Assessment of Heavy Metals Pollution in Sediments of the Lower Orashi and Sombriero Rivers, Southern Nigeria: A Consensus-Based Sediment Quality Guideline Approach

The monitoring of the ecological health of rivers is key to ensuring optimal quality of water ecosystems. This study examined seasonal heavy metals (Cd, Cr, Cu, Pb, Zn and Fe) pollution in sediments of the lower Orashi and Sombriero Rivers, Southern Nigeria, using the consensus-based sediment quality guidelines (CBSQGs) (TEC and PEC) approach. A quasi-experimental research design approach was adopted for the research. The study area was divided into 10 grids, which constituted the sampling points (Ozuochi, Emesu, Ogbema Corridor, Ogonokom, Hulk-Transition Zone, Atala-Degema Waterfront, Opulogoloboko, Idama Flow Station, Minjidukiri, Ebemaboko). Samples from were collected from each of these 10 grids in the dry and wet seasons in 15ml plastic containers, treated with concentrated nitric acid (HNO3) to adjust pH to preserve the oxidation states of the metals, and thereafter transported to the laboratory for analyses. PinAAcle 500 Flame Atomic Absorption Spectrometer (AAS) described in API-IA 3111B and ASTM D3651 was used to determine heavy metals concentrations. CBSQGs were used for sediment quality comparison. Results revealed that dry season concentrations of heavy metals were higher in Orashi River compared to the Sombriero River, with concentration values for Cd and Zn exceeding their TEC values in most sites. Wet season concentrations of Cd, Cr and Zn were higher in Sombriero River than Orashi River and transition zone; while concentrations of Pb, Cu and Fe were higher in Orashi River. With exception of Zn whose wet season concentration were above TEC guideline in most stations, concentrations of other metals were below their TEC. All dry and wet seasons concentrations of metals were below their PEC guidelines. This study concludes that Zn is arguably a significant pollutant in the rivers that needs to be monitored to avoid widespread contamination of the entire rivers systems. It is therefore recommended that regular monitoring of both rivers should be prioritised by concerned authorities to track slightest changes.