Abstract
Burkina Faso, a Sahelian country, has a predominantly Sudano-Sahelian climate, marked by a contrast between the arid north and the rainier south. Around 80% of the territory is based on bedrock formations (granitoids and metamorphic rocks), whose aquifers constitute the main sources of water for domestic, agricultural and pastoral uses. Against a backdrop of increasing climatic and anthropogenic pressures, understanding the dynamics, vulnerability and functioning of groundwater resources in these heterogeneous aquifers is crucial, particularly at all scales where they are mobilized to supply populations.This study assesses the groundwater resources of basement aquifers in the Sudano-Sahelian zone, taking into account the effects of rainfall variations and human pressures. It uses a combination of geological, hydrogeological, hydrochemical and isotopic methods to map and characterize groundwater productivity, recharge dynamics and quality. It also provides a comparison with similar studies in the more arid Sahelian zone.Two methodological approaches have been developed. The first is to exploit lithological data from drilling (cuttings), traditionally under-utilized, to refine the geological mapping of basement zones. This approach, tested in the Sanguié province, is based on the correlation between field observations and the silica content of groundwater. The second is a method for estimating groundwater recharge, known as the “combined Thornthwaite-Horton approach”, which couples the Thornthwaite water balance with the Horton runoff model. This approach makes it possible to efficiently assess renewable water resources in arid contexts.The results show that silica content is a good discriminant of the main lithologies of basement aquifers. Furthermore, despite their lithological diversity, basement aquifers exhibit similar hydrogeological characteristics, particularly in terms of saprolite depth, drilling depth and observed flow rates. The most productive part of the fractured horizon is generally located in the first 30 meters under the saprolite, with a notable decrease in productivity at depth. Productive boreholes in the basement zone should be sought at the level of the thick saprolite thicknesses, which constitute a criterion for the productivity of boreholes in the basement zone.In the Sudano-Sahelian zone, particularly at Dassa and Kyon, groundwater recharge is mainly diffuse, with an average of 65 mm/year. The Dassa and Kyon aquifers show a high piezometric level close to the ground, with little impact from domestic and agricultural withdrawals, unlike the Sahelian zones, which are more affected by resource scarcity. However, this high piezometric level is accompanied by a gradual deterioration in the chemical quality of the water, from deep aquifers to surface waters. Surface water, particularly around gold panning sites, shows high levels of pollution. Groundwater quality remains good overall, although well water is more altered than borehole water, mainly due to the nature of the alteration profile, which tends to concentrate more metals and rare earth elements (REEs) in its upper horizons.The study highlighted water points with above-standard concentrations of arsenic and lead, while cyanides, although detected, remain at non-critical levels. Well water generally has higher levels of trace metals than boreholes, with the exception of some boreholes that are no longer in service. Water-rock interaction explains the presence of major elements (HCO₃-, Mg²⁺, Ca²⁺, Na⁺, SO₄²-) as well as some trace metals (As³⁺, Sr²⁺, B³⁺, Mo⁶⁺, Sb³⁺), with calcite and dolomite saturation indices also enabling the relative chronology of these interactions to be assessed. Other elements (REEs, thallium, etc.) are attributed to human activities, such as gold panning and agricultural practices.This study highlights the importance of regular monitoring of groundwater quality, to guarantee the sustainability of water resources and guide water management policies adapted to the region's socio-economic and environmental challenges, all the more so in view of emerging pollutants such as rare earths and thallium, for which no potability standards have yet been defined.