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