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Geochemistry of marine sediments in the Mejerda River delta, Tunisia

Abstract Superficial sediments were collected from the Mejerda River delta between October and November 2008. Samples were analysed for 15 trace and major elements (Cd, Pb, Zn, Mn, Ni, Co, Cr, Cu, Sr, Fe, Al, Ca, Mg, Na, and K). Fine fractions, carbonates, total organic carbon, minerals and acid volatile sulfide (AVS) were also analysed to explain the spatial distributions of heavy metals. There are two theories of metal spatial distribution: metals concentrated in coastal sediments (Ca, Mn, and Sr) and metals concentrated in offshore sediments (Al, Fe, Mg, Na, K, Pb, Zn, Cr, Co, Ni). Ca, Mn and Sr show similar distributions to those of carbonates, quartz and calcite; these elements show their greatest concentrations in the coastal sediments. However, Al, Fe, Pb, Zn, Cr, Co and Ni show a large concentration in offshore sediments, such as in the clay and silts (< 20μm). None of these metals has a visible affinity with the organic carbon or AVS, indicating that the carbonates and fine fraction are the essential factors which control the distribution of heavy metals in the delta. With regard to other heavy metals studied in the Mediterranean Sea, Cd, Pb and Zn constitute the main pollutants in the delta.

Vertical Profiles of Simultaneously Extracted Metals (SEM) and Acid-Volatile Sulfide in a Changed Sedimentary Environment: Ghar El Melh Lagoon, Tunisia

The Ghar El Melh lagoon is located in the Gulf of Tunis. It constituted the old mouth of the Mejerda River before the construction of dams (1950). Three cores were studied to define the role of sulfides in retaining heavy metals in the sediment. Sediments are characterized by two distinct layers with different geochemical characteristics. According to color and grain-size distribution, sediments are due to the contributions of Mejerda in the sub-base and endogenous sedimentation in the upper layer. Therefore, the two layers gave information about this phenomenons history. Potential redox (Eh) increases with depth whereas organic carbon (TOC) and monosulfides (AVS) decrease. SEM/AVS molar ratio (reactive metals: AVS) was less than 1 revealing the importance of sulfides in scavenging metals in these sediments and essentially in the upper layer.

Evaluation of metal contamination in a changed sedimentary environment: Ghar El Melh Lagoon, Tunisia.

Abstract Three cores were collected in the lagoon of Ghar El Melh (Northern Tunisia) and the sediments were analysed for trace and major elements, sulfides acid volatile sulfides (AVS) and pyrite and total organic carbon (TOC). The sediments are composed of black-mud in the upper layer (0–10cm depth) and grey-mud underneath. Based on the lagoon history, it was believed that the black-mud is endogenic, while the grey-mud is exogenic (derived from old contributions of the Mejerda River before its diversion). The concentrations of TOC and AVS decrease with depth, while the redox potential (Eh) is negative in the black-mud and positive in the grey-mud. The Eh measurements thus revealed the singularity of the sedimentation mode in this lagoon. The North American Composite Shale (NASC) normalisation indicated that Cd, Zn and Pb were enriched through the entire profiles, indicating that this sediment was contaminated for a long time by mining activities and human pollution. Trace metal profiles of Fe, Cd, and Cu approximated that of TOC, while the profiles of Mn, Co, Pb, Ni and Zn followed the Eh. These results, confirmed by the principal component analysis (PCA), suggested that some metals can accumulate in the reduced sediment, while others accumulate in the sub-oxic sediment. Such inference is supported by the metal chemical speciation, which showed these metal–sediment component associations: Mn, Co, Ni, Pb and Zn to the Mn-oxi-hydroxide fraction, Fe to the residual and organic–sulfide-fractions, Cu to the organic–sulfide fraction and Cd to carbonates and sulfides.

Bioavailability and assessment of heavy metal pollution in sediment cores off the Mejerda River Delta (Gulf of Tunis): How useful is a multiproxy approach ?

Three core samples were taken from zones offshore from the Mejerda River Delta (Tunisia) and analyzed for major and trace elements to assess their relationships with organic matter, monosulfides and carbonates, as well as for pollution and bioavailability. Chemical speciation, ∑ SEM/AVS, the enrichment factor (EF) and the geo-accumulation index (I-geo) were used. Iron, cadmium, lead and zinc - the most frequently mined metals in the Mejerda catchment - were found as contaminants in the offshore areas. Estimations of trace element accumulation using the EF and the I-geo index show that lead, and to a lesser extent zinc, are the most polluting metals off the Mejerda outlet. According to their bioavailability, these metals are also the most toxic. Only cadmium is heavily present in delta sediment (EF>100) though deeply sequestrated (100% bound to the residual fraction) and thus presents no toxicity.

