Kamel Zouari

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.

Groundwater salinization processes in shallow coastal aquifer of Djeffara plain of Medenine, Southeastern Tunisia

Urban and industrial development and the expansion of irrigated agriculture have led to a drastic increase in the exploitation of groundwater resources. The over-exploitation of coastal aquifers has caused a seawater intrusion and has seriously degraded groundwater quality. The shallow coastal aquifer of the Djeffara plain, southeastern Tunisia constitutes an example of water resource suffering an intensive and uncontrolled pumping for irrigation. Intensive exploitation of the aquifer and climate aridity caused a decrease in piezometric level and an increase in salinity. According to the hydrochemical data (Cl−, SO42−, NO3−, HCO3−, Br−, Ca2+, Mg2+, Na+, K+) and the stable isotope composition (oxygen-18 and deuterium content), groundwater salinization in the investigated system is caused by three main processes: (i) salts dissolution especially in the central part of Jerba and around Medenine plain; (ii) evaporation process; and (iii) seawater intrusion which caused the increase in salinity in the peninsula of El Jorf, in Jerba and in the North of Ben Gardane.

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.

Relation entre nappes superficielles et aquifère profond dans le bassin de Sfax (Tunisie)

Abstract The study of the isotopic composition ( 18 O and 2 H) of groundwater collected in the Sfax basin (Tunisia), helped to understand the behaviour of the different aquifers. It showed that the groundwater in the deep aquifer is old, probably slow moving and recharged under a colder climate than at present. The increasing exploitation of the shallow aquifers probably favoured upward leakage from the deep aquifer. Isotope balance equations allowed us to estimate the contribution of the deep aquifer to the shallow aquifer recharge.

Integrated socio-hydrogeological approach to tackle nitrate contamination in groundwater resources. The case of Grombalia Basin (Tunisia)

Nitrate contamination still remains one of the main groundwater quality issues in several aquifers worldwide, despite the perduring efforts of the international scientific community to effectively tackle this problem. The classical hydrogeological and isotopic investigations are obviously of paramount importance for the characterization of contaminant sources, but are clearly not sufficient for the correct and long-term protection of groundwater resources. This paper aims at demonstrating the effectiveness of the socio-hydrogeological approach as the best tool to tackle groundwater quality issues, while contributing bridging the gap between science and society. An integrated survey, including land use, hydrochemical (physicochemical parameters and major ions) and isotopic (δ15NNO3 and δ18ONO3) analyses, coupled to capacity building and participatory activities was carried out to correctly attribute the nitrate origin in groundwater from the Grombalia Basin (North Tunisia), a region where only synthetic fertilizers have been generally identified as the main source of such pollution. Results demonstrates that the basin is characterized by high nitrate concentrations, often exceeding the statutory limits for drinking water, in both the shallow and deep aquifers, whereas sources are associated to both agricultural and urban activities. The public participation of local actors proved to be a fundamental element for the development of the hydrogeological investigation, as it permitted to obtain relevant information to support data interpretation, and eventually guaranteed the correct assessment of contaminant sources in the studied area. In addition, such activity, if adequately transferred to regulators, will ensure the effective adoption of management practices based on the research outcomes and tailored on the real needs of the local population, proving the added value to include it in any integrated investigation.

Hydrogeochemical investigations of recharge and subsequent salinization processes at Al-Raudhatain depression in Kuwait / Analyses hydrogéochimiques de la recharge et des processus associés de salinisation dans la dépression de Al-Raudhatain au Koweit

