Geophysical and Hydrogeological Investigations of Water Leakage from a Reservoir Dam to a Coastal Aquifer: The Lebna Case Study (Northeastern Tunisia)

In semiarid regions, dams are useful for surface water storage, sediment sequestration, and aquifer recharge. Built in 1987 on the Cap Bon peninsula (in northeastern Tunisia), the Lebna Dam is considered a good example of a multifunctional reservoir. The dam feeds two important irrigation networks, stores large sediment quantities, and allows a significant recharge flow to the underlying aquifer. This work suggests new leakage flow and dam-aquifer interaction characterizations through the development of an approach that combines a water balance calculation, geological field observations, groundwater monitoring, and geophysical research. The hydrological balance calculation performed over the 27-year monitoring period, from 1990 to 2017, shows that an estimated water volume of 3.7 Mm3y-1 has leaked from the Lebna reservoir to the coastal aquifer. Geological mapping of the Lebna Dam basin in summer 2019 revealed the existence of permeable layers of sands to sandstones exposed along the southern banks of the reservoir and extending to an elevation that included the water level when the dam is full; these rocks outcrop at approximately 16 m.a.s.l. A geophysical survey based on 67 vertical electrical soundings and 8 electrical resistivity tomography profiles in the area downstream of the reservoir was carried out to identify the lateral continuity of the recharge zones. Piezometric campaigns consisting of four field surveys in 2019 and 2020 were conducted in the region downstream of Lebna Dam, consisting of 71 water well samples. An interpretation of these geophysical data coupled with available borehole logging and piezometric measurements was used to define the leakage geometry from the reservoir dam to the coastal aquifer. The collected evidence led to the conclusion that concentrated recharge occurs in the downstream sections, especially on the right bank of the aquifer.

Syn-Collision Folding and Multi-Layers Detachment in the Molassic Basin, Northeastern Tunisia

Tunisia had been the place of various tectonic episodes which have drawn the actual structural map of the country. The Neogene period was the most active period in which the collision contributed to the mountain ranges formation such as the Atlassic chain. The contrast of mechanical behavior between the different sedimentary units of the cover and the substratum is a determining factor that has controlled the deformation style of many folding structures. Detachment folding has been studied on the basis of field and seismic data examples from the Northeastern Tunisia. We discuss different aspects of detachment to constrain the chronology of structural events and understand the cover‒substratum relationship during the major folding phases. In the present paper, we found that the deformation style admitted previously do not reflect adequately the tectonic structure of Kechabta Neogene basin (North of Tunisia). The cover-substratum relationship study at this area revealed two major shortening phases; the first had been happened during the Upper Tortonian and the second in the Lower Quaternary. The resulting folding style is characterized by disharmonic and multi-layers detachment phenomena produced by incompetent evaporitic sequences. The two described folding phases are superimposed and testify at a large scale the continental collision which controls the North African margin during the Late Miocene to the Quaternary.