a variation of the depositional environment. The base of this core is characterized by muddy sediment associated with bioclast remains (120e80 cm). It becomes higher in silt in its central part (80e20 cm), then is enriched in vegetable remains and shells of lamellibranches in its upper part. The average content of organic carbon varies from the base to the top of the core (6.01e6.14%, respectively) (Fig. 4). Al and Fe show some variations (5e10%) in two phases, one between 80 and 120 cm in which the contents of Al and
Median grain size (%) 0
20
0 20
Clays content (%) 0
40 1998
0
1973
20
80
1873 1848
100
40
depth (cm)
depth (cm)
1898
1923 1898
80
1873 1848
100 1823
1798 1773
140
1973
60
1823 120
1998
Time in years
60
Time in years
1923
40
1948
1948 40
20
120
1798
140
1773
Old-97-1 Old-97-2
Al (%) 0 20
5
10
Fe (%) 15
0 1998
0
1973
20
1948
80
1873
100
1848
40
depth (cm)
depth (cm)
1898
1973
20
1923
60
1898
80
1873
100
1823
1848 1823
120
1798
120
140
1773
140
1973 1948
40
1923
60
1898
80
1873
100
1848
Time in years
60
5,5 6,0 6,5 7,0 7,5 1998 0
Time in years
1923
Organic Carbon (%) 10 1998
1948
Time in years
40
5
depth (cm)
0
1823
1798
120
1798
1773
140
1773
Fig. 4. Profile of lithology of Old-97-1, median grain size, clay, organic carbon, Al and Fe contents in Old-97-1 and Old-97-2 cores.
B. Zourarah et al. / Estuarine, Coastal and Shelf Science 72 (2007) 359e369
Pb (mg/kg) 50
Zn (mg/kg) 0
100 1998
0
Depth (cm)
Depth (cm)
1848
100
40
1923
60
1898
80
1873 1848
100
1798
140
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1923
60
1898
80
1873 1848
100
1823
1823 120
1948 40
Time in years
80
1873
1973
20
Time in years
1898
100 1998
1948
Time in years
1923
60
50
0
1973
20
1948 40
Cu (mg/kg) 0
200 1998
0
1973
20
100
Depth (cm)
0
365
1823
120
1798
120
1798
140
1773
140
1773
Old-97-1 Old-97-2
Cd (mg/kg)
20
0,20
1973
1848 100 1823
Depth (cm)
Depth (cm)
80
1873
40
1923
60
1898
80
1873 1848
100
1973 1948
40
1898
80
1873
100
1823
1923
60
1848
Time in years
1898
20
Time in years
60
25,00 50,00 1998
1948
Time in years
1923
0,00 0
1973
20
1948 40
Cr (mg/kg)
Hg (mg/kg) 0,00 0,50 1,00 1,50 1998 0
0,40 1998
Depth (cm)
0,00 0
1823
120
1798
120
1798
120
1798
140
1773
140
1773
140
1773
Fig. 5. Profile of contents of Pb, Zn, Cu, Cd, Hg and Cr in Old-97-1 and Old-97-2 cores. 210
Pb activity in the first 10 cm of the core. It changes in a temporal way and shows higher rates at the top of the core, contrary to the base. The profile of 137Cs in the lagoon makes it possible to determine the age of the material. Two phases can be distinguished:
An area with an essentially marine influence, made of a sandy facies which is poor in metallic elements and organic matter but rich in biogenic carbonates. An area under a typically lagoonal influence characterized by silty to siltyemuddy facies with high concentrations of trace metals and organic matter, but poor in carbonates.
- a phase between 40 and 15 cm where the contents vary between 7.8 and 3.4 Bq/kg with a peak of 8 Bq/kg located at a depth of 19 cm in the core, - a phase located between 15 and 5 cm where the contents of 137Cs increase and vary from 3.54 to 8.9 Bq/kg with a peak of 10.6 Bq/kg around 5 cm.
