Drinking water supplies in the groundnut basin region of Senegal are mainly derived from Maastrichtian confined aquifer abstraction. However, water quality in this part of the aquifer is problematic with regard to health issue due to relatively high salinity and fluoride at levels exceeding WHO standards. From this fact, Water authority (Senegalese Ministry of Water Resources DGPRE) has decided to explore the potential of upper and middle aquifers (Quaternary, Eocene and Paleocene formations) where little is known about these hydrosystems. DGPRE hence launched a study to investigate the potential of the aquifer system overlying the saline Maastrichtian aquifer in order to build catchment facilities and/or explore dilution options to supply water that meets quality standards to this large regional population.
Field investigations including geophysical, hydrogeological, hydrochemical surveys were carried out focusing mainly on the aquifer system above the Maastrichtian formations. Results indicate moderately permeable but thick formations as well as variations in water quality from one sector to another. These investigations also reveal endorheic piezometric depressions, with a water level dropping to 30 m below sea level. This piezometric depression phenomenon is known in the Sahelian region and can be explained by several hypotheses including deep evaporation or ancient changes in sea level.
The presence of brackish water relics and piezometric depressions near the hypersaline waters of the Saloum River has raised concerns regarding the sustainability of water abstraction from these aquifers.
A numerical model was developed to serve as a tool for evaluating and managing this resource and for planning purposes. Various pumping scenarios were tested, in particular transferring the present water abstraction from the Maastrichtian aquifer to both upper and middle aquifers. The drawdown caused by pumping and the risk of water quality deterioration due to the migration of brackish water were evaluated in the long term. Results showed that exploiting this resource could be a pertinent solution. However, this would require preliminary field testing and increased monitoring of changes in salt concentrations during operation.
The combination of investigations, data analysis and numerical simulations made it possible to identify favorable locations for the creation of new shallow well fields. This work must be followed by pilot site tests, in order to verify in situ variations of the water table induced by pumping operations. If these tests confirm the potential of the upper and middle aquifers, it should then be possible to diversify resources tapped for drinking water supplies and thus improve the quality of distributed water.
The IDEV-Artelia consortium is grateful to the Senegalese Ministry of Water Resources (DGPRE) and its director, Mr Niokhor NDOUR for having accompanied the study and for his contribution to its success.