AQUIFERES SUPERFICIELS ET PROFONDS ET POLLUTION URBAINE EN AFRIQUE : Cas de la communauté urbaine de Niamey (NIGER)

The study was conducted in Southwestern Niger (near the Niger River), in a semi-arid region, on aquifers located in the city of Niamey and its suburbs. In the Niamey area, groundwaters are located in different aquifer types made up of quaternary alluvial deposits in the Niger River valley, "Continental terminal 3" (CT3) sandy to clayey sandstones in its left bank (Iullemmeden sedimentary basin), both aquifers overlying a fractured Precambrian basement (West African Craton). In the middle of 1980s, a severe drying up of the Niger River led to the drilling of 120 boreholes located in the fractured basement aquifers formed by hard-rocks to ensure the water supply of the city. Nitrate pollution, due to uncontrolled urbanization and deforestation, was highlighted. Indeed, the development of urban infrastructures (extension of sewerage and water supply) could not follow population uncurbed growth. In this case, the increase of groundwater pollution can be suspected while the water demand of the city is growing. The main objective of this thesis is to study the groundwater quality characteristics through combined piezometric and physicochemical data interpretations. The monitoring of more than one hundred wells over 3 years (70 seasonal piezometric surveys-1047-measurements, 80 physico-chemical surveys for each parameters (pH, conductivity and temperature), -1200 measurements- ; 232 samples for major ions analyses and 50 samples for bacteriological analyses) was undertaken. The study also includes a review of piezometric and chemical data from other earlier studies. Results obtained show that the seasonal fluctuations of groundwater levels in each well (ranging from 0.5 to 30 m below ground surface) indicate that the area is a hydrogeological recharge zone, which enhances the transfer of contaminants from the surface. Groundwaters are mainly recharged by indirect infiltration from surface waters (near ponds for example) and direct infiltration (on sandy dunes). Geochemical data (major ions) are used to define groundwater groups, to map their distribution in the study area, and to evaluate their origin. We combined multivariate statistical methods, scatter diagrams, ionic ratios, and Piper diagrams to decipher the processes affecting the water chemistry at large (Niamey) and local (wells) scale. The geochemical monitoring revealed high spatial and temporal variability in chemical parameters. Processes controlling the groundwater chemistry at large scales in the aquifers include atmospheric inputs (rainfall and dust) in the poorly mineralized formations, evapotranspiration in the soil layers, and alteration of silicates. Processes specific to individual wells are production of weak acids (H2CO3) and bases (HCO3-) in soil layers near pollutant sources, leaching of ions (Ca2+, Mg2+, Na+, K+, NH4+, NO3-, Cl-, HCO3-, SO42-) from pollutants, ion exchange reactions, and mixing by upward leakage from hard-rock semi-confined aquifers. Typically, water samples from aquifers formed by Precambrian altered hard-rock formations and Quaternary alluvial formations contain high amounts of bicarbonate, sodium, calcium and magnesium related to water-rock interaction and low flow in the aquifer. The chemistry of the groundwater in the Continental Terminal (CT3) sandy to clayey sandstone aquifer significantly differs from other groundwaters found in the altered and fissured hard rock aquifers. Higher levels of nitrate and sodium were observed in the CT3 aquifer due to contaminated recharge water. Nitrate is the main ionic contaminant and its content is seasonally or permanently above the World Health Organization guidelines for drinking water quality limit of 0.81 meq.L-1 in 44 % of sampled wells, and reaching 816 mg L-1, more than 16 times higher than the standard. Fourteen percent of sampled boreholes show an increase in the groundwater nitrate content over the last twenty years. As expected, samples from dug-wells located in the central city with numerous pollutant sources had the highest concentrations of leached elements indicating that land-use is the primary control on groundwater contamination. Key words: urbanization, water table, groundwater recharge, geochemistry, nitrates, semi-arid area, Niamey, Niger.