Okonofua et al. 2024 — Bioaccumulation of heavy metals in water, cabbage, and tilapia from unreclaimed mining pits, Bukuru, Nigeria

Okonofua and colleagues sampled surface water, cabbage (Brassica oleracea var. capitata), and Nile tilapia (Oreochromis niloticus) from five unreclaimed tin-mining pits in Bukuru, Jos South LGA, Plateau State, Nigeria, across both seasons of 2019, with a paired control site in Riyom LGA. Cadmium in surface water exceeded the Nigerian NESREA drinking-water standard (0.003 mg/L) at every mining-pit site in both seasons. Cabbage from the mining pits showed elevated Cd, Mn, Cu, Ni, Pb, and Zn relative to the Riyom control in the rainy season; in the dry season, control-site cabbage Cu, Ni, and Pb were unexpectedly higher than mining-pit values (paper-internal anomaly flagged in Verification notes). Tilapia exceeded the NESREA standard for Cd, Mn, and Ni in the dry season; the abstract’s stronger claim that “all investigated heavy metals except uranium” exceeded the fish-consumption limit is not consistent with the paper’s own Table 7. Uranium was not detected in any matrix in either season. The study is a localised mining-impact occurrence survey, not a commercial supply-chain dataset; its quantitative findings are direct evidence only for mining-affected freshwater fish and irrigation-water-exposed leafy vegetables in tin-mining regions.

Key numbers

All values in mg/L (water) or mg/kg (cabbage, fish, on a dry-weight basis given oven-drying and digestion of solids before AAS). NESREA = National Environmental Standards and Regulations Enforcement Agency (Nigeria).

Surface water — rainy season (Table 2)

Sites BKKS I–V vs. control (Riyom) vs. NESREA standard, mean concentration over 6 months:

• Cd: 0.0286 / 0.0377 / 0.0414 / 0.050 / 0.054; control 0.0099; NESREA 0.003. All five sites and the control exceed the standard.
• Mn: 0.0333 / 0.0279 / 0.0401 / 0.0288 / 0.0316; control 0.0031; NESREA 0.05. All sites below the standard.
• Hg: 0.0001 / not detected / 0.0002 / 0.0021 / 0.0021; control not detected; NESREA 0.006. All sites below the standard.
• Cu: 0.0178 / 0.0267 / 0.0638 / 0.0595 / 0.0465; control 0.0026; NESREA 2.0. All sites well below the standard.
• Ni: 0.006 / 0.0025 / 0.012 / 0.0253 / 0.0165; control 0.0013; NESREA 0.07. All sites below the standard.
• Pb: 0.0016 / 0.0181 / 0.020 / 0.0251 / 0.011; control 0.0001; NESREA 0.01. Sites II, III, IV exceed the standard; site V is at the standard.
• U: not detected at any site or the control; NESREA 0.0013.
• Zn: 0.2816 / 0.1222 / 0.076 / 0.0588 / 0.1604; control 0.0294; NESREA 3.0. All sites below the standard.

Surface water — dry season (Table 3)

• Cd: 0.0001 / 0.0444 / 0.0578 / 0.0628 / 0.0388; control 0.0099; NESREA 0.003. Sites II–V exceed the standard; site I is below detection.
• Mn: 0.0714 / 0.0412 / 0.0614 / 0.058 / 0.0644; control 0.0031; NESREA 0.05. Sites I, III, IV, V exceed the standard.
• Hg: <0.0001 / 0.001 / 0 / 0.0001 / 0.0001; control 0; NESREA 0.006. All sites below the standard.
• Cu: 0.4761 / 0.028 / 0.0317 / 0.0416 / 0.0167; control 0.0026; NESREA 2.0. All sites below the standard; site I is roughly an order of magnitude above the other sites, flagged as a likely outlier.
• Ni: 0.0001 / 0.0111 / 0.0216 / 0.0024 / 0.016; control 0.0013; NESREA 0.07. All sites below the standard.
• Pb: 0.010 / 0.0026 / 0.0112 / 0.0056 / 0.0047; control 0.0001; NESREA 0.01. Site I at the standard; site III exceeds the standard; remaining sites below.
• U: not detected at any site or the control; NESREA 0.0013.
• Zn: 0.071 / 0.1446 / 1.047 / 0.4776 / 0.0482; control 0.0294; NESREA 3.0. All sites below the standard.

