Zhang et al. 2026 — Trace metal contamination in five crops near a manganese mining area in Chongqing, China
Zhang and colleagues measured nine trace metals (Mn, Cd, Cu, Zn, Ni, Pb, As, Cr, Sb) in roots, stems, leaves, shells, and edible grain/tuber portions of five crops (rice, maize, peanut, soybean, sweet potato) grown near an active manganese mining and smelting area in Xiushan County, Chongqing. Rice showed the highest accumulation of As and Cr in edible grain portions among the five crops, with grain-specific As concentration of 0.2944 ± 0.0132 mg/kg (294 ppb) and Cr of 1.9800 ± 0.1682 mg/kg (1,980 ppb, exceeding China’s GB 2762-2017 limit of 1,000 ppb for total Cr in grain). Health risk assessment found Cr-THQ of 3.09 (adults) and As-THQ of 10.72 (children), indicating potential chronic risk for children consuming locally-grown rice, driven by high rice intake combined with the elevated As and Cr levels in this mining-affected area.
Key numbers
Grain/edible-portion concentrations (mean ± SD, mg/kg; ICP-MS, Agilent 7900):
Rice grain (n=16):
- As: 0.2944 ± 0.0132 mg/kg (294 ppb) — tAs
- Cr: 1.9800 ± 0.1682 mg/kg (1,980 ppb, exceeds GB 2762-2017 limit of 1,000 ppb)
- Pb: pollution index Pi < 1 (no pollution in grain); exact grain concentration not extracted
- Cd: Pi < 1 in grain (exact grain concentration not extracted)
- As-THQ children: 10.72 (chronic toxic effects threshold >10); adults: 4.90
Sweet potato root (n=15):
- THQ < 1 for all metals (all species); lowest heavy metal accumulation among the five crops
Soybean grain (n=15), Maize grain (n=15), Peanut kernel (n=10):
- THQ < 1 for both adults and children for all analytes
Mining-area context: Mn (2.27–3.37x higher in rice vs other crops), Cr root concentration up to 78.47 ± 27.11 mg/kg (rice roots), As root concentration 49.80% of total plant As in rice roots.
Single-factor pollution index for rice grain: As Pi = 1.5569 (China limit 0.5 mg/kg; wait — note: actual grain As 0.2944 < limit of 0.5 mg/kg but THQ still elevated due to low RfD), Cr Pi = 1.9659 (grain Cr 1.98 mg/kg > limit 1.0 mg/kg GB 2762-2017).
Note: As concentration 0.2944 mg/kg is total As. The As Pi uses national threshold of 0.5 mg/kg for total As in grain per GB 2762-2017.
Methods (brief)
ICP-MS (Agilent 7900) for all nine metals. Samples collected from Xiushan County, Chongqing near active Mn mining/smelting. Full plant sampling (roots, stems, leaves, shells, edible grain). Quality control: NIST 2711a (soil) and NIST 8436 (corn leaves) certified reference materials; spiked recovery 90–110%; relative standard deviation ≤5%. Statistical analysis: SPSS 19.0, one-way ANOVA with Tukey HSD. As speciation not performed; all As values are total arsenic. Cr values are total chromium (Cr-VI not speciated).
Implications
Certification: This is a mining-impact scenario, not representative of general commercial agricultural production. Rice from this area shows tAs ~294 ppb in grain and Cr ~1,980 ppb, both significantly higher than typical Chinese commercial rice (national median Cd ~60 ppb; As in commercial rice typically 50–200 ppb). Data are relevant for understanding contamination extremes near Mn mining operations in southwest China but should not be used as a general rice contamination estimate.
Courses: Strong case study for supply-chain geography effects on heavy metal contamination; illustrates that Cr (a metal often overlooked in food safety) can exceed regulatory limits when sourced from mining-impacted regions.
App: Flag: values from mining-impacted source area; not appropriate for general rice risk estimates. tAs ~294 ppb and Cr ~1,980 ppb in edible grain represent extreme-end contamination from a geologically anomalous site.