Tracing groundwater nitrate sources in the Dakar suburban area: An isotopic multi-tracer approach

The Dakar region is a mega city with multiple contaminant sources from urban expansion as well as industrial and agricultural activities. The major part of the region is underlain by unconfined sandy aquifer, which is vulnerable to contaminants derived from human land use. At present, the contaminated groundwater which extends over a large area in the suburban zone of Thiaroye poses a threat to the future of this valuable resource, and more specifically, a health threat. This study focuses on nitrate pollution occurrences and associated processes using nitrate isotope data ( 15N NO3, 18O NO3) combined with environmental isotopic tracers ( 18O, 2H, and 3H). Samples from 36 wells were collected to determine the level, distribution, and sources of contamination in relation to land use. Results indicate that shallow groundwater in the urbanized area of Thiaroye shows distinct evidence of surface contamination with nitrate as much as 300 mg/l NO 3-. In rural area not serviced by water supply distribution network, much higher NO 3- contents were found in few wells due to household and livestock feedlots. In most groundwater samples δ 15N values ranged from + 10 to + 22‰, indicative of predominantly human and animal wastes. This was confirmed by environmental isotope data which suggest a mixture of polluted recharge waters. By using the dual δ 15N vs δ 18O as well as δ 15N vs NO 3- approach, denitrification may occur to some extent but it is blurred by mixing with new infiltrated nitrates and cycling derived from continuous leaky septic system. Results suggest that nitrate contamination of the aquifer is a consequence of unregulated urbanisation (homemade latrines), continuing contaminant transfer in shallow water depth where aerobic conditions prevail. © 2011 John Wiley & Sons, Ltd.