Three sediment cores from dredged and undredged areas of the Northern Lake of Tunis, a Mediterranean lagoon in northern Tunisia, were used to investigate the behavior of nutrients related to diagenetic reactions in the sediment and to assess the release of reduced nitrogen and phosphorus from the surface sediment to the water, 30 years after dredging. The degradation of organic matter and the resulting N and P fluxes are greater in the undredged area than the dredged area, mainly due to the lability of fresh organic matter in the undredged site compared to the older refractory organic matter at the bottom of the dredged sites. The biological stability of microbial communities in sediment at the undredged site may play a significant role in the microbial mineralization of organic matter. A comparison with the pre-dredging sediments shows that the influence of dredging is very remarkable over time. It has effectively reduced organic matter content (TOC levels after dredging were 2 to 9.5 times lower) and, consequently, the fluxes of reduced nitrogen and phosphorus species across the sediment-water interface. Fluxes of ammonium and phosphorus decreased approximately 150 times after dredging. The improvement in the water quality of the lagoon following dredging and the resulting oxygenation of the water promote the nitrification of ammonium ions into nitrites and nitrates in the surface layer of the sediment, thereby trapping the reducing species resulting from the anaerobic mineralization of organic nitrogen at depth.

Diagenetic reactions occurring in the sediments of Bizerte Lagoon along with their seasonal variation were assessed through examining the concentration profiles of nutrients and dissolved Fe and Mn in the supernatant water and pore waters, as well as the profiles of total organic carbon, carbonates, Fe and Mn contents in the particulate phase. The results indicate that these diagenetic reactions followed the classical sequence of oxidant reduction following the degradation of organic matter with a successive reduction of NO3−, MnO2, Fe (OH)3 and SO42−. In particular, a delay was observed in the winter sequence which occurred at greater depth due to higher Eh in surface layer. Generally, NO3−, NH4+ and PO43− seem to originate from sediments as reflected by their measured flux from sediments to the water column while the reverse was quoted for SO42− as reported from other Mediterranean environments. In addition, oxygenation of the water column reduces the diffusive fluxes of (NO3−, NH4+, PO43−) released into the pore water during mineralization of organic matter.

Bizerte Lagoon is a vital Mediterranean ecosystem subjected to intense anthropogenic pressure. The potential ecological risk caused by certain metals (Zn, Cu, Cr, Cd, Ni, Pb and Mn) is assessed from the data carried out in the sediment and pore water at two sites along with identifying the effects of diagenetic processes on the vertical distribution of these metals and their resulting diffusive fluxes. Using various ecological indices our results reveal a high ecological risk to benthic organisms from metals chiefly Cd, Pb and Ni accumulated in the sediment at both sites. Metals derived from organic matter degradation (Cu and Cd) and/or reduction of Mn-Fe-oxyhydroxydes (Pb, Ni, Zn, Cr) due to early diagenetic processes in sediment. The resulting concentration gradients between pore water and overlying water induce diffusive fluxes of metals to the water column. The estimation of the potential ecological risk caused by dissolved metals in pore waters by application of the Interstitial Water Criteria Toxic Units index indicates a slight ecological risk by Cu that was not identified in sediment. The ecological risk posed by dissolved metals is evidenced from -6 cm depth, which reduces the possibility of contamination of benthic species living above the water-sediment interface by diffusive fluxes of these metals.

Deltaic sediments are important for biogeochemical metal cycling since they are hotspots for metal inputs. In addition, they are potential sites for diagenetic processes leading to either the burial of inorganic contaminants or their release. Diffusive fluxes of certain metals (Fe, Mn, Pb, Zn, Cu and Cd) in the sediments of the Mejerda River Delta (MRD) (Gulf of Tunis, Tunisia) were quantified by modeling the available concentration profiles in the pore water. The metals’ burial and sedimentation fluxes were also calculated using both the asymptotic concentrations of available metal profiles and sediment trap results. These fluxes were assembled with the exchange fluxes at the sediment-water interface in order to develop complete metal transfer budgets. The results showed that budgets of Cu and Zn are almost neutral. The sediment appears to be a good trap for iron since its average burial flux at the three studied stations is about 332.6 g m−2 year−1. Organic matter degradation, carbonate dissolution, and oxyhydroxide reduction are the main mechanisms which accelerate the release of metals associated with the suspended particle matter once they reach the pore water in the seabed.

