| Mercury pollution has increasingly become an environmental problem globally. In China, the government has paid great attention to mercury pollution control and remediation since the implementation of the Minamata Convention on Mercury. Guizhou Province has a 600-year history of mercury smelting. Currently 14 industries are associated with mercury in the province which has the most typical of mercury pollution issues in China. As an important source of drinking water in Guiyang, Baihua Lake is located in the central region of Guizhou Province, and is 16 km from Guiyang City. The lake was contaminated by the organic chemical waste from a mercury processing plant in the early period. The mercury concentration in the sediment ranged from 0.26 to 39.8 mg kg-1, with an average of 6.5 mg kg-1, which is substantially exceeded the soil background value in Guizhou(0.26 mg kg-1). Since the closure of the mercury processing plant, mercury released from the lake sediment became the most important sources of mercury pollution in water. Therefore, the main objective of this study was to evaluate the feasibility of employing in situ capping technology to control the mercury release from sediment, reduce the water pollution risk, and ensure the safety of the drinking water in Guiyang City. This study mainly includes the following three parts.1. To evaluate the release of mercury in sediment and pore water, sediment samples from four representative sites from upstream to downstream were taken and analyzed. The average mercury concentration in sediments was 6.084 mg kg-1, which is 21.2 times higher than the background value of soil. The average mercury concentration of the pore water was 23.7 ng L-1. Under similar hydrological conditions, the sediment mercury concentration in the downstream was significantly lower than that in the upstream. For the different hydrological conditions, the sediment mercury concentration in the back bay was higher than that in other areas of the lake. The overall concentration of mercury in the sediments decreased since the dredging of the upstream in 2008. However, the sediment and pore water mercury concentrations in the entrance of the lake have slightly increased since the dreging. This small increase of the mercury concentration is unlikely to have adverse impact on the water quality and ecosystems.2. In the present work, we tested 3 different capping materials, including zeolite, kaoline and hematite, for their effectiveness in controlling the release of sediment mercury. The laboratory simulation experiment showed that 2 cm was the optimal thickness of capping for Baihua Lake. The reduction rates of mercury release of zeolite, hematite and kaoline were 47.1%, 64.9% and 62.3%, respectively. Kaoline was found to be the most appropriate capping material. It was found that after capping the sediment with kaoline for seven days, there was no significant effect of the disturbance on mercury release.3. In the pilot scale experiment, a 22000-m2 area was severely contaminated with mercury within Baihua Lake which was isolated with tarpaulins. Kaoline powder was spread three times using a barge till the thickness reached 2~3 cm. After the capping layer settled and stabilized, concentration of mercury in the overlying water and pore water was monitored monthly for 6 months. The results showed that the kaoline capping resulted in the reduction rates of sediment release of mercury by 47.1% and phosphorus by 57%. |
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