As in many African coastal cities, the 1.5 millions of inhabitants of Cotonou city (Benin, West Africa) are supplied with drinking water thanks to groundwater. More than 90,000 cubic meter of groundwater are pumped daily in a coastal sedimentary aquifer, but the lack of knowledge of the aquifer properties is a strong limitation for a fear and sustainable management of this critical resource. To enhance the hydrogeological knowledge of this Mio-Pliocene aquifer, we characterized 43 sites distributed over the 2,200 km² surface area of the aquifer. Based on the joint use of two geophysical methods  (i.e. the Time domain Electromagnetic (TEM) and the Magnetic Resonance Sounding (MRS)), we have been able to describe the geometry of the aquifer and the spatial distributions of the specific yield and transmissivity. We also calculated the actual groundwater storage and the potential yield that could be abstracted.
We found that the Southern part of the aquifer is twice thicker than the Northern part (i.e. 85 and 45m respectively) The Transmissivity is ranging between 6.10-4 and 4.10-3 (m²/s) and is higher is the Southern part of the aquifer. The specific yield is homogeneous in the South but it is quite heterogeneous in the North; its average value is 9.5%. The potential yield calculated from the relationship we found between transmissivity and specific capacity is ranging between 20 m3/h in the Western part of the aquifer and 200 m3/h in its Southern part. The current groundwater storage calculated from the specific yield and the aquifer thickness is ranging between 2,000 and 10,000 mm. The total groundwater currently stored in the 2,200 km² surface area of the aquifer is 13 km3, which is enough to supply the whole inhabitants of Benin for 25 years with 100 l per capita and per day.
KEYWORDS: urban groundwater, sedimentary aquifer, hydrogeological properties, groundwater reserve
Descloitres, M., Chalikakis, K., Legchenko, A., Moussa, A.M., Genthon, P., Favreau, G., Le Coz, M., Boucher, M., Oï, M., 2013. Investigation of groundwater resources in the Komadugu Yobe Valley (Lake Chad Basin, Niger) using MRS and TDEM methods. J. Afr. Earth Sci. 87, 71–85. https://doi.org/10.1016/j.jafrearsci.2013.07.006