Coastal lagoon sediments are important for the biogeochemical carbon cycle at the land-ocean transition, as they form hotspots for organic carbon burial, as well as potential sites for authigenic carbonate formation. Here, we employ an early diagenetic model to quantify the coupled redox cycling of carbon, iron and sulphur in the sediments of the shallow Ghar El Melh (GEM) lagoon (Tunisia). The model simulated depth profiles show a good correspondence with available pore water data (dissolved inorganic carbon, NH4+, total alkalinity, Ca2+, Fe2+ and SO42-) and solid phase data (organic matter, pyrite, calcium carbonate and iron (oxyhydr)oxides). This indicates that the model is able to capture the dominant processes influencing the sedimentary biogeochemical cycling. Our results show that sediment of the GEM lagoon is an efficient reactor for organic matter breakdown (burial efficiency < 10%), with an important role for aerobic respiration (32%) and sulphate reduction (61%). Despite high rates of sulphate reduction, free sulphide does not accumulate in the pore water, due to a large terrestrial input of reactive iron oxides and the efficient sequestration of free sulphide into iron sulphide phases. High pyrite burial (2.2 mmol FeS2 m(-2) d(-1)) prevents the reoxidation of reduced sulphide, thus resulting in a low total oxygen uptake (4.7 mmol m(-2) d(-1)) of the sediment and a relatively high oxygen penetration depth. The formation of pyrite also generates high amounts of alkalinity in the pore water, which stimulates authigenic carbonate precipitation (2.7 mmol m(-2) d(-1)) and leads to alkalinity release to the overlying water (3.4 mmol m(-2) d(-1)). Model simulations with and without an N-cycle reveal a limited influence of nitrification and denitrification on overall organic matter diagenesis. Overall, our study highlights the potential role of coastal lagoons for the global carbon and sulphur cycle, and their possible contribution to shelf alkalinity, which increases the buffering capacity of the coastal ocean for CO2 uptake.

Tunis’s Lake North (LNT), located on the Mediterranean Sea, and Tunisia’s most important lagoon due to its economic value and its strategic position within the city, has recently undergone a vast sanitation project. To study the lagoon’s level of metal pollution, three sediment cores were taken and the sediments were analyzed for trace and major elements, acid volatile sulfides (AVS), total organic carbon (TOC), and minerals. Results showed that TOC concentrations (0.2–3.1%) decreased following the lagoon’s restoration. In addition, in comparison to historical data, concentrations of Cu (16–69.5 μg g−1), Zn (60.6–191.4 μg g−1), and Pb (13–100.9 μg g−1) also decreased. Enrichment factor calculation with respect to the crust and local background showed that the sediment had long been contaminated by human pollution and especially by Pb, Zn, and Cu. The AVS to simultaneously extracted metal (SEM) ratio revealed values generally less than 1 indicating no sediment toxicity risks. Statistics revealed a detrital origin for certain metals and a diagenetic origin for FeS2 and carbonate minerals.

To examine the state of pollution of Bizerte Lagoon which is exposed to intense anthropogenic pressure, two sediment cores were taken at two sites, one undergoes the dual effects of both marine waters arriving from the Mediterranean Sea through the Channel, and also of freshwater from the Tinja River; the other core is located at the center of the lagoon where water depth is maximal (12 m). Heavy metal concentrations in the two cores were assessed, with calculated enrichment factors and geo-accumulation indexes. Core sediments were also studied for chemical speciation and their monosulfide contents were measured. Results from enrichment factors and geo-accumulation indexes show an accumulation of Cd, Zn, Cr, and Pb, while chemical speciation revealed a risk only from Cd and Mn. Comparison of sequential extraction values with those of acid volatile sulfides revealed that non-toxic effects may be caused by any of the studied metals in the sediment.

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.

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.

A study of suspended particulate matter (SPM) fluxes along with their associated metals, organic matter and carbonates, was conducted off the Mejerda River outlet in May 2011 and in March and July 2012 at depths of 10, 20 and 40m using sediment traps. SPM fluxes are more significant near the Mejerda outlet, especially in winter, but dissipate further offshore. Normalization reveals that the Mejerda is a major source of Pb, Zn, Cd, Cu, Ni, and Co, all of which are the result of human activities. In contrast, Fe, Mn and N are of authigenic origin. The enrichment factor shows that Pb, Zn and especially Cd are the most highly polluting metals off the Mejerda outlet. This confirms the trend observed on the shores of the Mejerda prodelta and is consistent with the type of mining activities conducted in the Mejerda catchment.

Fluxes of suspended particulate matter (SPM) and their associated metals were performed off the Mejerda River Delta during both the wet (March) and the dry (July) seasons in 2012, using sediment traps at study stations at depths of 10, 20 and 40 m. Fluxes nearest to the Mejerda outlet were more significant, especially during winter (36 g m(-2) day(-1)), but dissipated further offshore, 24.5-6 g m(-2) day(-1) at the 20 m and 21.8-4.8 g m(-2) day(-1) at the 40 m stations. Many variations observed in seasonal and spatial metal fluxes are similar to those of SPM, in particular Pb and Zn, probably because they are associated with the mining activity characteristic of the Mejerda catchment. Chemical speciation reveals that most of the metals (20-100%) are bound to the residual fraction. The most toxic metals (Pb, Zn) are bound in part to the exchangeable fraction (20-50% for Pb and 5-15% for Zn) making them relatively bioavailable and therefore potentially toxic. While Cu and Cd fluxes are not always clearly established according to season, both metals are apparently sequestered deep in the sediment, bound especially to clays (40-80% for Cd and up to 100% for Cu). (C) 2016 Elsevier Ltd. All rights reserved.

