Urban Runoff Contribution to Surface Sediment Accumulation for Polycyclic Aromatic Hydrocarbons in the Cotonou Lagoon, Benin

Abstract

The carcinogenic and mutagenic properties of some polycyclic aromatic hydrocarbons (PAHs) and their widespread presence in the environment have led to many scientific investigations during the past three decades. In this study, surface sediments from six sampling sites in the Cotonou lagoon (three sites in the channel and three in Lake Nokoue) during the highest stormy season (May, June, and July) were used to assess the urban runoff contribution of PAH accumulation in this ecosystem. This assessment showed relatively high PAH loads (140-625 ng/g) during the rainy season and high inputs of organic materials (total organic carbon contents between 2.5 and 9.5% dry weight). Generally known as originating from different processes, PAHs detected in most of the Cotonou lagoon sampling sites were characterized by the predominance of the high-molecular-weight PAH over the low-molecular-weight PAH, which is an indicator of pyrolytic origin PAH inputs. PAH contamination in the Cotonou lagoon surface sediments was mainly from heavy urban traffic linked to the use of diesel vehicles and to the motorbike taxis proliferating across and around the city. Regression analysis carried out between total PAH concentration and total organic carbon contents showed, excluding the rural sampling site, a positive linear relationship for most samples ( R 2 = 0.9996), and confirmed the pyrogenic input of PAHs in the Cotonou lagoon. Mitigation measures were suggested to ameliorate the PAH pollution in the lagoon.