Understanding the Bioaccumulation of Mercury in Rice Plants at the Wanshan Mercury Mine, China: Using Stable Mercury Isotopes

Rice can accumulate a substantial amount of mercury (Hg). At the Wanshan Mercury Mine (WMM), the bioaccumulation of Hg by rice was complicated by historical large-scale mining and active artisanal smelting activities. This study investigated the concentration and isotopic composition of Hg in rice roots, leaves and paddy soil collected from abandoned Hg mining sites and active artisanal smelting sites at the WMM. At all sites, correlations between delta Hg-202 and 1/THg were observed in soil, suggesting a mixing of Hg from waste calcine and unroasted Hg ore. Significantly positive correlations were also observed in THg concentrations between roots and soil (r(2) = 0.84, p < 0.01) and between leaves and total gaseous mercury (r(2) = 0.87, p < 0.01), reflecting that roots and leaves receive Hg mainly from soil and atmosphere, respectively. Large differences of -0.87 +/- 0.57 parts per thousand and -1.85 +/- 0.57 parts per thousand in delta Hg-202 between the root and soil were observed at abandoned Hg mining sites and active artisanal smelting sites, respectively, suggesting that roots preferentially accumulate lighter Hg isotopes from the soil. Offsets of 0.05 parts per thousand and 0.27 parts per thousand in Delta Hg-199 between rice roots and leaves, with higher Delta Hg-199 values in roots than leaves, were observed at abandoned Hg mining sites and active artisanal smelting sites, respectively. The Hg-mass-independent fractionation signal is believed to be caused by Hg(II) photoreduction in paddy water; however, the extent of photoreduction is variable among rice paddies. Overall, this study suggests that Hg isotopes can be a useful tool to understand the sources and bioaccumulation of Hg in rice paddies.