The accumulation of organic carbon is native and linked to the development of an abundance of phanerogam in the upstream zone, mostly from the decomposition of macro-algae. It also reflects low hydrodynamics which favours accumulation
Based on both sedimentation accumulation rate and date of recovery of the core (1997), the two peaks of the two phases would correspond, respectively, to the years 1975 and 1989. 4. Discussion The distribution of the sedimentary facies within the lagoon is mainly governed by the marine dynamics. A granulometric study, carried out by the authors, indicates dynamic environment with coarse facies (sands) close to inlets and channels, and finer sediments towards the lagoon floor. This study distinguished two areas:
Table 3 Activity values of
210
Pb (Bq/kg) and the error rate for each level
Depth (cm)
Activity values of
0 1 3 5 7 9 11 13 15 17 19 35 39
1281 1308 1223 1095 1082 977 969 920 841 672 662 264 214
210
Pb
Error rate (þ/) 28 28 26 23 27 20 17 21 15 17 17 10 8
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366
500
0
1000 1500
5
10
15
0
0
0
5
5
5
10
10
10
15
15
15
20 25
Depth (cm)
0
Depth (cm)
Depth (cm)
0
Sedimentation rate (cm/y)
137Cs (Bq/Kg)
210Pbxs (Bq/Kg)
20 25
25
30
30
35
35
35
40
40
40
45
45
45
210
Pb,
10
20
30
Fig. 6. Excess
5
137
Cs and sedimentation rate in Oualidia lagoon.
of organic carbon mainly due to settlement of organic-rich fine sediments. In addition, the low energy of the tidal surge allows the formation of fine and stable sediments which permits the settlement of an abundant fauna. Organic carbon concentrations are generally dependent on sediment grain size distribution, and the correlation is better with clays than with the total fines. The high content of organic carbon results from the faecal debris of the oyster farms. The sediments of the lagoon of Oualidia are richer in organic carbon than those of other similar ecosystems such as the Sidi Moussa lagoon (Maanan et al., 2004) and the Nador lagoon (El Alami et al., 1998). The spatial distribution of the metallic elements is variable and their content increases from the upstream to the lagoon downstream area. The enrichment of surface sediments of NW Oualidia lagoon in Fe, organic carbon, and trace elements most likely results from the high surface area of fine-grained material. In order to define the possible affinities among the analyzed metals with the mineral and/or organic constituents, we have adopted a linear correlation approach. The correlation between the median grain size, metallic elements (Al, Fe, and Cr) and organic carbon and clays (Table 4) is statistically significant ( p < 0.05). These correlations
probably indicate an association between clays, Al and Cr, in the form of metaleclay complexes of continental origin (Maanan et al., 2004). Furthermore, these metals are influenced by the chemical composition and the grain size. Pb, Zn, Cu and Hg show statistically significant ( p < 0.05) correlation between themselves. They probably have the same origin and are influenced by the same factors and processes. The strong correlation between these elements and organic carbon underlines an association in the form of organometallic complexes (Baptista Neto et al., 2000). Hierarchical R-mode clustering (Fig. 7) makes it possible to identify these two major groups of elements with a group made of Al, Fe, Cr and clays representing an aluminosilicate supply of continental origin, and a second group made of Pb, Zn, Cu and Hg primarily related to the organic matter coming from the oyster farm discharges and the illegal discharges. The main sources of heavy metals in soils from anthropogenic activities are atmospheric deposition, the corrosion of building materials and traffic-related emissions in urban centres (Maanan et al., 2004). For agricultural soils, the most important concentration comes from use of agrochemicals.
