Declining groundwater resources in semi-arid areas are often cited because of anthropization and climate change. This is not the case in Nouakchott (Mauritania) where the water level has risen by 1 to 2m over the last 40years in parallel with urban expansion (+1 million inhabitants in 60 years). Using former and new data, primarily water table measurements and chemical indicators (major ions, bromide, ¹⁸O, ²H), we show that the groundwater level rise is mainly a consequence of the rapid population growth in the Nouakchott area, while the global sea level rise only has a limited impact. The increased supply of domestic water (currently 120,000 m³/day) and the lack of waste water networks have added large amounts of water to the Quaternary aquifer. In this metropolis where 60% of the total area is at an elevation of less than 1m asl, the rise in the groundwater level has dramatic consequences, including the abandonment of flooded districts, and the emergence of new diseases.