Ecological Risk Evaluation of Biological and Geochemical Trace Metals in Okrika Estuary

Concentrations of heavy metals, aluminium (Al), chromium (Cr), copper (Cu), iron (Fe), nickel (Ni), vanadium (V) and zinc (Zn) were analysed in the tissues of two estuarine fish species; Tilapia (Sarotherodon melanotheron) and silver catfish (Chrysichthys nigrodigitatus) and also in the surface sediments from Okrika estuary in the Niger Delta area of Nigeria. The sampling was conveyed in two seasons, October/November, 2015 and January/February, 2016, representing the climax of both wet and dry seasons, respectively. The levels of the metals in the fish species and surface sediments were analysed by inductively coupled plasma-atomic emission spectroscopy (ICPAES). The sequence of the metal accumulations in Tilapia was; Fe>Al>Zn>Cu>Cr>V>Ni, in silver catfish; Fe>Al>Zn>Cu>Cr>Ni>V and for sediment; Al>Fe>Zn>Cr>V>Ni>Cu. Levels of the metals in the fish species and sediment were higher than the international permissible limits based on the criteria or application of relevant pollution indices and standards. This indicates that the two fish species were not fit for human consumption. A two-way analysis of variance was used to test the difference in metal concentrations in the fish tissues and sediment. Mean differences were separated by the use of t test. Although, some degree of pollution was found in the reference or control site because Niger-Delta is strongly localised and congested with oil-refining and other industrial activities thereby making it difficult to get a clean estuary. In comparison to the metal accumulations in the tissues of the fish species from oil-impacted creeks (Ekerekana, Okari and Ogoloma) and less oil-impacted counterpart (control site), higher levels (p<0.05) of the metals were observed in the impacted than the less impacted creek. For most of the metals, S. melanotheron had higher concentrations than the C. nigrodigitatus. Seasonal variations reflected that there was higher accumulation of the metals in the two fish species in the rainy season than the dry season. For sediment, there was significant difference for most of the metals between these two creeks except for Al. The multivariate tools of principal component analysis, Pearson correlation analysis and the use of vanadium/nickel ratio were used to confirm the possible source of the metals in the environmental matrices from oil pollution. Therefore, it is mandatory and expedient to ensure regular monitoring of the trace element loadings in these creeks and to take appropriate measures to alleviate the incident to safeguard the health of the public.