Table 4 Pearson correlation coefficients between metal elements, clays and organic carbon [(n ¼ 10) significant correlation marked (*p < 0.05)]
Al Fe Pb Zn Cu Cd Cr Hg Organic carbon Clays
Al
Fe
Pb
Zn
Cu
Cr
Hg
Organic carbon
Clays
1 0.89* 0.26 0.24 0.4 0.02 0.85* 0.15 0.01 0.85*
1 0.05 0.03 0.36 0.25 0.6 0.25 0.06 0.65*
1 0.96* 0.87* 0.03 0.36 0.44 0.9* 0.49
1 0.83* 0.06 0.27 0.48 0.83* 0.42
1 0.21 0.4 0.68* 0.82* 0.55
1 0.16 0.1 0.93*
1 0.57 0.28
1 0.25
1
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367
Fig. 7. Hierarchical R-mode clustering.
et al., 1996). In addition, 210Pb data were characterized by a great variability with depth. Radioisotope dating with the 210Pb made it possible to determine a sediment accumulation rate similar to that found in other ecosystems such as the basin of Arcachon in France (Carruesco and Lapaquellerie, 1985). This sedimentation rate is small at the base of the core (0.6 cm/y), and becomes more important towards the top (1 cm/y). These results confirm the findings from sedimentological studies of cores in the Oualidia lagoon (Bidet and Carruesco, 1982) characterized by the presence of coarse marine sands at the base and a finer lagoonal mud towards the top part. This model for evolution of the lagoon is similar to that of Bird (1994). The sedimentation rates ranged from 0.6 to 1 cm/y corresponding to different sedimentary sequences. So calculation of a mean sedimentation rate for the whole profile gives only a coarse estimate. Episodes with different rates of sedimentation have occurred at the same place, separated by erosion events. The plot of 137Cs activity against depth for the Oualidia sediment core is displayed in Fig. 6. Two peaks of 137Cs activity are identified at 7 and 17 cm depth. Uncertainties in the interpretation of 137Cs depth profile may arise from possible post-depositional processes related to 137Cs mobility in the sediment. Although 137Cs is known to strongly bind mainly to the clay lattice, it can also be sorbed exchangeably to the
The heavy traffic along the coastal road between El Jadida and Safi, close to the Oualidia lagoon, can also likely contribute metals to the sediments via atmospheric deposition and runoff. Cochran et al. (1998) showed that the history of atmospheric fluxes confirms the trends in Venice lagoon sediments. Furthermore, Green-Ruiz and Pa`ez-Osuna (2001) showed that the increase of heavy metal levels in coastal systems in the SE Gulf of California is associated with intensive agriculture under the influence of urbanisation. The distribution of the factor of enrichment (EF) (Fig. 8) shows relatively high values (higher than 2.5) for Pb, Zn and Cu at stations S-1, S-3, S-12 and S-18. The standardization of the contents compared to Al confirms the anthropogenic components of these elements as stated before. The <1 values of the geo-accumulation index for Pb, Zn, Cu and Hg (Fig. 9) in the superficial sediments of the lagoon show that the lagoon is unpolluted to moderately polluted at maximum. Comparison of these values with the standard values suggested by the US Environmental Protection Agency (in Hamdy and Post, 1985), suggests that the sediments in this scale would rank as only ‘slightly polluted’. Measurements of 210Pb activities in the Oualidia lagoon sediment were performed in the bulk sediment and appeared to be influenced by grain size. This result is not surprising since adsorption of 210Pb is known to be greater for fine particles (Monna
Pb
5
Zn Cu
4
Hg
EFs
3
2
1
0 S1
S3
S5
S7
S8
S12
S14
S16
S18
S20
Stations Fig. 8. Enrichment factors for heavy metals in superficial sediment of Oualidia lagoon.
