| Mercury is a global pollutant,which is highly toxic to human health.Since the occurrence of Minamata Disease in the early 1950s,understanding mercury pollution is an important issue in the realm of environmental science.A wide range of hazards are associated with mercury pollution,making it difficult to control.Once released,mercury is prone to migration and may undergo biogeochemical conversion. Through microbial activity,inorganic mercury can be transformed into more toxic methyl mercury,which is biomagnified through the aquatic food chain,and is harmful to human health.Wujiang River is the largest tributary in the upper reaches of the south bank of the Yangtze River.The Wujiang River originates in the foothills of western Guizhou Province,near Wumeng Mountain,and crosses the central and northeastern part of Guizhou,from the Yanhe county before entering the area of Chongqing,and flowing into the Yangtze River.The total length of the main stem is 1037 kilometers.The watershed is 88,267 square kilometers,including 67500 square km in Guizhou Province.In China,13 cascade hydropower stations are under development in the Wujiang River Basin,including one in Puding Reservoir,located at the first level. Puding Reservoir was created in 2007 summer and 2008 spring and is considered an intermediate-stage reservoir,since it was created before the most recently created reservoirs,but is not as old as the oldest reservoir in the area.In order to understand mercury migration,transformation,and accumulation in an intermediate stage reservoir,surface water,pore water and sediment samples were collected in the wet season(August 2007) and the dry season(April 2008).Mercury species were determined in surface water samples,including dissolved mercury,particulate mercury,reactive mercury,and methylmercury.In pore water, total mercury and methylmercury were determined.Sediment profiles included organic matter,total mercury and methylmercury content.Diffusive flux was also calculated.This study includes the following results:1.The concentration of total mercury and particulate mercury were significantly correlated,as particulate total mercury strongly affects the migration and distribution of mercury.The ratio of methylmercury to total mercury averaged as 11%in the wet period,and during the dry season averaged 13%.Concentrations of methylmercury and total mercury were not significantly related to each other. DOC and dissolved mercury were also not correlated.2.The concentrations of total mercury,dissolved mercury,reactive mercury, methylmercury,dissolved methylmereury concentrations in wet period were higher than the dry season.This was mainly due to changes in rainfall,suggesting the importance of exogenous mercury inputs.3.The total mercury concentration of sediment ranged from 0.06～0.29 mg/kg, and averaged 0.18mg/kg,which is higher than that of other non-contaminated lake sediments.The total mercury levels in four sampling sites did not vary seasonally. Sediment methylmercury ranged from 0.04～2.73ng/g,and averaged 1.06ng/g.An obvious seasonal variation occurred,with higher methylmercury levels during the rainy season compared to the dry season.The percentage of methylmercury of total mercury was 0.59%,which was lower than the river(1～1.5%),also lower than the proportion measured in other nearby uncontaminated sites,including Hongfeng Lake (5.1%),and Aha Reservoir(0.91%).4.The concentrations of total mercury in pore water ranged from 4.33～47.35ng/L,and averaged 14.03ng/L;methylmercury concentrations ranged from 0.12～9.43ng/L,and averaged 1.70ng/L.Pore water depth profiles indicated a large peak at 10cm,which may be related to sediment organic matter and the concentration of S2-.The pore water flux of mercury was lower than flux measured at the Aha reservoir,Hongfeng lake,Wujiangdu Reservoir and Dongfeng reservoir.
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