Sediment contamination and ecological risk assessment in the upper reaches of the Bonny Estuary, Niger Delta, Nigeria

Sediments from an estuarine creek were assessed in order to ascertain heavy metal contamination and ecological risk status. Seventy-two samples were collected in triplicate per site (July-December 2016), put in labeled polythene bags and preserved in ice chests before laboratory analysis. Air dried samples were digested with Equia-Regia (mixture of HCI and HNO₃, ratio of 3:1) and metals analysed using Atomic Absorption Spectrophotometery. Ecological risk indices were used to evaluate sediment quality/status. Mean metal concentration decreased as Zn>Pb>Cr>Ni>Cu>Cd and were lower than WHO and USEPA limits, implying minimal contamination concerns. Mean value of Cr at site AZ1 (dry season) exceeded probable effect level (PEL) but less than threshold effect level (TEL), Pb level equaled TEL but below PEL while Zn, Ni and Cd concentrations were less than TEL and PEL. Monomial-Eⁱ_r and multinomial-R_I potential ecological risk indices indicated low ecological risk for metals as Cd>Pb>Cu>Zn>Cr, sites-AZ1>AZ3>AZ2>OK (wet season) and AZ1>AZ2>AZ3>OK (dry season). Monomial (ERM–Qi) biological risk index indicated medium-low priority in order of Zn>Pb>Cr while Cu indicated low-priority risk. Multinomial (mERM–Q) biological risk index gave low-priority risk. Contamination factor-C_f across sites decreased as AZ1>AZ2>AZ3>OK with higher values during the wet season. Degree of contamination-C_d was low in order of Cd>Pb>Zn>Cr>Cu>Ni while index of geo-accumulation-I_geo values (-5.460 - 0.552) for metals showed Cd>Pb>Zn>Cu>Cr>Ni and sites (AZ1>AZ2>AZ3>OK). Pollution load index values-PLI also indicated no pollution. The study concluded that the sediment quality of the Azuabie creek was within acceptable limits but relatively deteriorated compared to the control site.