Tracing groundwater nitrate sources in the Dakar suburban area: an isotopic multi-tracer approach M. Di´ edhiou, 1,3 S. Ciss´ e Faye, 1 O. C. Diouf, 1 S. Faye, 1 A. Faye, 1 V. Re, 2 S. Wohnlich, 3 * F. Wisotzky, 3 U. Schulte 4 and P. Maloszewski 5 1 University Cheikh Anta Diop (UCAD), Geology Department, B.P 5005 Dakar Senegal 2 Universit` a C` a Foscari, Dipartimento di Science Ambientali, Venezia, Italy 3 Ruhr University of Bochum (RUB), Department of Applied Geology, Universitaetstr. 150, D-44801 Bochum, Germany 4 Ruhr University of Bochum, laboratory of sediment and isotope geology, Bochum Germany 5 Helmholtz Center Munich, Institute of Groundwater Ecology, Ingolst¨ adter/Landstraße 1, D-85764, Neuherberg, Germany Abstract: 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 ( 15 N NO3 , 18 O NO3 ) combined with environmental isotopic tracers ( 18 O, 2 H, and 3 H). 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 υ 15 N values ranged from C10 to C22‰, 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 υ 15 N vs υ 18 O as well as υ 15 N 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. Copyright  2011 John Wiley & Sons, Ltd. KEY WORDS nitrate; environmental isotopes; nitrate isotopes; denitrification; Dakar; Senegal Received 8 June 2010; Accepted 9 May 2011 INTRODUCTION Groundwater located in the Dakar (Capital city of Sene- gal) suburban area is an important and valuable resource for potable water supply, agricultural and industrial uses. Extraction of this resource which started in the 50s, had been progressively increased during the period 1976–2000 to satisfy the increasing water demand with regard to the city’s expansion and the rapid demographic growth (2Ð7%). However, from the mid to end 80s, sign of urban impact became evident (Collin and Salem, 1989) especially nitrogen contamination in the suburban area of Thiaroye. At present, due to the high pollution level in the suburban area, most of the wells are withdrawn form use as public water supply, and the pumping rate lowered to 4000–5000 m 3 /d. This latter situation has put more stress in the system since lowering of pumping causes recur- rent flooding through rising groundwater table (Dieng, 2009). * Correspondence to: S. Wohnlich, Ruhr University of Bochum (RUB), Department of Applied Geology, Universitaetstr. 150, D-44801 Bochum, Germany. E-mail: stefan.wohnlich@rub.de Previous studies in the Thiaroye area (Collin and Salem, 1989; Fall, 1991; Tandia et al., 1999; Tandia, 2000; Ciss´ e Faye, 2001; Ciss´ e Faye et al., 2004) reported high nitrate levels which deteriorate water quality. Their occurrence was attributed to the high aquifer sensitiv- ity to surface-derived contamination because of the high permeability of the sandy aquifer and shallow water table conditions (Ciss´ e Faye, 2001; Madioune, 2005). This is associated with land use trend in the ground- water catchment which seems to be remarkably con- sistent with nitrate evolution into the system. In fact, historical and current trends in nitrate concentrations measured in the Thiaroye well field have evolved from 5–34 mg/l between 1966 and 1972 (below WHO admis- sible limits of 50 mg/l) to presently reach an average of 400–450 mg/l through a steady increase during the period 1972–1997 (Sow, 2009). This trend perfectly relates to the rapid expansion of unplanned urban and peri-urban settlements in farming lands since 1942. The effects of suburban development have been recently char- acterized (Collin and Salem, 1989; Bassel, 1996; Tandia, 2000; Ciss´ e Faye, 2001; Madioune, 2005; Dieng, 2009; Sow, 2009) and the increased nitrate levels and frequent Copyright  2011 John Wiley & Sons, Ltd. HYDROLOGICAL PROCESSES Hydrol. Process. Published online in Wiley Online Library (wileyonlinelibrary.com) DOI: 10.1002/hyp.8172 27 June 2011 , – 26 2012) 760 770 (