Najiba Chkir

Humidity changes in southern Tunisia during the Late Pleistocene inferred from U–Th dating of mollusc shells

Abstract Calcareous deposits, mainly consisting of mollusc shell accumulations, which have been dated by the U/Th disequilibrium method, mark the shorelines of paleolake highstands in the Great Chotts Area of Southern Tunisia. The 5 sites studied consist of discontinuous accumulations of fossils of marine-like organisms e.g.: Cerastoderma glaucum, Melania tuberculata, Melanopsis praemorsa, Cerithium rupestre. U/Th isochron plots and age frequency histograms for 39 shell samples are reported here. Limited variations for U content and 234U/238U activity ratios (AR) of shells support the hypothesis of closure of the geochemical system with respect to this element. It is remarkable that 234U/238U AR of shells collected in Chott Fejej or Chott Jerid are clustered around different values, reflecting probably different groundwater recharge from the Continental Intercalaire (CI) or Complexe Terminal (CT) aquifers. Furthermore waters collected near Wadi el Akarit show 234U/238U AR values comparable to those observed for shells. 14C determinations made on aliquots of some of these samples suggested an age distribution between 18 and 34 ka BP. The U/Th data of these 39 shell samples imply that 4 distinct flood episodes of these lakes occurred at about 30, 95–100, 130–150 and 180–200 ka. For the episode centred around 30 ka, the frequency histogram of ages shows a multimodal age group that could represent the existence of several humid pulses rather than a unique event. Moreover, the comparison of δ13C and δ18O with those of older humid Pleistocene phases, when very large palaeolakes have been recorded, suggests that these young carbonate shells are not related to a true highstand lake. It is suggested that they represent a period of less humid climatic conditions with carbonate accumulation in minor water ponds in which intensive biological activity could have taken place. It should be noted that this period was less arid than the present.

Uranium isotopes in groundwater from the continental intercalaire aquifer in Algerian Tunisian Sahara (Northern Africa)

The disequilibrium between (234)U and (238)U is commonly used as a tracer of groundwater flow. This paper aims to identify uranium contents and uranium isotopic disequilibria variation in groundwater sampled from deep Continental Intercalaire aquifer (southern Algeria and Tunisia). Large variations in both U contents (0.006-3.39ppb) and (234)U/(238)U activity ratios (0.4-15.38) are observed. We conduct a first assessment in order to verify whether the results of our investigation support and complete previous hydrogeological and isotopic studies. The dissolved U content and (234)U/(238)U activity ratio data were plotted on a two-dimensional diagram that was successfully utilized on sharing the CI aquifer into different compartments submitted to different oxidising/reducing conditions and leads also to distinguished two preferential flow paths in the Nefzaoua/Chott Fejej discharge area. Uranium isotopes disequilibrium indicate that ranium chemistry is mainly controlled by water-rock interaction enhanced by long residence time recognised for this aquifer.

Tomographic, hydrochemical and isotopic investigations of the salinization processes in the oasis shallow aquifers, Nefzaoua region, southwestern Tunisia

An electrical imaging tomography survey was carried out to identify the lateral and vertical salinity distribution in the oasis shallow aquifers of the Nefzaoua region located in southwestern Tunisia. In addition, hydrochemical and isotopic data were examined to determine the main factors and mechanisms controlling the groundwater chemistry and salinity. Locally, with respect to salinization processes, electrical imaging tomography results show that the storage basins of irrigation excess-water contribute to the increase of the salinity for the major part of the oasis nearby these basins. Major elements distribution and saturation indices indicate that dissolution of evaporites (halite, anhydrite and gypsum) is the main process controlling the groundwater mineralization. Isotopic data highlighted the effect of evaporation in the salinization of these waters. The correlation between the oxygen 18 and the chlorides data confirms the importance of evaporation effect and dissolution as main processes controlling the groundwater mineralization.

Nouvelles données paléoenvironnementales pour le Quaternaire récent en Tunisie centrale (bassin de Maknassy)

Abstract Sedimentary deposits outcropping on the Wadi Leben and Wadi Ben Sellam banks in the Maknassy Basin (central Tunisia) have registered paleoclimatic variations, and particularly humid episodes, already known for Upper Pleistocene and Holocene, in present hyperarid areas of northern Sahara. A pluridisciplinary study, including prehistory, sedimentology, mineralogy, ecology and radiochronology ( 14 C, uranium-thorium [U-Th]), improves the comprehension of paleoenvironmental interpretations and makes it possible to precisely determine the chronological framework of humid episodes during the Upper Quaternary in Tunisia.

