Enamorado-Montes et al. 2021 — Total mercury accumulation in three commercial rice varieties grown in La Mojana region soil, Colombia
This controlled greenhouse study evaluated total mercury (tHg) accumulation in roots, husks, and grains of three commercial Oryza sativa L. varieties (Fedearroz-473, Fedearroz-2000, Fedearroz-Mocarí) cultivated in soils from La Mojana, Colombia, spiked to ambient (130 µg kg−1), moderate (800 µg kg−1), and high (1500 µg kg−1) Hg concentrations. La Mojana is an ecologically significant wetland region downstream from artisanal gold-mining activity on the San Jorge and Cauca rivers, where background soil Hg in rice-growing sites already exceeds world-soil averages (~70 µg kg−1). Across all varieties and treatments, Hg partitioned strongly into roots (overall mean 739.04 µg kg−1 tHg), with substantially lower concentrations in husks (74.18 µg kg−1) and grains (17.02 µg kg−1), and translocation factors from root to grain were less than 0.13 in all cases. All measured grain concentrations remained below the hazard-quotient threshold for noncarcinogenic risk (HQ < 0.36), though the FA473 variety at the highest soil Hg level (1500 µg kg−1) exceeded both the European Commission maximum level for tHg in rice (10 µg kg−1) and the Chinese National Standard (20 µg kg−1).
Key numbers
Grain tHg by variety and soil treatment (µg kg−1, dry weight):
| Variety | Soil 130 µg kg−1 | Soil 800 µg kg−1 | Soil 1500 µg kg−1 |
|---|---|---|---|
| FA473 | 8.64 ± 1.28 | — | 26.15 ± 3.23 |
| FA2000 | all values below 20 µg kg−1; no significant effect of soil Hg | ||
| FAM | below Chinese threshold at 130 µg kg−1 | — | 20.09 ± 0.48 |
Root tHg: Overall mean 739.04 µg kg−1; highest value 1950.70 ± 347.15 µg kg−1 (FA2000, 1500 µg kg−1 soil treatment).
Husk tHg: Overall mean 74.18 µg kg−1; FA2000 at intermediate soil level reached 105.32 ± 3.51 µg kg−1.
Grain tHg overall mean: 17.02 µg kg−1 across all treatments and varieties.
Translocation factor (root→grain): <1 in all cases; highest was FA2000 at ambient soil Hg, 0.129.
Regulatory reference points: EU Commission Regulation (EU) 2018/73 sets a maximum tHg level of 10 µg kg−1 in rice. China GB 2762-2017 sets 20 µg kg−1. The ambient-soil (130 µg kg−1) native Hg concentration in La Mojana region is below Colombian/international agricultural-soil Maximum Allowable Concentration ranges (500–5000 µg kg−1) but above the crustal/world-soil average of ~70 µg kg−1.
Health risk: Hazard quotients (HQ) calculated using a La Mojana-specific weekly rice intake of 1218 g/week and JECFA PTWI of 4 µg Hg/kg body weight/week. HQ ranged up to 0.350 (FA473, 1500 µg kg−1 soil) — below the HQ = 1 threshold — at even the most extreme experimental soil conditions.
Biomass: Highest dry biomass at 1500 µg kg−1 soil was FA473 (308.76 ± 108.26 g); lowest was FAM (140.02 ± 2.36 g). Weight per 1000 grains ranged 17.64–20.62 g overall mean 19.16 g; significantly lower at 800 µg kg−1 soil for all three varieties.
Methods (brief)
Greenhouse pot experiment at University of Córdoba, Colombia (October 2014–February 2015). 3 × 3 factorial design: three varieties × three soil Hg levels, three replicates, 27 experimental units total. Surface soil (0–30 cm) from a La Mojana rice-growing site was homogenized, sieved (4 mm), and spiked to target concentrations using Hg(NO3)2 solution; 15-day stabilization before planting. Water management was aerobic (not flooded). Hg in roots, husks, and grains was analyzed by Direct Mercury Analyzer (DMA-80 TRICELL, Milestone) via thermal decomposition and amalgamation per EPA method 7473. Measurement is total Hg (tHg); the study does not speciate inorganic Hg and methylmercury. LOD = 0.01 ng tHg per sample. Method validated against certified tomato leaves (CRM 1753a, 34 µg kg−1); recovery 97–102%, RSD 5.35%.
Important limitations: This is a controlled greenhouse study under aerobic conditions at spiked concentrations up to ~11× the ambient La Mojana soil level. Aerobic conditions reduce MeHg production relative to flooded paddy conditions; results may underestimate tHg and MeHg grain accumulation under actual flooded-field management. The study measures tHg only; it does not partition MeHg from inorganic Hg in the grain, which is toxicologically important. The three varieties (Fedearroz series) are Colombian commercial cultivars; results do not directly generalize to other rice markets.
Implications
Certification: This paper establishes that commercially grown rice in mining-impacted Colombian regions can approach or exceed EU tHg limits (10 µg kg−1) at ambient soil concentrations for some varieties. For the HMT&C program, it is relevant to tHg in rice ingredients and rice-based products, particularly when sourcing from mining-adjacent origins. The study does not provide MeHg data; for MeHg exposure from rice, separate speciation studies are needed.
Courses: Illustrates the root-as-barrier mechanism limiting tHg translocation to grain, the importance of cultivar selection in contaminated-soil regions, and the relevance of water management regime (aerobic vs flooded) to Hg speciation and bioaccumulation.
App: The grain tHg values in this study (ranging from ~8.6 to 26.2 µg kg−1 under spiked-soil conditions) are above commercial US/EU market survey levels but demonstrate the upper range possible in mining-impact origin regions. The typical_ppb field for tHg in rice should remain anchored on broader market surveys rather than this single-origin, spiked-soil study.
Microbiome: Not directly addressed in this paper.