Accumulation and risk assessment of mercury in soil as influenced by mercury mining/smelting in Tongren, Southwest China

To investigate the influence of mercury (Hg) mining/smelting on the surrounding soil environment, ninety soil samples were collected around Hg mining/smelting areas in Tongren city, Guizhou Province, Southwest China. The total mercury (THg), methylmercury (MeHg), bioavailability and fractions of Hg in the soil and their potential risk were evaluated. The results showed that Hg mining/smelting significantly increased the soil pH and decreased the soil organic matter content (p < 0.05). The THg content in the surrounding soil was much higher than that at the control site, with almost all the samples exceeding the national standard in China (3.4 mg/kg, GB15618-2018). Similarly, the concentrations of MeHg (0.09-2.74 mu g/kg) and bioavailable Hg (0.64-62.94 mu g/kg) in these soil samples were also significantly higher than those in the control site. However, the MeHg/THg ratio was significantly lower in mining/smelting influenced soils (0.01-0.68%) than in control soils (0.60-3.72%). Fraction analysis revealed that residual (RES-Hg) and organic matter-bounded (OM-Hg) Hg accounted for more than 50% of the THg. Ecological risk assessment revealed that the potential ecological risk for most of the Hg mining/smelting-influenced soils (30.16 <= E-r <= 2280.02) were higher than those at the control site (15.12 <= E-r <= 27.1). In addition, these Hg mining/smelting-influenced soils posed acceptable noncarcinogenic risks to adults (except for two soil samples), with hazard indices (HIs) ranging from 0.04 to 1.11 and a mean HI of 0.44. However, children suffer serious noncarcinogenic risks, with HIs ranging from 0.34 to 7.43 and a mean HI of 3.10.