Distribution Characteristics Of Mercury In Soil Around Coal-fired Power Plant In The Southeast Coastal Area

As one of the most important anthropogenic sources, mercury (Hg) emissions from coal-fired power plants (CFPPs) can be transported and transformed in the atmosphere, leading to soil pollution via dry and wet deposition. The research of concentration level, chemical forms, spatial distribution and its influence factors of Hg in soil around CFPPs, is the basis for further analysis of migration and transformation rules of Hg in the atmosphere-soil-water-plant system. This study aims to solve three scientific issues based on the soil around the CFPP in Zhejiang Province: first one is to study the spatial distribution and its influence factors of total mercury (THg) through analyzing the concentrations of THg of 79 top soil samples around the CFPP; the second is to explore the vertical distribution characteristics and influence factors of THg and methyl mercury (MeHg) in eleven profiles soil samples through the concentrations analysis; last one is to explore the chemical forms and spatial distribution characteristics of Hg through analyzing different forms of Hg contents in soil by BCR three-step continuous extraction method. The main results are summarized as follows:(1) The concentration of THg in top soil ranged from 0.017 to 3.341 mg/kg, with the average of 0.186 mg/kg. Although THg in the areas showed an increasing trend in the top soil, it did not cause significant soil contamination. Besides CFPP, some coal stoves without air pollution control devices (APCDs) made a great contribution to the THg concentration in the local area. The spatial correlation of THg was quite weak, suggesting that the concentration of THg was little influenced by parent soil material, while affected by industrial pollution and human activities. Concentration of Hg in top soil around the CFPP displayed double peak features with the variation of distance to the chimney. It also showed noticeable geographical trend. Affected by the prevailing wind, the high concentrations appeared in the west-north-west(WNW), west(W) and west-south-west(WSW) direction. Influenced by the pollution characteristics of different Hg species, the maximum THg concentration areas appeared at a distance of 6-8 km away from the chimney, and the second higher concentration area appeared at 1-3 km(2) The concentration of THg in profile soil ranged from 17.26～152.78 ng/g, with an average of 67.51 ng/g. The concentration decreased with the depth of soil layer and significant difference was found in various soil layers. Meanwhile, low contents of THg in deep soil indicated that most of the THg in the top soil derived from anthropogenic pollution. The concentration of MeHg in profile soil ranged from 0.017 to 0.439 ng/g, with an average of 0.135 ng/g. The concentration decreased with the increasing of soil depth. However, there were significant differences between the top and middle soil layers. No significant differences were found between the middle and deep soil layers. The concentration of THg and MeHg was significantly and positively correlated with soil organic matter (SOM) and clay, while had weak negative correlation with pH. We speculated the main factors that affected the distribution of MeHg were THg and SOM. Land use pattern did not affect the concentration of THg in the top soil but affect the concentration of MeHg significantly. The concentration of MeHg in paddy soil was higher than that in vegetable soil and uncultivated soil. But no significant difference was found between vegetable soil and uncultivated soil.(3) Mercury exists in soil mainly as the form of final residue. The partition coefficient of mercury in each fraction was:Final residue (79.91%)>>Oxidizable (13.53%)> Reducible (5.91%) >> acid Exchangeable (0.64%). Land use pattern had no significant influence on the varied of mercury form in the top soil. Vertically, the concentrations of mercury in each fraction decreased with the increasing of soil depth, while the partition coefficients had irrelative variation. Mercury methylation rate in soil was between 0.05%～0.57% with an average of 0.18%, indicating only a minority of mercury taken part in the methylation process. Mercury in acid exchangeable had the most significant positive correlation with MeHg, which played a more important role in the process of mercury methylation. The partition coefficient of bioavailable mercury in soil was 0.64%, and the potential biological effective mercury was 22.37%, which showed only a small amount of mercury is biological effective in the soil.