Gill-tissue bioaccumulation of heavy metals in oreochromis niloticus and labeo coubie and associated drinking-water health risk in the lower river Niger, Lokoja, Kogi state, Nigeria

Abstract

Bioaccumulation of aquatic pollutants by fish in the lower River Niger at Lokoja can expose those who consume them to elevated risk, yet species specific bioaccumulation and consumption risk data are lacking for these fish communities. Samples of Oreochromis niloticus and Labeo coubie were collected monthly over six months spanning the dry season from January to March and the onset of the rains from June to August, and concentrations of cadmium, lead, chromium, zinc, mercury and nickel were measured in gill tissue by atomic absorption spectrophotometry. Bioaccumulation factors were computed from paired tissue and water concentrations collected at the same stations on the same occasions, and the estimated daily intake, hazard quotient, hazard index, margin of exposure and cancer risk were computed for the drinking water exposure pathway separately for adults and children. Gill tissue is the site of waterborne uptake rather than the edible portion, so the concentrations reported here are an upper bound on the edible portion burden and any inference drawn from them about consumption exposure is correspondingly conservative. Tissue concentrations of cadmium, lead, zinc and mercury remained below the WHO and FAO limits in every month and in both species, whereas chromium, at 1.04 to 1.28 mg per kg against a limit of 0.15, and nickel, at 1.14 to 1.25 mg per kg against a limit of 0.6, exceeded theirs throughout. Nickel then rose sharply in August, to 3.70 mg per kg in O. niloticus and 3.65 in L. coubie, approximately six times the permissible limit and the largest tissue exceedance recorded in the study. Bioaccumulation factors were below unity for lead, chromium, zinc and mercury, close to unity for cadmium, and above unity for nickel in both species, reaching 2.952 in L. coubie. In the health risk assessment chromium dominated the non carcinogenic profile, with an adult hazard quotient of 0.883 and a child hazard quotient of 1.68 in the dry season, rising to 0.918 and 1.74 in the wet season. Hazard indices exceeded unity for both age groups in both seasons and were approximately twice as high for children as for adults. The study concludes that chromium and nickel, rather than the metals most often monitored, determine risk at this reach, and that children are the population for whom the acceptable exposure threshold is crossed.