| 其他摘要 | This thesis examines the small-scale gold mining area of Tongguan County in Shanxi Province (Xiaoqinling region), as a case study of an area using amalgamation for gold extraction. The distributions of mercury and methylmercufy are studied in air, water, soil and some crops in that region affected by gold mining activities, from the point of view of environmental geochemistry. The environmental geochemistry behaviors and modes of transport involving the transformation and enrichment of mercury in gold mining environments are thoroughly discussed. The total mercury emissions due to gold mining activities in the past over twenty years in Tongguan County are approximated and extrapolated for the whole of China. In addition, feasible recommendations on how to improve the environmental management of gold mining areas are proposed. Through the detailed studies reported in this thesis, some main conclusions are as follows: 1. 1. The spatial variability pattern of airborne mercury is clearly presented in Tongguan County: the airborne mercury concentrations in the zone where gold mills are densely distributed are higher than that in other zones, and decrease with increasing distance away from the gold mills. The mean airborne mercury concentration is 18,047 ng/m~3, with the maximum of 33,080 ng/m~3, found in an amalgamation workshop of a larger gold mill. The average concentrations in areas near gold mills and in areas moderately distant from gold mills are 385 ng/m~3 and 77 ng/m~3 respectively; while the mean airborne mercury concentration is down to 29 ng/m~3 in areas distant from gold mills, and 2 ng/m~3 in background level areas along the banks of the Yellow River which are little affected by air movements from the gold mining region. In Tongguan County, every miner living in the study area would inhale an estimated 72,804～75,268 ng of mercury per day, and every local inhabitant (adult) would inhale 924～4,620 ng of mercury per day. 2. In Tongguan County, the mean total mercury concentration in spring and well water samples is 329.2 ng/L and the mercury concentrations of all different species of them are all extremely high, which possibly shows the higher mercury background levels in gold mine areas, and indicates that the local grdundwater has suffered mercury contamination from gold mining activities. Dissolved mercury concentrations in river water samples collected in cold and warm seasons and methylmercury concentrations in warm season water are several times higher than that in spring and well water'samples, and also several hundreds times higher for other mercury species. There is a positive correlation between the reactive and dissolved mercury concentration in water. The mean concentrations of reactive and dissolved mercury in water in cold season are marginally higher than that in warm season water, but the opposite: is true of particulate mercury. In a few water samples, reactive mercury levels are higher than dissolved mercury, but particulate mercury is the predominant species in total mercury levels and in most samples exceeds 50 %. 3. In sediment samples collected during cold and warm seasons, the. mean.'total mercury concentrations are up to 151.1 and 44.6 u.g/g, respectively, and the mean methylmercury concentrations are 7.1 and 3.6 ng/g, respectively. There is a positive correlation between the total mercury and methyl mercury concentrations in warm season sediments. Particles in water and sediments are derived mainly from waste residues and tailings emitted from gold mills, so they have a common lithological origin and both of them contain high heavy metals (Hg, Fe, Mn and Cu) levels. However, all heavy metals are found in particles characterized by small particle diameter, large surface area, organic material: and strong adsorption ability, and thus the mercury levels are obviously higher than that in ordinary sediments. There are positive correlations between Hg and other heavy metals levels in particles, but not in sediments, and the only tight correlation is found between the concentrations of Hg and Cu. 4. In Tongguan County, the mean mercury concentration in gpld ore is 6.9.2 ng/g, while the mean mercury concentration in tailings is 628.4μg/g,. exceeding 10,000 times the gold ore background concentration. The levels of other metals (Fe, Mn, Cu) have no significant difference between the gold ore and tailings, as they are little affected by gold mining activities. Loess can resist the disturbirig of gold mining activities because of its own special structure and character, so the heavy metal concentrations in loess can preferably show their background,levels. The total mercury and methyl mercury levels in other soils are about 50 time Higher than that in loess respectively, while there is no remarkable'difference for other. heavy metals among all soil samples. The methylmercury concentrations contain small ratios in comparison to total mercury concentrations in soil (ranging from 0.001 % to 0.57 %), possibly related to the lower methylation level of mercury in soil. 5. High mercury concentration in local moss reflects the significant precipitation of airborne mercury in the gold mining region. High concentrations of total mercury and methylmercury in algae in a stream show the high enrichment ability of mercury in organisms. In Tongguan County, mercury in most crops is adsorbed mainly from airborne mercury and mercury concentrations in almost all crops exceed seriously Chinese edible criterion, by as much as 60 times national limits. Methylmercury concentrations contain ratios ranging from 0.12% to 12.34% total mercury, while methylmercury concentrations in vegetables are higher than that in wheat. In total, there is a negative correlation between the total mercury and methylmercury concentrations in crops. In Toungguan County, the local inhabitants would intake 54,720 ng total mercury per day through eating rice (wheat), which is much higher than the quantity of mercury inhaled through breathing. And these data have exceeded the mercury intake criteria to people recommended by international experts. 6. In Tongguan County, methylmercury concentrations are very low compared with total mercury concentrations in water, sediment, soil and crop samples, possibly due to the impact of the geological background of quartz veins gold ore. Specifically, minerals with Fe and Mn can astrict the ions of mercury, reduce or restraining methylation of mercury to some extent. 7. After confirming the outputs of gold extracted by amalgamation techniques in Tongguan and mercury release factors from the amalgamation activities., mercury releases due to gold mining activities could total 1476.6 t during the period from 1980 to 2003 in China, with 78 % into atmosphere cycle. And there could be 116.5 t mercury emissions from Tongguan County alone, representing 7.9% of releases. Due to the 1996 ban on amalgamation technique and improvements in amalgamation technologies, mercury emissions have steeply decreased. The mean annual emission of mercury due to gold mining activities in whole country is now about 30 t, including 3.9 t from Tongguan County, representing approximately 13 %of Chinese emissions. |
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