Li et al. 2017 — Total mercury in vegetables and grains near coal-fired power plants, China
This study measured total mercury (tHg) in vegetables, grain crops, and soils at six locations within 10 km of two newly operational coal-fired power plants in China (Power Plants A and B, both <5 years of operation at time of sampling), finding that 79% of vegetable samples and 67% of grain samples exceeded China’s GB 2762-2012 food safety standard (10 µg/kg fresh weight for vegetables, 20 µg/kg for grains). Mercury concentrations in soil and plant tissue were negatively correlated with distance from the power plants (R² = 0.82, P < 0.001), and the distribution pattern with leaves containing more mercury than roots indicated atmospheric deposition of particulate mercury rather than primarily soil uptake. Water rinsing reduced leaf mercury concentrations by 19–63%, consistent with surface deposition of fly ash. The study identifies coal-fired power plant proximity as a significant and quantifiable risk factor for mercury contamination in local food production.
Key numbers
All food concentrations in µg/kg fresh weight (FW). China standard GB 2762-2012: vegetables 10 µg/kg FW, grains 20 µg/kg FW.
Soil mercury (µg/kg, mean ± SE):
- A1 (1 km from Plant A): 305.10 ± 47.97
- A2 (3 km from Plant A): 157.81 ± 20.52
- B1 (1 km from Plant B): 383.23 ± 32.59
- B2 (3 km from Plant B): 294.91 ± 15.67
- B3 (5 km from Plant B): 179.14 ± 13.53
- B4 (10 km from both plants): 124.58 ± 6.14
- Control site (>55 km): 32.01 ± 1.30
Background soil Hg for China: ~37 µg/kg. Both power plant sites exceed background by 4–10×.
Vegetable tHg (µg/kg FW) — selected crops across locations A1–B4:
| Crop | A1 | B1 | Max reported | Control |
|---|---|---|---|---|
| Lettuce | 39.04 ± 4.41 | 21.03 ± 0.16 | 39.04 | 0.35 ± 0.10 |
| Amaranth | 46.40 ± 2.33 | 29.29 ± 5.06 | 46.40 | 0.28 ± 0.21 |
| Water spinach | 86.69 ± 2.16 | 54.46 ± 4.55 | 86.69 | 0.85 ± 0.22 |
| Tomato (fruit) | 71.80 ± 11.95 | 76.33 ± 5.47 | 76.33 | 0.73 ± 0.36 |
| Eggplant | 42.37 ± 4.24 | 43.36 ± 1.71 | 43.36 | 0.43 ± 0.39 |
| Pepper | 49.66 ± 1.40 | 62.09 ± 3.22 | 62.09 | 0.93 ± 0.84 |
| Cucumber | 38.45 ± 1.40 | 18.21 ± 1.19 | 38.45 | 0.87 ± 0.24 |
| Cowpea | 56.31 ± 4.03 | 57.30 ± 9.24 | 57.30 | 0.93 ± 0.13 |
Fold-exceedance of China 10 µg/kg FW limit: lettuce 2.1×, amaranth 2.9×, water spinach 5.4×, tomato 7.6×, eggplant 4.3×, pepper 6.2×, cucumber 1.8×, cowpea 5.7×. Control samples did not exceed the limit for any vegetable.
Grain tHg (µg/kg FW):
| Crop | A1 | B1 | Max | Control |
|---|---|---|---|---|
| Rice | 62.95 ± 3.88 | 59.21 ± 4.36 | 62.95 | 0.55 ± 0.48 |
| Maize | 21.02 ± 1.98 | 21.18 ± 0.67 | 21.18 | 0.55 ± 0.63 |
Rice exceeds the grain limit (20 µg/kg FW) by ~3.0×; maize by ~1.1×.
Tomato organ distribution (B3 location, 5 km from Plant B): leaves 116.17 µg/kg, fruit 29.07 µg/kg, stems 18.35 µg/kg, roots 13.64 µg/kg. Higher leaf > fruit > stem > root pattern indicates atmospheric (foliar) deposition pathway.
Water rinsing effects on leafy vegetables: Lettuce leaf Hg reduced 19–41%; amaranth reduced 25–63% (B1 location: 56.06 unwashed → 29.29 washed; B2: 20.5 → 7.50; B3: 14.91 → 5.52; B4: 4.47 → 3.64).
Estimated weekly mercury intake for local residents (95th percentile concentration): vegetables 70.83 ng/g FW → PWI 2.674 µg/kg bw/week; grains 62.02 ng/g FW → PWI 1.687 µg/kg bw/week; total 4.36 µg/kg bw/week. WHO PTWI: 1 µg/kg bw/week. Estimated total intake is 4.36× above PTWI if all food is sourced from the plant vicinity. Assumptions: vegetable consumption 301 g/person/day, grain 217.6 g/person/day, body weight 55.9 kg.
Methods (brief)
Field locations within 10 km of two coal-fired power plants, Jiangsu area, China, sampled in 2015. Plant A: two 1030 MW units, operating since 2012, emitted ~437.8 kg Hg in 2013. Plant B: two 660 MW units, operating since 2013, emitted ~268.7 kg Hg. Vegetables and soils dried at 55°C, ground to fine powder. Digestion: EPA Method 7473. Analysis by atomic fluorescence spectrophotometry (AFS-230E). Species measured: total mercury (tHg) only; MeHg fraction not speciated. Pooled samples (~20 individual plants per pool). Control samples from grocery store >55 km from any power plant.
Critical species note: All mercury measurements are total Hg (tHg). The paper explicitly distinguishes tHg from methylmercury (MeHg) in the text, noting that terrestrial plant Hg is predominantly Hg(0) and Hg(II) with low MeHg fractions (unlike fish/seafood), but no speciation data for MeHg in these vegetable samples are reported. The high tHg values near power plants therefore cannot be directly compared to MeHg-based intake guidance without speciation; the health risk calculations use tHg as a conservative upper bound.
Implications
Certification: Proximity to coal-fired power plants is a supply-chain sourcing risk factor for leafy vegetables, rice, and other crops. A sourcing requirement specifying distance from coal combustion facilities, or lot-level tHg testing for crops from high-coal-density agricultural regions (eastern China in particular), is supported by this evidence. The data bracket the contamination gradient: 1 km gives ~60–87 µg/kg FW in leafy vegetables, 10 km gives ~7–24 µg/kg FW, and control (>55 km) gives <1 µg/kg FW.
Courses: Demonstrates foliar deposition as the dominant pathway for Hg in terrestrial vegetables near coal sources, contrasting with the root-uptake and methylation pathway dominant in rice-paddy and aquatic contexts. Water rinsing provides meaningful but incomplete decontamination (19–63% reduction), unlike for Pb and Cd where washing effects are smaller.
App: For geographic risk-scoring of leafy vegetables and rice, the model should flag crops described as sourced from or near high-coal-energy regions in eastern China. The supply-chain flag is distance- and operational-duration-dependent; short-operating plants in this study (3–4 years) already produce 4–10× background soil Hg.