Abdallah Ben Mammou

Groundwater recharge study in arid region: An approach using GIS techniques and numerical modeling

To demonstrate the capabilities of Geographic Information System (GIS) techniques and numerical modeling for groundwater resources development in arid areas, specifically for the demarcation of suitable sites for the artificial recharge of groundwater aquifers, a study was carried out in the Maknassy basin, which is located in Central Tunisia. Thematic maps were prepared using a Hydrogeological Information System. All of the thematic layers were integrated using an ARCVIEW based model, enabling a map showing artificial recharge zones to be generated. Meanwhile, a ground water model, MODFLOW-2001, was used to estimate the effect of such water recharge on the piezometric behavior of the hydrological system. Additionally, these simulations helped manage ground water resources in the study area. The GIS-based demarcation of artificial zones developed in this study was based on logical conditions and reasoning, so that the same techniques, with appropriate modifications, could be adopted elsewhere, especially in similar aquifer systems in arid areas where the occurrence of groundwater is restricted and subject to a greater complexity. The efficiency of artificial recharge may be tested using hydrogeological modeling by simulating the effect of a potential groundwater refill.

GIS-based subsurface databases and 3-D geological modeling as a tool for the set up of hydrogeological framework: Nabeul–Hammamet coastal aquifer case study (Northeast Tunisia)

The subsurface data are a basic requirement for the set up of hydrogeological framework. Geographic information systems (GIS) tools have proved their usefulness in hydrogeology over the years which allow for management, synthesis, and analysis of a great variety of subsurface data. However, standard multi-layered systems are quite limited for modeling, visualizing, and editing subsurface data and geologic objects and their attributes. This paper presents a methodology to support the implementation of hydrogeological framework of the multi-layered aquifer system in Nabeul–Hammamet (NH) coastal region (NE, Tunisia). The methodology consists of (1) the development of a complete and generally accepted hydrogeological classification system for NH aquifer system (2) the development of relational databases and subsequent GIS-based on geological, geophysical and hydrogeological data, and (3) the development of meaningful three-dimensional geological and aquifer models, using GIS subsurface software, RockWorks 2002. The generated 3-D geological models define the lithostratigraphy and the geometry of each depositional formation of the region and delineate major aquifers and aquitards. Where results of the lithologic model revealed that there is a wide range of hydraulic conductivities in the modeled area, which vary spatially and control the groundwater flow regime. As well, 17 texturally distinct stratigraphic units were identified and visualized in the stratigraphic model, while the developed aquifer model indicates that the NH aquifer system is composed of multi-reservoir aquifers subdivided in aquifers units and separated by sandy clay aquitards. Finally, this study provides information on the storing, management and modeling of subsurface spatial database. GIS has become a useful tool for hydrogeological conceptualization and groundwater management purposes and will provide necessary input databases within different groundwater numerical models.

Characterization of Recharge Mechanisms and Sources of Groundwater Salinization in Ras Jbel Coastal Aquifer (Northeast Tunisia) Using Hydrogeochemical Tools, Environmental Isotopes, GIS, and Statistics

Groundwater is among the most available water resources in Tunisia; it is a vital natural resource in arid and semiarid regions. Located in north-eastern Tunisia, the Metline-Ras Jbel-Raf Raf aquifer is a mio-plio-quaternary shallow coastal aquifer, where groundwater is the most important source of water supply. The major ion hydrochemistry and environmental isotope composition (δ18O, δ2H) were investigated to identify the recharge sources and processes that affect the groundwater salinization. The combination of hydrogeochemical, isotopic, statistical, and GIS approaches demonstrates that the salinity and the groundwater composition are largely controlled by the water-rock interaction particularly the dissolution of evaporate minerals and the ion exchange process, the return flow of the irrigation water, agricultural fertilizers, and finally saltwater intrusion which started before 1980 and which is partially mitigated by the artificial recharge since 1993. As for the stable isotope signatures, results showed that groundwater samples lay on and around the local meteoric water line LMWL; hence, this arrangement signifies that the recharge of the Ras Jbel aquifer is ensured by recent recharge from Mediterranean air masses.

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.

