Suspended particulate matter and water fecal pollution in the Niger River at Niamey

130 million residents use the Niger River water for agriculture, fishing, transport and domestic activities. Suspended particulate matter (SPM) and other waste, washed into the river during rainy events,  constitute a favorable environment for bacteria proliferation since it provides protection from ultraviolet radiation, offers nutrients and ideal living environment. The objectives of this work are to: characterize the concentration and nature of SPM in the middle Niger River and analyze their spatio-temporal variability; study the link between SPM and indicator of fecal bacteria contamination (E. coli); quantify the part of fecal pollution that the Niamey city brings into the Niger River.

Two regular monitoring sites were set up on the Niger River and two periurban lakes east of Niamey. Measurements of SPM were made by filtering water samples acquired during two full years. Water color was monitored by above-surface radiometer operating in red and infrared bands and Sentinel-2 satellite. E. coli content and physico-chemical water parameters were also monitored and SPM were analyzed using electronic microscopy and laser granulometry.

The analysis of SPM revealed extremely fine (0.2 µm) particles of mineral origin (kaolinite) with some iron oxide/hydroxide during the red flood. Radiometric measurements by both the in-situ radiometer and Sentinel-2 were found to be well suited to monitor SPM dynamics in this area. SPM and discharge showed a complex relationship : SPM increases before the discharge increase at the beginning of the red flood but they are at their lowest during the black flood. SPM concentration and E. coli somewhat co-vary at the beginning of the rainy season but E. coli values remain relatively high during the black flood, despite low SPM at that time. E. coli concentration at the downstream station reveals an important fecal pollution from the Niamey city with E. coli values on average 10 times higher than the upstream station. Maintaining and developing monitoring tools in this area is particularly important in the context of  global environmental changes, as for example the increase in the red flood discharge observed  in Niamey since the 90’s, that may deeply modify the transfer of contaminants in surface waters.