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Norwegian University of Science and Technology (2013)

Nitrate Pollution of the Thiaroye Aquifer in the Suburban Parts of the Dakar Region, Senegal

Brandvold, Vibeke

Titre : Nitrate Pollution of the Thiaroye Aquifer in the Suburban Parts of the Dakar Region, Senegal

Auteur : Brandvold, Vibeke

Université de soutenance : Norwegian University of Science and Technology

Grade : Master thesis 2013

Résumé partiel
The current report, with the title Nitrate pollution of the Thiaroye aquifer in the suburban parts of the Dakar Region, Senegal ? Using chemical and stable isotope analyses to identify the sources of nitrate and the processes at the groundwater/surface water interface, constitutes the master thesis of Vibeke Brandvold. The thesis is made up by a total of 102 pages + appendices and was written in the 2013 spring semester. Flooding is a major problem in the departments of Pikine and Guédiawaye, in the suburban parts of the Dakar Region, Senegal. The region is very densely habited, but no public wastewater system is developed. Sanitary facilities consist of pit latrines and permeable septic tanks. The groundwater of the Thiaroye aquifer is, in this region, polluted with concentrations of nitrate that largely exceeds the WHO regulatory limit of 50 mg-NO3/L for drinking water. Today, drinking water is transported from Lake Guiers, in the northeast of Senegal, and from the Sebikotane aquifer, southeast of Dakar. Some families have a hand-operated pump, called Diambar pump, which extracts water from the uppermost part of the groundwater. This water is mainly used for domestic purposes like washing and given to animals, but it is not suited as drinking water. Almost all of the wastewater in the region infiltrates into the ground and recharges the aquifer. Precipitation is the only other source of recharge. Many areas are flooded throughout the year and especially during the rainy season from July to October, when surface runoff collects in low-lying areas. To reduce the extent of flooding, several retention pools, 3-4 m deep, have been constructed. In the region there are also areas of urban agriculture, called the ?Niayes ?.During the work on this master thesis, the water qualities of the groundwater of the Thiaroye aquifer and of different surface waters in the suburban areas of the Dakar Region were studied. 34 samples from Diambar pumps, dug wells, flooded zones, Niayes and retention pools were analysed for the contents of anions, cations, organic and inorganic carbon, together with stable environmental isotopes of water (2H/1H and 18O/16O), nitrate (15N/14N and 18O/16O) and algae (15N/14N). The research was especially focused on the pollution of nitrate, in hopes of being able to identify the sources of nitrate and the processes that happen at the ground-/surface water interface. The surface waters have in general higher conductivities and concentrations of salts than the groundwater, due to more extensive evaporation. The isotopic composition of water was used to quantify the extent of evaporation from both the groundwater and the surface waters, and it has given new insight to the hydrological connectivity at the ground-/surface water interface. The hypothesis that there is a good connection between the waters, and that the groundwater recharges the surface waters during the rainy season, have been strengthened.

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Page publiée le 4 mars 2015, mise à jour le 6 février 2018