Extremely Elevated Total Mercury and Methylmercury in Forage Plants in a Large-Scale Abandoned Hg Mining Site: A Potential Risk of Exposure to Grazing Animals.

Abstract

Ninety-five wild forage plants (belonging to 22 species of 18 families) and their corresponding rhizosphere soil samples were collected from wastelands of a large-scale abandoned Hg mining region for total Hg (THg) and methylmercury (MeHg) analysis. The forage plant communities on the wastelands were dominated by the Asteraceae, Crassulaceae, and Polygonaceae families. The THg and MeHg concentrations in the forage plants varied widely and were in the range of 0.10 to 13mg/kg and 0.19 to 23μg/kg, respectively. Shoots of Aster ageratoides showed the highest average THg concentration of 12 ± 1.1mg/kg, while those of Aster subulatus had the highest average MeHg concentrations of 7.4 ± 6.1μg/kg. Both the THg and MeHg concentrations in the aboveground plant parts exhibited positive correlations with the THg (r = 0.70, P < 0.01) and MeHg (r = 0.68, P < 0.01) concentrations in the roots; however, these were not correlated with the THg and MeHg concentrations in their rhizosphere soils. The species A. ageratoides, A. subulatus, and S. brachyotus showed strong accumulation of Hg and are of concern for herbivorous/omnivorous wildlife and feeding livestock. Taking the provisional tolerable weekly intake (PTWI) values for IHg recommended by the Joint FAO/WHO Expert Committee on Food Additives (JECFA in Summary and conclusions of the seventy-second meeting of the joint FAO/WHO expert committee on food additives Rome, Italy, 2010) for human dietary exposure of 4ng/g into account, the daily intake of IHg by a 65kg animal grazing on 1.0kg of forage (dry weight) would be between 190 and 13,200μg, three to five orders of magnitude higher than the permitted limit, suggesting a potential risk of exposure.