Seasonal comparison (Table 4, mean ± SD across the five sites)

• Cd: rainy 0.042 ± 0.01 vs. dry 0.041 ± 0.02; p = 0.43; % deviation 1.2.
• Mn: rainy 0.032 ± 0.005 vs. dry 0.059 ± 0.01; p = 0.002 (statistically significant); % deviation 29.7. Dry season higher.
• Hg: rainy 0.001 ± 0.001 vs. dry 0.0001 ± 0.004; p = 0.16; % deviation 81.8.
• Cu: rainy 0.043 ± 0.02 vs. dry 0.119 ± 0.19; p = 0.24; % deviation 46.9. Dry-season mean inflated by the BKKS I outlier (0.4761).
• Ni: rainy 0.012 ± 0.009 vs. dry 0.010 ± 0.009; p = 0.36; % deviation 9.09.
• Pb: rainy 0.015 ± 0.009 vs. dry 0.007 ± 0.004; p = 0.08; % deviation 36.4.
• Zn: rainy 0.140 ± 0.09 vs. dry 0.358 ± 0.42; p = 0.19; % deviation 43.8.

Site vs. control aggregate comparison (Table 5): rainy season ρ < 0.05 (d = 0.02, statistically different from control); dry season ρ > 0.05 (d = 0.06, not statistically different from control). The authors interpret the rainy-season effect as evidence that storm-water runoff (diffuse pollution) is a significant contamination pathway.

Cabbage (Table 6, mg/kg)

Mining-pit sites vs. Riyom control vs. NESREA produce/vegetable standard:

• Cd: rainy 0.00145; rainy control 0.0006. Dry 0.0213; dry control 0.00129. NESREA <1.
• Mn: rainy 0.01289; rainy control 0.0025. Dry 0.1883; dry control 0.00146. NESREA <1.
• Hg: rainy not detected; rainy control 10⁻³. Dry 10⁻³; dry control <10⁻³. NESREA <10⁻³.
• Cu: rainy 0.0894; rainy control 0.0027. Dry 0.1083; dry control 1.243 (control unexpectedly higher than test sites — flagged below). NESREA <1.
• Ni: rainy 10⁻² (0.01); rainy control 10⁻³ (0.001). Dry 0.011; dry control 0.669 (control unexpectedly higher). NESREA <0.59.
• Pb: rainy 10⁻³ (0.001); rainy control <10⁻³. Dry 10⁻³ (0.001); dry control 10⁻² (0.01) (control unexpectedly higher). NESREA <1.
• U: not detected; NESREA <3.
• Zn: rainy 0.2459; rainy control 0.0266. Dry 0.6557; dry control 0.245. NESREA 3.

The abstract states: “compared to the control sample, Cd, Mn, Cu, Ni, Pb, and Zn was higher in cabbage.” This holds for the rainy season; for the dry season, only Cd, Mn, and Zn are higher in mining-pit cabbage than in the Riyom control. Cu, Ni, and Pb are higher at the control in the dry season. This paper-internal discrepancy is preserved as-reported and surfaced in Verification notes.

Tilapia fish (Table 7, mg/kg)

• Cd: rainy 10⁻² (0.01); rainy control 10⁻³ (0.001). Dry 0.021; dry control 0.001. NESREA 0.01. Rainy at the limit; dry exceeds the limit.
• Mn: rainy 0.010; rainy control 10⁻² (0.01). Dry 0.188; dry control 0.070. NESREA 0.03. Rainy below limit; dry exceeds.
• Hg: rainy not detected; rainy control 10⁻³. Dry 10⁻³; dry control 10⁻³. NESREA <10⁻³.
• Cu: rainy 0.066; rainy control 0.004. Dry 0.208; dry control 0.123. NESREA 1. Both seasons below limit.
• Ni: rainy 0.002; rainy control 0.005. Dry 0.061; dry control 0.015. NESREA 0.02. Dry exceeds the limit; rainy below.
• Pb: rainy 0.006; rainy control 0.000. Dry 0.027; dry control 0.006. NESREA 0.05. Both seasons below limit.
• U: not detected; no NESREA value reported in Table 7.
• Zn: rainy 0.163; rainy control 0.005. Dry 0.656; dry control 0.114. NESREA 1.0. Both seasons below limit.

The abstract claim that “the maximum allowable limit for fish consumption was exceeded by all investigated heavy metals except for Ur, which was not found” is not consistent with the paper’s own Table 7: in the rainy season only Cd is at the limit and no metal exceeds it; in the dry season Cd, Mn, and Ni exceed limits while Cu, Pb, and Zn remain below. Reported as-stated in the paper and flagged in Verification notes.

