Urban wet weather discharges and pollutant dynamic in the Djibi River (Abidjan – Côte d’Ivoire)

African cities have little or no sanitation or rainwater drainage network. The runoff draining diffuse pollution, combined with domestic and industrial wastewater is discharged directly into the receiving environment without treatment. In the Abidjan district, the densest neighborhoods in the Abobo municipality, are drained by the Djibi River. Contaminants transported during floods present an environmental risk, particularly for the Aghien Lagoon which receives the river. This lagoon, the largest freshwater waterbody near Abidjan, has been identified as a potential resource for the production of drinking water to face the tremendous difficulties in water supply of the city.

Urban wet weather discharges are made up of all the water discharged from urban zones and their drainage systems during and a few hours after a rainy event. The origin of the pollutants contained in these urban discharges is varied: atmospheric pollution, leaching of dry weather deposits accumulated in the basin, erosion and resuspension of the pollutants present in the sanitation and drainage systems.

To better understand and quantify pollutant transfers from the urban zones to the Aghien Lagoon, 8 representative floods were sampled (12 samples for each flood) in the Djibi River at 3 sites (2 sites downstream from urban zones, 1 site just upstream from the Lagoon) between 2019 and 2021. Physico-chemical parameters and suspended solid, nutrient, dissolved and total trace element concentrations as well as discharge were measured to evaluate their environmental impact on the Aghien Lagoon.

During floods, concentrations of pollutants increased systematically, reaching very high values. The concentration peaks were observed at the rising limb of the flood, before the peak of discharge, for the majority of the analyzed elements. We also observed a massive remobilization of pollutants from storage areas. The most obvious is located in urban zones. In addition of the clean-up of urban areas, we also observed a remobilization of the pollutants accumulated, during low flow conditions, in the river bed itself.

The pollutant loads transferred to the Aghien Lagoon are very important and contribute significantly to the reduction of its water quality which could increase the treatment cost of a pumping under construction.