Abstract Fresh groundwater is uniquely present in the northern parts of Kuwait at Al-Raudhatain and Umm Al-Aish depressions, where it occurs in the form of lenses floating on top of Kuwait Groups brackish water. Earlier studies have provided a wealth of data on these depressions. Nonetheless, information on the recharge process is both scarce and scattered among different studies. Based on existing information, chemical and isotopic investigations, and geochemical modelling, a coherent description of the recharge and salinity evolution processes at Al-Raudhatain freshwater lenses is presented. Radioactive isotope (14C, 13C and 3H) results indicate that the groundwater sampled at the depression and its vicinity contain significant portions of recharge from recent (less than 500 years) rainfall events. Furthermore, the results of stable isotopes (18O and 2H) specify that recharging rainfall events took place during the last three to four decades. Examining the relationship of 18O/2H along with the Emirates meteoric water line (i.e. the nearest available meteoric water line) reveals that samples are showing sings of insignificant evaporation, which indicates rapid infiltration. Given the established low infiltration rates at the depression and the lack of surface runoff at the wadis, it was concluded that most of the recharge comes from infiltration outside the depression, followed by subsurface runoff to the depression where recharge takes place. Reverse geochemical modelling showed that the main processes controlling the geochemistry of the water in the depression are halite, albite, illite dissolution, as well as K-feldspar weathering and calcite precipitation. These processes take place during infiltration, subsurface runoff and percolation to the groundwater. The contribution of the recharge water in the makeup of the body of the lens versus the regional brackish groundwater was estimated according to their contribution in total dissolved solids concentrations using the salinity stratification and the results of 18O.

Hydrogeochemical Analysis of Groundwater Pollution in an Irrigated Land in Cap Bon Peninsula, North-Eastern Tunisia

Situated in semi-arid regions of northern Africa, the Nabeul-Hammamet basin, North-eastern Tunisia, is a typical alluvial plain, where rural population relies exclusively on groundwater as a water-supply source. Major ions hydrochemistry was used in conjunction with hydrogeological data to understand the aquifer hydrodynamic functioning and to identify natural and anthropogenic salinisation processes. An attempt has been made to recognize these processes using Principal Components Analysis (PCA) and bivariate diagrams of major element data. The groundwater composition is extensively modified by the water–rock reaction in the subsurface, that is, dissolution of halite, gypsum, and/or anhydrite; and ion-exchange with phyllosilicates. Nevertheless, overall groundwater samples are characterized by relatively high nitrate contents suggesting that the return flow of irrigation waters is a significant source of the groundwater contamination.

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.

Deciphering groundwater flow between the Complex Terminal and Plio-Quaternary aquifers in Chott Gharsa plain (southwestern Tunisia) using isotopic and chemical tools

Abstract The Complex Terminal (CT) and Plio-Quaternary (P-Q) aquifers in the Chott Gharsa plain in southwestern Tunisia have been investigated with the aid of chemical and isotopic tools. It has been demonstrated that groundwater from the CT is mainly of palaeo-origin, especially in the western and central parts of the plain where the most negative values of δ18O and δ2H were observed (between −8.1 and −7.6‰ for δ18O, and −60 to −57‰ for δ2H), combined with low concentrations of radiocarbon (6.8–7.5 pmc) and absence of tritium. Modern recharge of the aquifer occurs only in the eastern part of the system where younger waters were observed, as indicated by their stable isotope composition, relatively high radiocarbon content and presence of tritium. Groundwater from the P-Q multi-layer aquifer represents mixtures of ascending deep CT waters and modern water recharging the P-Q aquifer system. Isotope mass balance was used to quantify mixing proportions. The calculations showed that the contribution of deep CT groundwater to the P-Q aquifer system reaches about 75% in the western and central parts of the plain where the CT aquifer remains strongly artesian. This contribution decreases to about 15% towards the eastern part of the plain, as a consequence of significant reduction of artesian pressure in this area of the CT aquifer. Chemical data suggest that mineralization of the studied groundwater systems is controlled mainly by dissolution of evaporative minerals (halite, anhydrite and gypsum) and cation exchange reactions with the matrix, possibly enhanced by recent anthropogenic disturbance of the system caused by lowering of the water table due to heavy exploitation and return flow of saline irrigation water into the P-Q aquifer. Editor D. Koutsoyiannis; Associate editor E. Custodio Citation Yangui, H., Abidi, I., Zouari, K., and Rozanski, K., 2012. Deciphering groundwater flow between the Complex Terminal and Plio-Quaternary aquifers in Chott Gharsa plain (southwestern Tunisia) using isotopic and chemical tools. Hydrological Sciences Journal, 57 (5), 967–984.