Characterization of the functioning of the basement aquifers in the area of Ouagadougou through a study of water chemical quality
Some 1800 traditional wells and 100 deep boreholes (40 to 65 m depth) contribute to the drinking water supply of Ouagadougou. In order to better know the origin of the mineralization, the concentration trends and to characterize the structure and the functioning of the aquifers, a hydrochemical study has been carried out. The city is located in the Sudano-Sahelian climate zone with a rainy season from June to September and a dry season occurring between October and May. The average rainfall is about 700 mm. The geology of the study area comprises crystalline formations overlain by a layer of weathered materials about 30 m thick. Hydrogeologically, two main aquifers are encountered here: the near surface aquifer located in the laterites and exploited through hand-dug wells, and the deep aquifer consisting of coarse sands and fractured bedrock. The latter aquifer, most of the time confined, is tapped by the boreholes and is isolated from the first by clayey formations. Regarding the chemistry, water from the boreholes shows a bicarbonate calcic and magnesium facies, whereas the well waters are characterized by a chlorinated and nitrate facies with a sodium trend. The dominant process in the deeper aquifer is the hydrolysis of silicates. The near-surface aquifer is highly influenced by superficial supplies. In this case, the mineralization of the anthropogenic organic matter encourages the dissolution of the silicates. The electrical conductivity and the alkalinity, easily measurable in the field, support this suggestion. The nitrate contents exceed WHO norms in nearly 60% of the 1800 inventoried wells. Their statistical analysis in relation to the distances between the different sampling points highlights the heterogeneity of the environment and the localized state of the influence of the anthropogenic supplies, confirming the hydrodynamic concept of a discontinuous environment typical of bedrock zones. The seasonal variations of nitrate concentrations in the wells reflect soil leaching phenomena during the wet season. The stable isotope (18O/2H) content of the waters allowed a differentiation between the wells characterized by evaporation processes and the boreholes showing contents close to multi-year mean values of the monsoon rainfalls. These observations suggest two types of recharge: the first type takes place directly through the weathered mantle and the second type, delayed, occurs through lakes and creeks under evaporation conditions.