Metal origin and Pb isotopes in water of the mine-draining Mejerda river system, north Tunisia

ABSTRACT Water samples (< 0.45 μm) from the Mejerda hydrographic system were analysed for trace metals and Pb isotopes, together with Pb isotope measurements of galena from the mines of the system. Water was collected (i) within the mines, (ii) in tributaries lying as close as possible to the mines, (iii) along the tributaries, (iv) along the main Mejerda River, and (v) in dammed lakes. All metal concentrations measured in the waters are below the limits given for health purposes of surface water, though some of them are close. Metals such as Al, V, Ni, Cu, Pb, and U are slightly enriched in the uppermost part of the system but they do not exceed 20 ppb (μg l−1), with the exception of Al reaching 120 ppb. In the lower Mejerda sector, a few high Zn concentrations of up to 550 ppb are observed, and up to 80 ppb for Ba. Lead isotopic compositions of 41 water samples show large variations, yielding ratios at 17.736–18.884 for 206Pb/204Pb, 15.504–15.977 for 207Pb/204Pb, and 37.135–39.992 for 208Pb/204Pb. In contrast, Pb isotopic compositions of 11 galena samples from different mines define a surprisingly narrow field, being systematically more radiogenic in 206Pb than water. Compared to water and galena, Pb in Tunisian gasoline has a very different and much less radiogenic Pb isotopic composition at 16.351, 15.493, and 36.135, respectively. No significant contribution of such gasoline Pb can be detected either in the main Mejerda River or the numerous tributaries. The same observation is made for galena Pb, with the particularity that water lying closest to the isotopic compositions of galena was sampled in the lower part of the system where mines are absent. For the upper part of the system, minor contributions from the mines cannot be ruled out. The full set of Pb isotopic data substantiates the ultimate sources of Pb to have evolved with large variations in time-integrated U/Th/Pb ratios, including differences in mantle extraction ages. Simple mixing between two end-members can be ruled out for most of the water Pb data. It is therefore suggested that Pb and other metals of Mejerda water originate from local soils, from where they can be leached from alteration phases. In contrast to water showing a wide variation of ultimate source lithologies, the particularly narrow field of galena Pb isotopic compositions requires Pb extraction from a surprisingly homogenous and strongly U- and Th-depleted reservoir. This reservoir is most likely the Triassic evaporites, essentially gypsum, forming in a well mixed water volume devoid of significant detrital input.

Trace element accumulation and elutriate toxicity in surface sediment in northern Tunisia (Tunis Gulf, southern Mediterranean)

Metal concentrations in sediments were investigated in the Gulf of Tunis, Tunisia, in relation to anthropic activities along the Mejerda River and Ghar El Melh Lagoon, with effluents discharged into the gulf. Distribution of grain size showed that the silty fraction is dominant with 53%, while sand and clay averages are 34 and 12% respectively. Zn concentration increased in the vicinity of the Mejerda River while Pb was at its highest levels at the outlet of Ghar El Mehl Lagoon. Sediment elutriate toxicity, as measured by oyster embryo bioassays, ranged from 10 to 45% abnormalities after 24h, but no relation was found between metal concentration and sediment toxicity. The AVS fraction that represents monosulfide concentrations in the sediment was higher in the central part of the gulf than in the coastal zone. The results reveal the influence of AVS, TOC and grain size on metal speciation and sediment toxicity.

Sulfide influence on metal behavior in a polluted southern Mediterranean lagoon: implications for management

The degree of pyritization and degree of trace metal pyritization (DTMP) were investigated in sediments from Ghar El Melh Lagoon (northern Tunisia) in order to study metal deposition. A sediment core and 28 samples were thus taken in summer 2008, and metals and sulfate were analyzed in pore water/pyrite. Acid-volatile sulfide and metals were simultaneously extracted from these two fractions and the role of pyrite in the metal cycling studied. To examine pyrite presence and mineralogical form in sediments, X-ray diffraction of the washed and decarbonated sediment was performed along with scanning electron microscopy. Results showed that pyrite is present in fromboidal and euhedral forms. Thermodynamic calculation highlighted the formation of metallic sulfides and the co-precipitation of metals with iron sulfides. The DTMP increases with depth, indicating that these metals are either sequestered as sulfides or that they co-precipitate with pyrite into the deep sediment.

Validity of Drastic and SI Vulnerability Methods

The phreatic aquifer of Oued Gueniche (prefecture of Bizerta, northeast of Tunisia) which occupies an area of 83 km2 has a great economical importance because it is used for irrigation and domestic consumption. The area of the aquifer is essentially occupied by agricultural zones, characterised by an increasing use of chemical fertilizers. Those chemical fertilizers threaten the quality of the ground waters. The study of the vulnerability to pollution of this aquifer was made by applying two vulnerability methods: the generic DRASTIC which is an intrinsic vulnerability method, and the Susceptibility Index (SI) which is a specific vulnerability to agricultural pollution method. This study employed the Geographical Information System (GIS) technology as a system for the acquisition, storage, analysis and display of geographic data. The validity of the two methods to agricultural pollution by nitrates was verified by comparing the distribution of nitrates in the groundwater with the distribution of the different vulnerability classes. That comparison demonstrated that the SI method is the more valid method in the studied system.