Summary
This study measured zinc, lead, iron, magnesium, chromium, and cadmium concentrations in three commercially important fish species (Nile tilapia, African sharptooth catfish, and Flathead grey mullet) from two slaughterhouse-wastewater-impacted rivers (Transamadi and Mgbuosimiri) and one control site (Iwofe) in Rivers State, Nigeria. Flathead grey mullet (Mugil cephalus) from Mgbuosimiri had the highest lead (1.50 ± 0.05 mg/kg) and cadmium (0.41 ± 0.02 mg/kg), exceeding EU maximum levels for fish muscle (lead 0.30 mg/kg, cadmium 0.05 mg/kg) by 500% and 800%, respectively. Principal component analysis (PCA) explained 77.5% of the variance, with lead and cadmium clustering as anthropogenic sources while magnesium loaded independently, consistent with a geogenic tracer role. The target hazard quotient (THQ) for lead approached unity in Flathead grey mullet (0.88), and THQ for cadmium reached 0.97; the hazard index (HI) exceeded 1.0 in all species from Mgbuosimiri, peaking at 2.07 in Flathead grey mullet.
Key numbers
| Measure | Value | Units | Matrix | Source location |
|---|---|---|---|---|
| Max lead (Pb), Flathead grey mullet (Mgbuosimiri) | 1.50 ± 0.05 | mg/kg | Fish muscle | Abstract |
| Max cadmium (Cd), Flathead grey mullet (Mgbuosimiri) | 0.41 ± 0.02 | mg/kg | Fish muscle | Abstract |
| EU ML, lead (Pb) in fish muscle | 0.30 | mg/kg | Reference | p. 3 |
| EU ML, cadmium (Cd) in fish muscle | 0.05 | mg/kg | Reference | p. 3 |
| lead (Pb) exceedance (% over EU ML) | 500% | percentage | Flathead grey mullet | Abstract |
| cadmium (Cd) exceedance (% over EU ML) | 800% | percentage | Flathead grey mullet | Abstract |
| THQ for lead (Pb), Flathead grey mullet | 0.88 | dimensionless | Consumers | Abstract |
| THQ for cadmium (Cd), Flathead grey mullet | 0.97 | dimensionless | Consumers | Abstract |
| HI (all metals), Flathead grey mullet (Mgbuosimiri) | 2.07 | dimensionless | Consumers | Abstract |
| PCA variance explained | 77.5% | percentage | All species and sites | Abstract |
| CR, Flathead grey mullet | 3.97 × 10^-4 | dimensionless | Consumers | Abstract |
Methods (brief)
Water and fish samples collected during dry season (March 2025) from three sampling sites along a 15 km river stretch in Rivers State. Metal concentrations measured by atomic absorption spectrophotometry (AAS). Two-way ANOVA assessed species and location effects. PCA used for source apportionment. EDI, THQ, HI, and CR calculated following USEPA guidelines using JECFA reference doses. Control site (Iwofe): a hydrologically comparable river body with no upstream abattoir operations.
Evidence fitness
The lead and cadmium exceedances in Flathead grey mullet from the slaughterhouse-impacted Mgbuosimiri site are substantial (500% and 800% above EU MLs) and represent a genuine food safety hazard for populations consuming fish from this specific source. The carcinogenic risk approaching the upper acceptable limit (3.97 × 10^-4 vs. 10^-4 to 10^-3 range) is an additional concern. This study is an impacted-site assessment; the findings are specific to slaughterhouse-effluent-impacted rivers and cannot be used as general Nigerian freshwater fish contamination benchmarks. The PCA approach using magnesium as a geogenic tracer adds scientific rigor to source apportionment.
Limitations
Dry season only; metal concentrations may vary with seasonal flow. Single sampling event. No certified reference material recovery data reported in the available pages. Market-level fish (from distribution networks rather than point-of-capture) may dilute or amplify contamination depending on supply chain mixing.
Related evidence
Update history
No substantive edit history is available in this build. The full commit record is available in git.