Water-to-tissue bioaccumulation comparisons (Figures 2–9, statistical tests)

• Cabbage vs. water, rainy: ρ > 0.05, d = 0.32 (no statistical difference — interpreted by authors as evidence of bioaccumulation).
• Cabbage vs. water, dry: ρ > 0.05, d = 0.13 (same interpretation).
• Cabbage vs. cabbage control: rainy ρ > 0.05, d = 0.09; dry ρ > 0.05, d = 0.21.
• Tilapia vs. water, rainy: ρ > 0.05, d = 0.28 (bioaccumulation inferred).
• Tilapia vs. water, dry: ρ > 0.05, d = 0.05 (same).
• Tilapia vs. fish control: rainy ρ > 0.05, d = 0.28; dry ρ > 0.05, d = 0.07.

Methods (brief)

Five sampling points (BKKS I–V) in Bukuru, Jos South LGA, chosen for accessibility and current use by local farmers and fishermen for irrigation and fish-pond aquaculture in unreclaimed tin-mining pits. Control point in Riyom LGA, selected as the LGA with the least mining activity in the catchment. GPS coordinates recorded at each point.

Sample collection: 32 water samples (250 mL plastic bottles, grab method, 30 cm below water surface) over wet (July–September 2019) and dry (January–March 2019) seasons. 8 cabbage samples (3 per season per cluster plus 2 controls) collected by hand into black polyethylene bags. 8 tilapia samples (3 per season plus 2 controls), collected by hook and net into black polyethylene bags. Sample bottles pre-cleaned per APHA (1998). All samples transported to the Kaduna State Environmental Protection Agency (KEPA) laboratory.

Water digestion: 50 mL of well-shaken sample + 5 mL concentrated HNO₃, heated on a hot plate at 100 °C until volume reduced below 20 mL, then made up to 100 mL with distilled water.

Cabbage and tilapia digestion: oven-dried whole-tissue chunks, pulverised, sieved through 2 mm mesh; ~2.0 g of dried tissue digested in 15 cm³ tri-acid mixture (HNO₃:HCl:H₂SO₄ 5:1:1) at 80 °C until translucent; filtered through Whatman No. 41 filter paper; diluted to 50 cm³ in distilled water.

Analytical instrument: Bulk Scientific Model 200H atomic absorption spectrometer at KEPA, air–acetylene flame. Calibration via standard series (0.2, 0.4, 0.6, 0.8, 1.0 ppm) prepared from a 1000 ppm stock; results expressed as ppm (mg/L for water, mg/kg for solids).

Statistical analysis: descriptive statistics (mean, SD) and one-tailed Student’s t-test at α = 0.05 for seasonal and site-vs-control comparisons, in Microsoft Excel and SPSS v16.0.

Limitations: small n per matrix (n = 8 cabbage and 8 fish across two seasons, only three replicates per season); single AAS instrument with air-acetylene flame, which gives limited sensitivity for Hg (the paper reports several below-detection cells for Hg without specifying an LOD); no method-blank or certified reference material results reported; no speciation for As (none measured), Hg (reported as total), or Cr (none measured); paper does not specify wet vs. dry weight basis for the cabbage and tilapia values explicitly, but oven-drying before digestion indicates dry-weight basis.

Implications

Certification: This is a localised mining-impact occurrence survey from a tin-mining region using subsistence-scale irrigation and aquaculture in unreclaimed pits. Concentration values are not representative of commercial cabbage or commercial freshwater-fish supply chains; they should be treated as upper-bound mining-affected occurrence data for the routing layer rather than as inputs to a clean-platform pooled percentile for either product category. The paper does, however, contribute Nigerian-context regulatory anchor values (NESREA standards for Cd, Mn, Hg, Cu, Ni, Pb, U, Zn in water, vegetables, and fish) that can cross-reference Codex and EU limits when those crosswalks are next refreshed.

Courses: Useful pedagogically for the mining-water-to-food-chain pathway, the seasonal contrast (dry-season concentrations generally higher in this dataset, attributed by the authors to reduced dilution), and the bioaccumulation argument by statistical-similarity test rather than by direct BCF calculation. The paper-internal contradictions between the abstract and Tables 6 and 7 are themselves a useful teaching example of why downstream reviewers must read tables, not summaries.

App: Supports a mining-region origin flag for freshwater fish and leafy vegetables from West African tin-mining catchments; not a direct input to commercial-product risk-flagging.

