Walid Oueslati

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.

Trace elements in different marine sediment fractions of the Gulf of Tunis (Central Mediterranean Sea)

Abstract Over several decades, the mouth of the Mejerda River (northern Tunisia) has received large amounts of mining tailings mainly containing Pb, Zn and Ba. We have measured the concentrations of major and trace elements (Al, Ti, P, K, Mg, Na, Ca, S, Fe, Cl, V, Cr, Co, Ni, Cu, Zn, Ga, Br, Rb, Sr, Y, Zr, Nb, I, Ba and Pb) in three different fractions of 45 surficial sediments using wavelength dispersive X-ray fluorescence. The size distribution of fine particles (<2 μm, 2–20 μm, 20–63 μm) revealed that clay and silt compounds were predominant (75–95%) in the deeper estuarine region. These condensed clay phases were mainly dioctahedral smectite, illite and kaolinite. The total organic carbon (TOC) concentration varies from 0.08 to 1.37 %. The C:N ratio varies from 1 to 17 indicating that the organic matter has a mixed marine and continental origin principal component analysis was used to determine the associations of trace elements with organic and/or inorganic phases of the sediment fractions. The correlations of Fe2O3, MnO2 and P2O5 with trace elements showed that amorphous Fe and Mn were the most active scavengers of dissolved metals from the Mejerda River. Cluster analysis clearly distinguished local trace element accumulations according to grain size fractions (< 2 μm and 2–20 μm) near the Ghar el Melh Lagoon and th River.

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.

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.

Methodology for the extraction of carbonate-bound trace metals from carbonate-rich soils: application to Lakhouat soils, Tunis, Tunisia

Abstract This study aims to optimise the method by which carbonate-bound trace metals are extracted during sequential extractions of carbonate-rich contaminated soils. Specifically, we aim to optimise the volume and concentration of sodium acetate that must be added to samples to completely dissolve carbonate, and hence to release into solution all trace metals associated with this phase. We apply a range of volumes and concentrations of sodium acetate to fixed masses of two raw soil samples (H1) and (H2) from the mining region of Lakhouat (Gouvernorat of Siliana Tunisia). The soils contain >80% carbonates, and carbonate-phase trace metals are therefore expected to be a major fraction of the total trace metal contamination. The applied range of volumes and concentrations of sodium acetate typifies those used in existing sequential extraction protocols. By X-ray diffraction analysis of the solid phase before and after extraction, we show that complete carbonate removal is guaranteed by application of 2 M sodium acetate, at 40 mL per gram of soil. Lower concentrations and volumes fail to completely dissolve soil carbonate. Atomic absorption analysis of the extracts shows concentrations of Zn and Pb to be highest in the samples from which the carbonate is completely removed, confirming the association of these metals with the carbonate phase.

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.

Trace Metals Accumulation in Surface Sediments of the Gulf of Gabes, Tunisia: Usefulness of Metal Sediment Fractionation

The gulf of Gabes is a special area in the Mediterranean known by its abundant marine wealth. The geological setting of the southern coast of Tunisia is characterized by an extended continental shelf.