Shumba et al. (2025) assessed total mercury (tHg) concentrations in tilapia (Oreochromis niloticus), soil, and sediments from copper mining and non-mining areas along the Kafue River in Kitwe District, Zambia. Tilapia in the mining area had a mean tHg of 0.015 mg/kg (15 µg/kg) in muscle tissue compared to 0.007 mg/kg (7 µg/kg) in the non-mining area — a statistically significant difference. Soil and sediment Hg levels in the mining area exceeded USEPA reference values, but fish tHg concentrations and estimated daily intake (EDI) from fish consumption remained below USEPA and WHO/FAO maximum tolerable limits. The Kafue River drains a region where copper smelting has operated for over 80 years, and mercury enters the system primarily from smelting operations and artisanal gold mining.
Key numbers
Fish muscle tissue tHg (ICP-MS, wet weight):
- Mining area tilapia (n=32): mean 0.015 mg/kg (15 µg/kg ww)
- Non-mining area tilapia (n=47): mean 0.007 mg/kg (7 µg/kg ww)
- Difference statistically significant: p ≤ 0.05
Soil tHg:
- Mining area: mean 1.066 mg/kg (exceeds USEPA reference of 0.3 mg/kg)
- Non-mining area: mean 0.041 mg/kg
Sediment tHg:
- Mining area: mean 1.304 mg/kg (exceeds USEPA reference of 0.2 mg/kg)
- Non-mining area: mean 0.034 mg/kg
Health risk:
- EDI from fish consumption (mining area): below USEPA/WHO/FAO PTDI of 0.1 µg/kg/day and 0.23 µg/kg/day
- THQ < 1 (no adverse health effect risk at current consumption rates)
- Correlation between fish length and Hg accumulation: r = 0.232, p = 0.1166 (not statistically significant)
Reference standards used: NIST SRM-2710, NIST-2709 (soil/sediment), DORM-3 (fish); recoveries 95–101%.
Methods (brief)
Cross-sectional study, October 2023 (dry season). Purposive sampling of medium-sized tilapia (12–15 cm) using gill nets and fishing rods. Fish muscle tissue digested in UltraCLAVE at 190°C with HNO₃; 1 mL HCl added to prevent Hg loss. Soil and sediment digested with aqua regia at 180°C. All matrices analyzed by ICP-MS (Agilent 7700). Detection limit for Hg: 0.000001 mg/L. Quality control: gold (50 µg/L) added to all samples to amalgamate Hg; rhodium (Rh) used as internal standard; recoveries 100–101%.
Note: tHg reported (not speciated MeHg). For freshwater fish, the majority of fish tissue Hg is typically MeHg, but this study did not perform speciation.
Implications
Certification: Tilapia from copper-mining-impacted freshwater sources in sub-Saharan Africa carries tHg at 15 µg/kg — well below regulatory limits but measurably elevated compared to non-mining areas. Provenance documentation for tilapia sourcing is warranted.
Courses: Illustrates how artisanal mining and industrial smelting co-contaminate freshwater fish with Hg even when final fish concentrations remain below regulatory limits; sediment serves as a persistent Hg reservoir.
App: Tilapia from Zambia/copper-belt region: elevated tHg flag relative to non-mining areas; still below WHO limits at current fish size and consumption patterns.