Improvement of Groundwater Recharge Mechanisms Knowledge in Rural Sahelian Zones: Case Study of Tougou Catchment, Burkina Faso

The lack of adequate information on groundwater recharge mechanisms in the basement rock area of Sahelian regions does not allow to estimate recharge rates. Thus, this study, which aims to improve the knowledge of the groundwater recharge mechanisms of the Tougou (catchment of 37 km² representing a basement rock in Sahel of West Africa) aquifers was initiated. The first step was to characterize the geology in terms of geometry and structure. The ERT profile (1.2 km length) crossing perpendicularly the river and lithologs from 10 observation wells (Average depth: 25m) and 1 borehole (Depth: 60 m) were used to make the correspondence between geological and geophysical data. The second step was to characterize vertically and laterally aquifers recharge mechanisms under the ephemeral river and two river banks. Hence, hourly to daily groundwater levels, electrical conductivity, and temperature of groundwater have been measured in those 10 observation wells and 1 borehole (Period: 2016-2020). The river water levels and the rainfall were also collected. The cross-correlation function was used between the rainfall or river water levels and the hydraulic heads time series. The geological characterization showed from top to bottom:

• Residual soils: 1 m to 2 m thick, present in the riverbed and on the right bank;
• Laterite (lateritic clays and lateritic cuirass): 2 m to 14 m thick, absent in the riverbed and present on the two banks;
• Laterally continuous clayey saprolite: 10 m to 22 m thick;
• Weathered schist: 32 m thick in the river. A bedrock was found at a depth of 55 m.

This geological conceptual model was a grounding for interpreting the results incurred from other data collected. It was ascertained that the weathered schist aquifer below the river is semi-confined (Average water depth: 9.5 m < top: 25 m) and vertically recharged by the saprolite aquifer. Laterally, the clayey saprolite aquifer is recharged by two main flows from:

• The river: the electrical conductivity and temperature of the groundwater in the clayey saprolite aquifer below the river vary at the same time as the water table increases during the rainy season. In addition, mean hydraulic head differences of +0.3 m and +2 m have been observed between the piezometer located in the river and respectively, the piezometer located at 20 m from the river on the left bank and other piezometers located on the right bank (up to 600 m from the river). A maximum good cross-correlation between hydraulic heads and river water levels rather than with rain was found in all piezometers, but mostly in the one located in the river (cross-correlation = 0.56). These indicate an indirect recharge process.
• The left bank: An mean hydraulic head difference (+3 m) which is related to a transfer of hydraulic pressure from probably a nearby recharge area was noted between the piezometers located at 300 m and the riverbed.

For further studies, the information obtained will be used to estimate the recharge through different methods including numerical modeling.