Multi-contaminant Pollution Burden in Nigerian Freshwater Ecosystems: a Systematic Review of Ecotoxicological Evidence from Heavy Metals, Hydrocarbons, Pesticides, and Microplastics for Integrated Risk Assessment

Nigerian freshwater ecosystems are subjected to multiple exposures of heavy metals, petroleum hydrocarbons, organochlorine and organophosphate pesticides and microplastics. This co-exposure makes risk assessment of contaminants alone more complex and underestimates the combined ecological exposure and impact on aquatic organisms and human communities. The study objectives are to summarize the available ecotoxicological data from 2020 to 2026, which was obtained from the Scopus, Web of Science, PubMed and Google Scholar databases. Eighty-seven peer-reviewed studies were evaluated that fulfilled the pre-defined inclusion criteria. The Niger Delta, Lagos Lagoon basin, Benue and Cross River systems and Ogun River catchment are ecologically important and anthropogenically stressed areas reviewed. In the studies reviewed, 68% of the surface waters and sediments contained levels of cadmium, lead, arsenic and chromium that were higher than the permissible limits set by WHO and NESREA. Concentrations of total petroleum hydrocarbon (TPHC) in the waterways of the Niger Delta ranged from 12 to 4,200 mg/kg in sediments and the level of polycyclic aromatic hydrocarbon (PAH) carcinogenic equivalent quotients (CEQ) in fish tissues exceeded threshold levels. Even though national bans have been placed on the use of organochlorine pesticides, residues of these chemicals, especially dichlorodiphenyltrichloroethane and its metabolites, remained in sediments and biota. At least 14 river systems were found with abundances of microplastics between 340 and 8,200 particles per cubic metre. Clarias gariepinus, Oreochromis niloticus and Synodontis spp. were the commercially important fish species with bioaccumulation factors greater than unity. These multiple classes of contaminants occur together and thus can lead to synergistic effects at organism and population levels in terms of toxicity and endocrine disruption. Lack of multi-contaminant monitoring in one package and weak enforcement of discharge standards is a structural gap that hinders ecological risk management. There is a need for coordinated regulatory reform and adoption of mixture toxicity assessment protocols. This study aims to assess the levels and sources of pollutants in the Nigerian freshwater environment, identify the toxic effects of pollutants, and develop an integrated risk assessment framework for the Nigerian freshwater environment.The objectives of this study are to determine the levels and sources of pollutants in the Nigerian freshwater environment, identify the toxic effects of pollutants and develop an integrated risk assessment framework for the Nigerian freshwater environment.