B. Zourarah et al. / Estuarine, Coastal and Shelf Science 72 (2007) 359e369
368 2
Pb Zn
Geo-index
1,5
Cu Hg
1
0,5
0 S1
S3
S5
S7
S8
S12
S14
S16
S18
S20
Station Fig. 9. Geo-accumulation index values for heavy metals in superficial sediment of Oualidia lagoon (0, not polluted; >0e0.9, slightly polluted; 1e1.9, moderately polluted; Mu¨ler, 1979).
sediment (Smith et al., 2000). However, this result must be treated with care because the propagation of the Chernobyl radioactive cloud was very random on the Maghreb. 5. Conclusion The present study of trace metal abundances and distributions in 137Cs dated sediment cores from the Oualidia lagoon confirms the historic (<50 years) metal pollution due to former anthropogenic activities in the lagoon. The sediments showed well-preserved 137Cs peaks estimated to have occurred in 1975 and 1989 as a result of atmospheric deposition. High mean sedimentation rates of 0.6e1 cm/y were calculated for the sediments in the core. These sediments clearly record the temporal evolution of the heavy metal pollution. The study of the evolution of the contents of metals through time allowed the description of two groups of metals: - a first group of metals consisting of Al, Fe and Cr, contents of which varied a little during the last seven decades and origins are exclusively natural. These metals form the aluminosilicate group. - a second group is made of Pb, Zn, Cu and Hg, which presents an enrichment from 20 to 30 cm of depth; what would correspond to the period between 1960 and 1975 which is characterized by the installation of the first suburbs and the oyster farms in the lagoon. The increase in these pollutant metals could thus come from anthropogenic activities. The enrichment factor and the geo-accumulation index show that currently the lagoon of Oualidia is, at most, only moderately polluted. Table 5 Comparison of contents of Pb, Zn and Cu (mg/kg) in the Oualidia lagoon
Bidet and Carruesco (1982) (n ¼ 48 samples) Present study (n ¼ 10 samples)
Pb
Zn
Cu
8 (4e20) 54 (30.56e88)
63 (18e107) 227 (260e202.56)
23 (6e37) 36 (20e90)
The comparison of the sediment metal content with former work on the same lagoon (Bidet and Carruesco, 1982) (Table 5) shows that the content of Pb, Zn and Cu increased in relation to the socio-economic development of the area. If the current state of contamination is not worrying, the touristy and industrial development in this littoral zone require a thorough study and a regular follow-up of the sedimentological and geochemical conditions. It would, thus, be necessary to be attentive to the threshold beyond which the disturbances undergone by the lagoon ecosystem are likely to become irreversible. Acknowledgements The authors gratefully acknowledge reviewers for their critical scientific suggestions and comments; we appreciate their effort in bringing the text to a publishable state. This study was carried out in the framework of a LagMar/ REMER program financed by the Ministry for National Education, the Higher Education and Scientific Research (Morocco) and the Service of Cooperation and Cultural Action (SCAC) of the Embassy of France in Rabat. This study was also supported by grants from integrated action MoroccanFrench ‘‘MA/04/101’’. References Balachandran, K.K., Lalu Raj, C.M., Nair, M., Joseph, T., Sheeba, P., Venugopal, P., 2005. Heavy metal accumulation in a flow restricted, tropical estuary. Estuarine, Coastal and Shelf Science 65 (1e2), 361e370. Baptista Neto, J.A., Smith, B.J., McAllister, J.J., 2000. Heavy metal concentrations in surface sediments in a near shore environment, Jurujuba Sound, Southeast Brazil. Environmental Pollution 109, 1e9. Bellucci, L.G., Frignani, M., Paolucci, D., Ravanelli, M., 2002. Distribution of heavy metals in sediments of the Venice lagoon: the role of the industrial area. Science of the Total Environment 295, 35e49. Benninger, L.K., Aller, R.C., Cochran, J.K., Turekian, K.K., 1979. Effects of biological sediment mixing on the 210Pb chronology and trace metal distribution in a Long Island sound sediment core. Earth and Planetary Science Letters 43, 241e259. Bidet, J.C., Carruesco, C., 1982. Etude se´dimentologique de la lagune de Oualidia (Maroc). Oceanologica Acta N Sp., 29e37.
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