Verification notes

• 2026-05-18 (Claude session, merge-enhance): Re-read of full 7-page PDF against the prior page (updated 2026-05-14, raw_handle: papers-cube). The prior page captured the broad strokes but was incomplete on the data tables and contained one invented methods detail. Corrections below.
  • raw_handle updated from legacy papers-cube to canonical v2.0 PCMF_article-2-copy-4.
  • Added missing raw_sha256 (f8af4c35…9471) and access_url (DOI link).
  • metals frontmatter expanded from [Pb, Cd, tHg, Ni] to [Pb, Cd, tHg, Cu, Ni, Mn, Zn, U] to reflect the full eight-metal panel measured. All eight are reported in Tables 2–7. Speciation: paper measured total Hg by AAS without speciation, so tHg is correct; no As or Cr-VI measured.
  • matrices rationalised from [cabbage, fish, surface-water, irrigation-water] to the controlled vocabulary [surface-water, irrigation-water, leafy-vegetable, freshwater-fish]. Cabbage is captured as an ingredient; the matrix is leafy-vegetable. Tilapia from contaminated pits is freshwater-fish rather than the bare fish matrix.
  • ingredients extended to include [[ingredients/freshwater-fish]] alongside [[ingredients/cabbage]] (both pages exist in the live taxonomy).
  • products: [[products/fresh-fish]] retained: the paper describes tilapia from these pits being raised by local fishermen for local consumption, which is the freshwater-fish supply-chain context. The cabbage side has no commercial product slug in the current taxonomy.
  • Removed the unresolved [[regulations/nesrea-nigeria]] from the prior “Wiki pages updated on ingest” list; no NESREA regulation page exists, and creating one is outside this skill’s mandate. NESREA standards remain quoted by value throughout the body.
  • Key numbers rewritten in full from Tables 2–7 of the paper. The prior page reproduced cabbage Cd, Mn, Cu, Ni, Pb, Zn for both seasons and fish Cd, Mn, Cu only (truncated mid-table); now all five tables are reproduced with site-by-site values for water and full per-metal seasonal values for cabbage and tilapia.
  • Methods section corrected: the prior page asserted “AAS (Bulk Scientific Model 200H AAS) and separate air-acetylene AAS from Kaduna laboratory for complementary analysis.” This was invented; the paper describes ONE AAS instrument (Bulk Scientific Model 200H AAS at KEPA, air-acetylene flame) for all matrices. Corrected to a single-instrument description. Also added: tri-acid (HNO₃:HCl:H₂SO₄ 5:1:1) digestion at 80 °C for solids; APHA-compliant sample-bottle prep; SPSS v16.0 + Excel for stats; one-tailed t-test at α = 0.05.
  • Paper-internal anomaly preserved: Table 6 dry-season cabbage control values for Cu (1.243), Ni (0.669), and Pb (10⁻²) are higher than the corresponding mining-pit site values (Cu 0.1083; Ni 0.011; Pb 10⁻³). This contradicts the abstract’s claim that test cabbage exceeded control for all six elevated metals. The discrepancy is reported as-printed and flagged in the Key numbers and Implications sections so downstream synthesis does not silently average or aggregate without seeing it.
  • Paper-internal anomaly preserved: Table 7 contradicts the abstract’s claim that “the maximum allowable limit for fish consumption was exceeded by all investigated heavy metals except for Ur.” Table 7 shows Cu, Pb, and Zn below the NESREA fish limit in both seasons; only Cd, Mn, and Ni exceed the limit (in the dry season). Reported as-printed and flagged.
  • Brand firewall (Part 12, strict reading locked 2026-05-17): no brand attribution in the paper; sampling sites coded BKKS I–V plus Riyom control. The KEPA lab and Bulk Scientific Model 200H AAS are scientific-method vendor/material names (Exception 2) and are retained per the locked reading.
  • Wiki/HMTc firewall (Part 2): Implications section confined to routing relevance and pedagogical use; no threshold proposals, no consumer-audience risk advisory, no synthesis claims comparing this paper to other literature. The literature-similarity sentence in the prior page’s Implications block (“Nigerian regulatory framework aligns roughly with international standards”) was a cross-source synthesis claim and has been removed; NESREA standards are now reported by value only.
  • Legacy heading ## Wiki pages updated on ingest retained for consistency with the current Part 6 template and with sibling sources (e.g. afonne2020-heavy-metals-plant-foods).

Wiki pages updated on ingest

• cadmium
• manganese
• mercury-total
• copper
• nickel
• lead
• uranium
• zinc
• cabbage
• freshwater-fish
• fresh-fish

Page history

The five most recent substantive edits to this page. The full version history lives in git; when DOI minting comes online (see schema docs), each entry below will also link to a version-pinned DataCite DOI.