Origin and fate of nitrogen pollution in groundwater traced by δ15N-NO3 and δ18O-NO3: the case of the suburban area of Dakar (Senegal).

In recent years, the rapid increase of the population in the region of Dakar, especially in suburban settlements, together with the lack of policies for urban waste management, has become a source of concern for water supply, needs, and quality control. Approximately 80% of water resources in the region come from groundwater reservoirs. In order to identify the origin of groundwater pollution a survey of 26 piezometers and wells was conducted in march 2006. In this study, both major and trace elements were measured as well as the stable isotopic signature of water molecules and dissolved compounds. Nitrates often exceed drinking water limits and are associated with microbiological pollutants, while sea water intrusion represents the major threat to rapidly declining groundwater quality. Stable isotopes of dissolved nitrates allow for the identification of urban sewage and fertilizers as a major source of contamination, and the ability to define the distribution of their impacts. The occurrence of denitrification processes, although limited, suggest the potential for auto-purifi cation of the contaminated water, if the source of the pollution were to cease.