Investigation by multivariate analysis of groundwater composition in a multilayer aquifer system from North Africa: A multi-tracer approach
Highlights
► We investigate the hydrodynamics and hydrochemistry of a multilayer aquifer system. ► We examine the geochemical evolution, the origins and the circulation patterns of groundwater. ► The mineralization is controlled by water-rock interaction and return flow process. ► Groundwater derives from palaeoclimatic and modern end-members. ► Mixing by upward and downward leakage occurs between these two end-members.
Introduction
In most North African regions, the hydrogeology is largely controlled by tectonics, which created several multilayer aquifer systems. In these systems, faulting activity, which has been most active, has played a significant role in the lateral compartmentalization of the superposed aquifers. This compartmentalization, enhanced by high hydraulic heads and anthropogenic processes such as overexploitation and irrigation practices, has disturbed and complicated the hydrodynamic functioning of water tables, originally obeying a “Piston flow” circulation model, especially through the mixing of different groundwaters. Indeed, the potentiometric conditions and flow patterns of most exploited multilayer aquifer systems in North African regions at the present-day differs significantly from those found prior to the onset of drilling and the considerable borehole exploitation. Large-scale drilling generally disrupts the initial condition so that water tables are no longer in a steady state with hydraulic gradients largely reflecting topography. In contrast, water pumping has caused steadily declining water-levels and reduced potentiometric pressure over time, which has disturbed the stratification of groundwater bodies. Consequently, mixed groundwaters are always obtained during the sampling of pumped groundwaters creating problems for groundwater resource management.
In this study, hydrogeologic, hydrochemical and isotopic information from the aquifer system will be integrated and used to determine the main factors and mechanisms controlling the chemistry of groundwaters in the study area and to identify the origins of water bodies and their migration pathways. The main issues that will be addressed by this study include: (1) the relative importance of geologic and hydrogeologic factors in controlling the groundwater circulation within the aquifer system; (2) the validity of statistical clustering techniques to characterize the geochemical features and the isotopic signatures of groundwaters; (3) the geochemical effects and age of leakage influencing groundwaters and (4) the appraisal of the value of chemical and isotopic approaches to delineate flowpaths and evaluate evolutionary processes.
Section snippets
Physiographic setting
Tunisia is located in the Western Mediterranean, which represents a climatic transition zone open to the influence of the cool North Atlantic air masses and the warm Mediterranean air masses (Rindsberger et al., 1990). Moreover, specific geomorphologic characteristics of Tunisia i.e. the absence of high mountains and the relatively limited geographic extension allow the integration of Saharan air masses into the atmospheric circulation (Celle-Jeanton et al., 2001a). However, hydrometeorological
Geology and hydrology
As shown in the geological map (ONM, 1985) (Fig. 1) and the lithostratigraphic column (Dassi, 2004) (Fig. 2), the lithologic units in the study area extend from the Cretaceous to the Quaternary. The Cretaceous series, which outcrops in the surrounding Mountains of Semmaama, Douleb, Mrhila and Rheradok are constituted mainly of clay, marl, limestone, marly limestone and sandstone marine deposits. These series belong to the Sidi Khralif, Melloussi, Bou Hedma, Fahdène, Annaba, Biréno, Alèg and
Analytical procedure
The sampling campaign was undertaken in the study area during June 2000. A total of 40 groundwater samples (Fig. 1) were collected from springs and selected private and public wells for chemical and isotopic analyses (Table 1).
Samples destined for major and minor element analyses were filtered through 0.45-μm filters and collected in acid-rinsed 100-mL polyethylene bottles. Chemical analyses were undertaken using ion chromatography and ICP MC where calibrations were performed using appropriate
Hydrochemical study
A preliminary characterization, carried out using the Piper triangular diagram (Piper, 1944), is shown in Fig. 4. This diagram is used to obtain a preliminary geochemical characterization through the identification of different types of groundwaters. It shows that most groundwater samples are characterized by mixed water type with a slight tendency to Cl, SO4 and/or Ca water-types.
To accurately identify the main processes governing the hydrochemistry of groundwaters in the system a factor
Summary and conclusions
The hydrochemical and isotopic features of the multilayer aquifer system of the Sbeïtla basin, central Tunisia, have been investigated using multivariate statistical analysis and bivariate conventional diagrams to determine the main hydrochemical processes of salinization and to identify the origin and circulation patterns of groundwaters.
The deep confined aquifer is characterized by artesian heads, which maintain a higher potentiometric level than the semi-confined and unconfined aquifers.
Acknowledgments
Acknowledgement is hereby given to the German Academic Exchange Service (DAAD) for financial support of this work. I greatly appreciate constructive comments of two anonymous reviewers and editorial handling by Dr. Asa Danielsson. I am also grateful to Prof. Ron Fuge for his helpful and constructive comments.
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