Rise of Urban Water Table as a Cause of Flooding: Improving Knowledge in the City of Niamey (Niger Republic)

Niamey, the capital of Niger, has experienced continuous demographic growth (+4%), accompanied by rapid urban expansion that is insufficiently controlled. This growth, combined with the effects of climate change as well as a drastic change in land use (urbanization of cultivated fields, deforestation of plateaus and erosion of slopes) disrupts the water cycle, thus leading to the superposition of three types of floods: 1) rain floods (monsoon period);2) river floods (Niger river);and 3) flooding caused by rising water table. In several neighbourhoods, the water table is now out in a sustainable manner and degrades already fragile sanitary conditions. This study aims to clarify the functioning of aquifers in the city of Niamey due to the combination of geological, geophysical and hydrogeological data. Hydrogeological investigations make it possible to identify, in areas flooded by the water table, a shallow aquifer with low capacitance (effective porosity of a few %) and low permeability (2 × 10-6 to 1 × 10-4 m/s), overlying at a level shallow clay (~10 m) and lying on the Precambrian base (schists, granites). This configuration limits flows and has led to the appearance of permanent pools created by the water table in the valley thalweg in and around the city. Thus, in Niamey, an increase of up to twenty to forty meters was observed between 1961 and 2021 with seasonal piezometric fluctuations of a pluri-metric order following the rainy season. Beyond the health impacts, this trajectory negatively impacts land and locally causes degradation or displacement of traffic axes. Containing the level of the water table appears to be essential in the long term for sustainable sanitation in the city of Niamey.

Physicochemical and Bacteriological Quality of Ground Water in the Basin of Gounti Yena Valley in Niamey City (Niger Republic)

The Gounti Yéna valley, the subject of this study, is a tributary of the Niger River, it is the main watercourse that divides the left bank of the city of Niamey into two unequal parts. The surface area of its watershed is estimated at about 62 Km2. The objective of the present study is on the one hand to evaluate the current state of the physico-chemical and bacteriological quality of the water of Gounti Yéna basin, and on the other hand to define the risks of water pollution and its origin in the face of the phenomenon of rising water table of this basin. In order to carry out this work, we proceeded to a study of the evolution of the physicochemical and bacteriological parameters of the water of the basin of Gounti Yéna during the period going from November 2020 to October 2021, at the level of four points of sampling chosen from the upstream to the downstream of the basin. The results obtained showed that the physico-chemical quality of the water in Gounti Yéna basin is influenced by the lithology of the watershed, climatic factors (rainfall and temperature) and anthropic actions. This study also highlighted the presence of fecal contamination in the water of the Gounti Yéna basin, with a greater degree of emphasis during the rainy season. This contamination remains a concern and may constitute a health risk.

Monitoring of the Physicochemical and Bacteriological Quality of the Water of the Gounti-Yéna Basin and Assessment of the Impact on the Health of the Populations

The Gounti Yéna valley is the main watercourse that divides the left bank of the city of Niamey into two unequal parts. The area of its watershed is estimated at about 62 km2. The objective of this study is on the one hand to evaluate the current state of the physico-chemical and bacteriological quality of the water of the Gounti Yéna basin and on the other hand to define the risks of water pollution and its origin in the face of the phenomenon of the rise in the water table of this basin by defining some health risks linked to these impacts. In order to properly carry out this work, we carried out a study of the evolution of the physicochemical and bacteriological parameters of the water of the Gounti Yéna basin during the period from November 2020 to October 2021, at three sampling points chosen from upstream to downstream of the basin then entomological surveys and among the health centers of the capital. The results obtained showed that the physico-chemical quality of water in the Gounti Yéna basin is influenced by climatic factors (precipitation and temperature) and anthropogenic actions. This study also highlighted the presence of faecal contamination in the water of the Gounti Yéna basin with a greater degree during the rainy season. This contamination remains worrying and constitutes a health risk causing waterborne diseases. The stagnation of the various bodies of water is also a favorable place for the development and spread of vector-borne diseases.

Physico-Chemical and Isotopic (18O, 2H and 3H) Characterization of the Upper Dallol Maouri Watershed (Niger)

Dallol Maouri is one of the fossil valleys on the left bank of the Niger River. Its watershed drains water from valleys in the Tahoua and Dosso regions of Niger. The watercourse is not continuous and has areas of surface water accumulation (ponds) and areas of spreading and infiltration. The surface water potential in this area is limited, with most of the ponds having a temporary to the semi-permanent regime. Groundwater resources are, therefore, the main sources of withdrawal for human food, agriculture, and livestock. Climate change through the variation in rainfall, the often-anarchic multiplication of catchment works (wells and boreholes) and agricultural activities can have impacts on the qualitative and quantitative evolution of surface and groundwater. The objective of this study is to characterize the surface and underground water of the upper dallol Maouri through the physicochemical parameters of the water and the use of the isotopes of the water molecule (18O, 2H and 3H). A total of 53 water samples were analyzed at the laboratory of the Regional Directorate of Hydraulics and Sanitation of Tillabéri (Niger) for chemical parameters and 73 samples were analyzed at the Radio Analysis and Environment Laboratory (LRAE) of the National School of Engineers of Sfax (ENIS) of Tunis for isotopy. The water of the ponds is characterized by three types of facies: calcium bicarbonate, calcium and magnesium bicarbonate, and calcium nitrate chloride. The groundwater is 75% of the chloride-nitrate-calcium facies and the CT3/CT2 groundwater is calcium and magnesium bicarbonate. The results of the stable isotopes made it possible to highlight the presence of four distinct water groups: ponds characterized by highly evaporated water, the sheets of Continental Terminal 1 and 2 (CT1 and CT2) with an isotopic cachet of old water, the water table consisting of a two-layer system in the southwestern part of the area. The water is not very mineralized (40 μS to 600 μS) and is slightly acidic (pH = 6.4). The analysis of the radioactive isotope (3H) content of the water from the water table has made it possible to identify the areas of recharge of the water table. These results also highlight: the presence of an upward drainage zone, and the contribution of lowland ponds to the recharge of alluvial aquifers ranging from 24% to 84%. No structure in this zone reaches the Continental Intercalaire and the Continental Hamadien, the depth of which is estimated at an average of 600 m. The relationship between the 18O and 3H contents of groundwater water shows two types of water with tritium contents higher than 2 TU for alluvial aquifers and tritium contents lower than 2 TU for groundwater (CT3). The analysis of the radioactive isotope (3H) contents of the groundwater has made it possible to identify recent recharges at the level of the alluvial aquifers and in the outcrop zone of CT3. There is no hydraulic continuity between the plateau ponds and groundwater.