Effects Of Eutrophication On The From And Distribution Of Mercury In Wanfeng Reservoir

As the minamata disease erupted in Japan, scholars pay extensive attention to methyl mercury pollution. The recent study in north American and European has found that the aquatic environment after impounding of reservoir may in favor of mercury methylation and biological accumulation. However, studies refer to the influence of eutrophication gives to the transformation of migration are severely limited in china nowadays. In addition, mechanism on the form distribution of mercury after eutrophication is unclear. In this paper, Wanfeng reservoirs in the Nanpanjiang Riveris is chosed to be the object of study.,and the transformation of migration infulunced by eutrophication in aquatic environment can be studied more deeply. There are four main research topics in present paper:(1) Water chemical parameter analysis; (2) Studies on the methyl mercury distribution in Wanfeng reservoir; (3)To ensure the extent of eutrophication in Wanfeng reservoir:(4)The relationship between Eutrophication level and methylmercury distribution. Through the analysis of measured data of autume and winter in Wanfeng reservoir, six main results are obtained as follow:1)An clear thermal stratification had been formed at the vertical profile of water column in Autumn and Winter, because of that, convection exchange in each layer of water is limited, dissolved oxygen and pH both show the same layered as temperature. In autumn, a double thermocline appeared in the depth of 8～12 m and 40～50 m, which effectively limited the convective exchange of upper, intermediate and bottom of the layer water,that make water chemistry in different layers difference significantly. In winter, the double thermocline appeared in the depth of 30～60 m as well, even though there was significant thermal stratification just in winter, but it was in instability state. Convection happened, which lead the Surface water thickened to 30m, oblique temperature layers disappeared, material exchange could find between upper water and the lower water directly. 2)The results of the tests showed that, in winter, the 60m above water was aerobic layer, and the low layer was anaerobic type, the degree of saturation of DO was below 5%. In winter, the water of lake can be divided into two layers:aerobic layer (0-70m) above, and anaerobic layer (below 80m). The stratification of DO and ORP was nonconformity, the latter on was under the interface of aerobic layer and anaerobic layer. The oxidation and reduction properties of the water could be determined by DO, oxide material and reductive materials in the water.3) In the autumn and winter, the total concentration of P (as the chlorophyll in autumn) in the upper later water of the lake Wanfeng was moderate eutrophication, the total concentration of N achieve entrophication level. Influenced by thermal stratification, the eutrophication was limited in the upper layer water and the bottom water. In the autumn, characterized by a great mount of planktonic algae, TSS, DOC concentrations were significantly higher than winter in the upper layer, In the bottom water, characterized by reducing environment, iron and Mn are hydrogenated, heavy metal Ion were released constantly.4) Total mercury, dissolve mercury, particle state mercury has similar distribution rule in wanfeng resevoir. In autumn, due to the influence of thermal stratification, plankton adsORPtion, and atmospheric mercury settlement, the inflex of THg, DHg and PHg were unreasonably higher. The anaerobic environment of the bottom water cause this consequence:mercury was released from sediments to upward water constantly.5)The average concentration of Dissolved Methylmercury (DMMHg) in the water were 0.153(in autumn),0.171(in winter) in Wanfeng resevoir. The same as the other water chemical parameters, DMMHg appears stratified phenomenon. And the same as the ORP, DMMHg can be divided into two layers, DMMHg content of upper layer was low, and DMMHg content would achieve max value in depth of 80m, and then fall rapidly. Influenced by many environmental factors, near the interface of ORP, DMMHg was high unreasonably, and near the interface of water-sediment, process of DMMHg was restrained in winter.6)Due to the eutrophication and tradional breeding, There were a lot of organic matter as the bottom sediment, the decomposition which makes bottom water turn into a reductive environment, and the methylation process in bottom water was promote. Thus it can be seen, the methylation process was promote by eutrophication and tradional breeding. As mentioned above, the conclusions are as follows:a) The main methylation places Was restricted at Bottom water; b) There was an abnormal methylation near the ORP interface; c) the methylation process was promoted, because the chemical characteristics of Wanfeng resevoir were changed by eutrophication.