Pollution and risk assessment of heavy metals in water, sediment and fish (Clarias gariepinus) in a fish farm cluster in Niger Delta region, Nigeria

The study determined the following heavy metals: cadmium (Cd), chromium (Cr), copper (Cu), manganese (Mn), lead (Pb) and zinc (Zn) concentrations in surface water and in fish pond (water, sediments and farmed fish (Clarias gariepinus)) from a fish farm cluster with the view of assessing its pollution level and associated human health exposure risk to these heavy metals through fish consumption. Samples were digested with aqua regia and metal concentrations were determined with an atomic absorption spectrophotometer equipped with an air acetylene flame. Mean concentrations of the heavy metals (ppm) in surface and pond water ranged as follows: cadmium (below detection limit (bdl): 0.298); chromium (bdl: 0.257); copper (bdl: 0.067); manganese (bdl: 0.163); lead (bdl: 0.736) and zinc (bdl: 0.078) while in sediment, the ranges (mg/kg dry weight) were as follows: cadmium (1.156-3.329); chromium (9.439-14.373); copper (26.710-121.819); manganese (3.143-37.202); lead (0.340-1.537) and zinc (87.681-371.060). The mean concentrations of the metals in surface water were significantly different (p<0.05) from those in sediment in all the sampling sites. The mean metal concentrations (mg/kg dry weight) in C. gariepnus were in the following ranges: cadmium (0.733-1.405); chromium (0.593-1.692); copper (175.882-245.321); manganese (bdl: 3.326); lead (0.221-0.263) and zinc (248.191-292.333). Some of the heavy metal concentrations obtained in water, sediment and catfish analyzed were above the permissible limit set by some National (DPR) and International organisations (WHO, FEPA and FAO). The pollution studies showed that lead, cadmium and chromium contaminated the surface water samples. Sediment from all sites showed low to considerable contamination by the heavy metals. The human exposure risk assessment of the metals showed that the total hazard index was less than one which indicates no probable adverse health effect from the consumption of fish from the ponds, although, this case is different for Pb since there is no estimation of oral reference dose (RfDo) for lead according to EPA (2020).