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.

In situ benthic flux of oxygen, alkalinity, and nutrients was assessed at the water–sediment interface using benthic chambers at three stations in the Medjerda River Delta (Tunisia) at 10, 20 and 40 m depth during March and August 2012. Simultaneously, three sediment cores were taken at the same locations to determine the diffusive flux of dissolved inorganic nitrogen (DIN = NO2 −, NO3 −, NH4 +) and PO4 3− with a Technicon Autoanalyzer III to estimate diagenetic mechanisms occurring below the sediment–water interface. Oxygen consumption at the interface is about 1.7–10 mmol/m2/day essentially controlled by organic matter degradation and oxidation of reduced elements. Nitrate concentration is relatively high in the sediment (above 140 μM for NO3 −), and its production did not always conform to the general scheme of early diagenesis. NH4 + and PO4 3− were released at a rate of 96.5 and 1.27 μmol/m2/day, respectively, consistent with those measured in benthic chambers, but amounting to <30 % of benthic fluxes for NH4 + and <5 % for PO4 3−.

An in situ benthic flux study of iron (Fe), manganese (Mn), lead (Pb), zinc (Zn), cadmium (Cd), copper (Cu) and cobalt (Co) was conducted at three stations at the Mejerda River outlet in the Gulf of Tunis, at depths of 10, 20 and 40 m in March and August 2012. Simultaneously, three sedimentary cores were taken at the same locations to evaluate the diffusive flux of these heavy metals and to determine the early diagenetic mechanisms occurring below the sediment–water interface and their impact on heavy metal mobility. The concentrations of Fe2+ in pore waters were similar to those observed in the rest of the Gulf of Tunis; flux, both benthic (275 µmol m−2 day−1) and diffusive (9 µmol m−2 day−1), confirmed that sediment was a source of Fe2+ for the water column, at least for the 10- and 20-m stations. The diffusive flux of Mn2+ was greater in August (11–14 µmol m−2 day−1) than in March (1–2.6 µmol m−2 day−1), representing 8–15 % of benthic flux and indicating that, as for Fe, sediment is a source of Mn2+ for the water column, especially in August. Mn2+ is produced before Fe2+, which corresponds to the overall pattern of early diagenesis. In March, the production of Mn2+ and Fe2+ takes place in the lower layers of the sediment, probably due to agitation and resuspension occurring at the sediment–water interface. The concentrations of Pb2+ and Zn2+ were higher in this study with reference to the corresponding concentrations in the coastal areas, revealing an anthropogenic activity on the Mejerda River, Ghar El Melh Lagoon (old Mejerda outlet) and the Khlij Channel as the main vectors.

The phreatic aquifer of Bekalta experienced a progressive degradation of water resources over time: using increasingly important waters for irrigation and drinking water, nitrate pollution, salinization... This aquifer is of great economic importance because it is used for irrigation and domestic consumption. Vulnerability map to nitrate pollution is a necessary tool to developing management to preserve the quality of groundwater. This study utilized the Geographic Information System technique and the DRASTIC model to assess the vulnerability of groundwater resources to contamination. The Geographic Information System (GIS) technology represents the best method to solve the main problems in the vulnerability survey. Indeed is allowed for swift organisation, quantification, and interpretation of large volumes of hydrological data with computer accuracy and minimal risk of human errors. The Visio model was exported and loaded into an ESRI Geodatabase in ArcCatalog as defined by the UML model. The purpose of this geodatabase is data harmonization process within modeling groundwater vulnerability to pollution. The resulting map shows evidence for three categories of vulnerability (low, middle and high). The resultant vulnerability map showed the predominant of moderately vulnerability class on the most of the Bekalta region which occupying an area of 68%. The low and high groundwater vulnerability classes occupy respectively an area of 30% and 2% of the total surface of the study area.

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.

DOI 10.4409/Am-006-10-0009 Abstract: The phreatic aquifer of Oued Guéniche located in the prefecture of Bizerta (northeast of Tunisia) and of the phreatic aquifer of Grombalia located in the prefecture of Nabeul (northeast of Tunisia) have a great economical importance because they are used for irrigation and domestic consumption. They occupy respectively areas of 83 km2 and 392 km2. Both aquifers encompass an area comprised mostly of 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 those aquifers 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 two aquifers with the distribution of the different vulnerability classes. That comparison demonstrated that the SI method is the more valid method in the studied systems.

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 (0-10 cm 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.

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 phenomenon's 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.

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.