Identification of major sources controlling groundwater chemistry from a multilayered aquifer system

Abstract The Maknassy basin is a typical example of an arid area in Central Tunisia. The geology of the area is mainly composed of Cretaceous calcareous formations. To identify the major process involved in controlling the groundwater chemistry, 53 groundwater samples for three different aquifer levels have been examined. The groundwater chemistry is dominated by SO4 (Na + Mg) and (Cl + SO4). The presence of (Ca + Mg) and (Na + K) with HCO3 indicates the domination of dolomitic rock weathering as the major source of cations. The plots for Na to Cl indicate a fine correlation and reflects their release from halite dissolution during groundwater transition. An ion exchange process is also activated in study area which is indicated by a shift to the right in a plot for Ca + Mg to SO4 + HCO3. The plot of Na – Cl to Ca + Mg – (HCO3 + SO4) confirms that Ca, Mg and Na concentrations in groundwater are derived from aquifer materials. Saturation indexes of groundwater samples indicate undersaturation for gypsum and anhydrite, and oversaturation for calcite, dolomite and aragonite confirming waterrock interactions by dissolution and dilution processes. In general, water chemistry is guided by weathering process, ion exchange and water-rock interaction during groundwater transition.

Aquifer structure identification through geostatistical integration of geological parameters: case of the Triassic sandstone aquifer system (SE Tunisia)

This paper presents an integrated modeling approach for constructing a geometric model of an Early to Middle Triassic sandstone aquifer system, commonly referred to as the Sahel El Ababsa aquifer, in southeastern Tunisia. The approach combines thorough interpretations derived from geological data along with various geostatistical methods to create accurate representations of the faulted bounding surfaces. The resulting architectural model was used to define the main aquifer system compartments and explain the nature of the hydraulic relations and communications within and beyond the aquifer system. A database was constructed based on multiple and various data sources, including water boreholes, exploration-petroleum wells, and seismic sections. These features were georeferenced and linked to the database used to build a stratigraphic framework of the area. The geological correlations based on wells information allowed a valuable observation on the structural trends and fault network as well as the lateral facies variation of the aquifer system. The derived interpreted faults were displayed on a georeferenced map which was later converted into a vertex database needed for the next modeling steps. A proper architectural modeling approach of the Early to Middle Triassic sandstone aquifer system was then performed using geostatistical methods that provide a variety of calculation techniques to model the reservoir bounding surfaces, through kriging methods including ordinary kriging and kriging with inequalities. The latter method was used to deal with the lack of data as it can retrieve information from the end of the boreholes that do not reach the surface to be calculated and integrate them in the modeling procedures. This enabled us to build the first architectural model with an accurate description of the bounding surfaces of the Early to Middle Triassic rocks. In fact, although it has been the center of geological and hydrogeological studies, the Upper Permian surface has not previously been the focus of an integrated geomodeling study. The generated models were analyzed, verified through kriging variance maps, then compared with the outcomes of the geological study. The results depict a new structural scheme of the Sahel El Ababsa area and provide an in-depth characterization of the complex compartmentalization affecting the aquifer system that allowed the definition and grouping of the hydrostratigraphic units. Therefore, this work is the first attempt through which an integrated architectural modeling approach is used to characterize and identify the Sahel El Ababsa aquifer system.

Hydrochimie et caractérisation qualitative des ressources en eaux d’un système aquifère multicouche en zone aride : application au bassin de Maknassy (Tunisie centrale)

L’evaluation qualitative des ressources en eau d’un systeme aquifere exige la connaissance prealable de ses caracteristiques geometriques et hydrodynamiques. Le systeme multicouche de Maknassy est forme de trois unites : la nappe phreatique logee dans des terrains detritiques du Mioplioquaternaire, la nappe profonde a deux niveaux constituee par les reservoirs carbonates du Cretace superieur (formations Zebbag superieur et Zebbag inferieur d’âge albien inferieur-turonien inferieur). L’hydrochimie permet la caracterisation de ces aquiferes et montre un facies sulfate pour la nappe phreatique, sulfate calcique et chlorure sodique pour la nappe profonde. Par ailleurs, cette analyse met en evidence l’intercommunication entre les niveaux aquiferes du reservoir de la formation Zebbag superieur et celle de la nappe phreatique. L’etude a revele une variete des processus d’acquisition de la mineralisation. L’effet de la dissolution de la croute gypseuse est appreciable pour la nappe phreatique. La variation des concentrations des ions majeurs des eaux des differents niveaux aquiferes est observee de l’amont vers l’aval. Cette variabilite de la mineralisation est fonction des caracteristiques lithologiques des aquiferes, des reactions d’echange, de la dissolution et du temps de sejours.