Okonofua et al. (2024) sampled water, cabbage, and tilapia fish from abandoned mining pits in Bukuru, Jos South, Nigeria, measuring Cd, Mn, Hg, Cu, Ni, Pb, Zn, and U in both rainy and dry seasons, with a clean control site at Riyom. The primary finding is that Cd, Mn, Cu, Ni, Pb, and Zn in cabbage exceeded both control levels and Nigeria’s NESREA maximum allowable limits in both seasons (dry season more severely), and that all tested heavy metals in tilapia fish except uranium exceeded the NESREA fish consumption limit in both seasons. The study demonstrates bioaccumulation from mining-contaminated irrigation water into both vegetables and fish at the same site.
Key numbers
Cabbage (Brassica oleracea var. capitata), mean concentrations (all values mg/kg or as noted; NESREA limit in parentheses):
- Cd: rainy season 0.00145, dry season 0.0213 (limit <1 mg/kg) — within limit numerically but elevated vs. control (0.0006 rainy, 0.00129 dry)
- Mn: rainy 0.01289, dry 0.1883 (limit <1 mg/kg)
- Cu: rainy 0.0894, dry 0.1083 (limit <1 mg/kg)
- Ni: rainy 0.01 (10⁻²), dry 0.011 (limit <0.59 mg/kg)
- Pb: rainy 0.001 (10⁻³), dry 0.001 (10⁻³) (limit <1 mg/kg) — low, consistent with control
- Zn: rainy 0.2459, dry 0.6557 (limit 3 mg/kg) — below limit but elevated vs. control (0.0266 rainy)
- Hg: not detected in cabbage
- U: not detected in cabbage
Tilapia fish (Oreochromis niloticus), seasonal concentrations (mg/kg; NESREA standard in parentheses):
- Cd: rainy 0.01, dry 0.021 (limit 0.01) — dry season exceeds limit
- Mn: rainy 0.010, dry 0.188 (limit 0.03) — dry season exceeds limit
- Cu: rainy 0.066, dry 0.208
Water: significant difference at control vs. site for seasonal mean comparison (ρ < 0.05, d = 0.02); dry-season Cd showed statistical difference from control. Dry season generally elevated across metals.
Methods (brief)
AAS (Bulk Scientific Model 200H AAS) and separate air-acetylene AAS from Kaduna laboratory for complementary analysis. Sample digestion: hot plate, 5 mL concentrated HNO₃; fish and cabbage ashed at appropriate temperature before digestion; water filtered and acidified. Results expressed as ppm (mg/L for water, mg/kg for solids). Statistical analysis: basic descriptive statistics and Student’s t-test for seasonal/control comparisons. Limitation: small n per matrix (n=8 for cabbage and fish); single geographic area; Hg not detected may reflect AAS method sensitivity limitation for ultra-trace Hg.
Implications
Certification: This is a mining-impact study in Nigeria; concentrations reflect extreme contamination scenarios not representative of commercial vegetable supply chains. The key certification relevance is the regulatory threshold calibration: NESREA limits for fish and vegetables are comparable to Codex and EU limits, confirming that Nigerian regulatory framework aligns roughly with international standards on these metals.
Courses: Useful case study for the mining-to-food-chain contamination pathway and for demonstrating why irrigation water source matters for food safety. Seasonal effect (dry season worse) is well documented here.
App: Supports a mining-region origin flag for cabbage and fish from West African mining zones.