Hydro-geochemical behaviour of two coastal aquifers under severe climatic and human constraints: comparative study between Essaouira basin in Morocco and Jeffara basin in Tunisia

Since surface waters are mostly irregular and rare, groundwater in arid and semi-arid regions are submitted to high human constraints enhanced by severe climatic conditions. Multiple isotope analyses and chemical tracing of groundwater from the Essaouira basin (South-Western Morocco) and from the Jeffara basin (South-Eastern Tunisia) reveal that salinisation processes are mainly related to natural conditions such as dissolution of evaporate rocks (gypsum and halite minerals) along the recharging outcrops. However, since these basins are coastal, seawater intrusion is one of multiple salinity sources that could affect the quality of groundwater if intense exploitation goes on. The comparison of hydro-geochemical and isotopic data available for the two basins provide a framework for a comprehensive diagnostic in which different states of risks could be defined according to climatic and human constraints. We argued that these heavily exploited coastal aquifers have exacerbated vulnerability given relatively low current recharge and salinisation processes.

Hydrochemical and Isotope Evidence of Groundwater Contamination of Cultivated Fields of Semi-Arid Environments in Tunisia

This present study, which uses geochemical and isotope hydrological approaches, attempts to provide appropriate scientific information about the dominant geochemical processes that influence groundwater chemical composition in a semi-arid region of cultivated field. Therefore, particular emphasis has been placed on (1) the evolution of solute concentrations within the hydrological system as well as in the vadose zone, (2) the contribution of anthropogenic processes to the groundwater contamination, and (3) the estimation of groundwater recharge deriving from return flow of irrigation water. It has been demonstrated that the excess of irrigation waters has significantly influenced the rate of groundwater recharge. The annual amount of groundwater recharge related to the irrigation practices is approximately 100 mm. Modification of the geochemical characteristics of the Zeroud aquifer groundwater is caused by water-rock interaction, including both the dissolution of evaporate and the cation exchange process. However, anthropogenic processes related to the return flow of irrigation waters have led to extremely high nitrate concentrations and flushing of solutes, such as chloride, sulfate, sodium, and calcium. The isotopic signature provides evidence of groundwater contamination. Indeed, the relatively enriched stable isotope concentrations and low tritium concentrations highlight the important contribution of irrigation waters to the local groundwater recharge, particularly in the Menzel Mhiri agricultural area.

Geochemical and isotopic investigation of the aquifer system under semi-arid climate: case of Essaouira basin (Southwestern Morocco)

Detailed hydrochemical and stable isotopic data of groundwater were used to understand the geochemical processes occurring in the Wadi Ouazzi basin in Southwestern Morocco. Hydrogeochemical investigations show that the evaporite dissolution (halite and gypsum), followed by cation-exchange reactions with reservoir matrix and by sea spray, constitute the main processes that control groundwater mineralization. Data inferred from 18-oxygen and deuterium isotopes in groundwater samples indicated recharge with modern rainfall. Water characterized by lower 18O and 2H values is interpreted as recharged by non-evaporated rainfall originating from Atlantic Ocean. Tritium contents, ranging between 0.18 and 3.43 TU, indicate that groundwater in both aquifers derive from post- and pre-nuclear recharge. The study of the carbon isotopes made it possible to estimate the water residence time of the two aquifers studied. Moreover, the calculated ages range from the modern to the late Pleistocene with the dominance of the modern age waters.

Improving the Hydrogeological Conceptual Model of the Sidi Merzoug-Sbiba Aquifer System (North-West of Tunisia) Using Hydrochemistry and Isotopic Tools

The aim of the study is to improve the knowledge of the hydrogeological systems in the Sidi Merzoug Sbiba Basin (Northwestern Tunisia), using chemical and isotopic tools. Three major aquifers have been identified in this semi-arid region by previous hydrogeological studies: the Cretaceous, the Miocene and the Plio-Quaternary aquifers. Its hydrodynamic regime is largely influenced by tectonics, lithology and recharge conditions. Given the heterogeneity of the multilayer aquifer system in a complex fractured zone, an hydrochemical and environmental isotope (2H, 18O,3H and14C) data were used to caracterised the groundwater flow and provide valuable information about the geochemical processes controlling groundwater quality and the circulation patterns of the different groundwater bodies. Three major processes control the chemical composition: i) dissolution of carbonate minerals, ii) cation exchange reactions and iii) evaporation process. Stable isotopes indicate that most groundwater samples originate from infiltration of modern precipitation. A significant infiltration before evaporation takes place, indicating a major recharge directly from Cretaceous and Miocene formations of surrounding mountains and infiltration of surface water in the El Breck and Sbiba rivers. Downstream, the isotopic signature of evaporated water clearly indicates a recharge from rivers, irrigation fields or Sbiba dam. Tritium and 14C contents confirm the existence of modern groundwaters in the South-Western border and in the North -Eastern part of the basin and confirm the stratification of the system.