ABSTRACT
ABSTRACT: Ouagadougou City is built on crystalline rocks dating back to the Proterozoïc era. These rocks are covered by weathered material. Demographic explosion, rapid extension of the town, and particularly its undeveloped peripheral slums, lack of appropriate sanitation networks are exposing aquifers to the risk of contamination. Aquifers contribute about 60% of the population’s water supply and especially to the poor. With this in mind, the present study evaluated the situation in order to develop an early warning system. The discontinuous nature of the area led to a comprehensive evaluation of 1800 wells and 510 boreholes, which would be used to obtain water for analysis (nitrate content). This would enable the assessment of the range of spatial pollution and its comparison with the theoretical vulnerability map. The surficial aquifer, tapped by wells, is currently the most contaminated. The main factor influencing vulnerability here is the thickness of the unsaturated zone. Regular monitoring indicates that bacteriological contamination is mainly derived from faecal or household origin, and that the contaminating products are periodically washed into the aquifers during months with the high rainfall, i.e. July, August and September. Nitrate concentrations increase before they are reduced by dilution or diffusion during this period. From this, suggestions can be made to amend the monitoring schedule.
