长荡湖围堰干法环保清淤工程实施效果评价

内源污染是造成湖泊富营养化和水体重金属风险的重要来源。以长荡湖围堰干法清淤工程为例，探讨了清淤工程实施前后的底泥营养盐和重金属质量分数及上覆水体水质变化，开展了清淤工程实施效果评价，提出了围堰干法清淤工艺适用性思考。结果表明，围堰干法清淤工程对底泥的部分营养盐去除效果良好，底泥中有机质含量的平均降低幅度为27.09%，14个采样点位中，有11个点位清淤后底泥有机质平均含量低于清淤前；清淤后的底泥TN质量分数比清淤前平均降低76.95%，底泥TN质量分数得到有效削减；清淤后底泥TP质量分数比清淤前提高了35.16%，原因可能与采样季节、采样方法、磷素沉积及清淤过程中的机械干扰有关。清淤工程有效降低了底泥重金属风险，对底泥镉的平均去除率为76.00%，清淤后没有点位底泥生态风险等级超过“中风险”，底泥生态风险显著降低。清淤工程实施前后上覆水体水质无明显变化，清淤后水质仍处于Ⅳ类―V类水，超标因子为TN和TP。围堰干法施工有效降低了长荡湖底泥中部分营养盐和重金属赋存量，但是软弱地基区域施工机械难以实施，机械操作可能造成下层底泥的翻动，部分区域难以完全实现预期清淤目标。该研究结果表明，应综合考虑清淤目标和施工机械、场地条件等影响因素合理选择清淤工艺和确定清淤深度。

Study on the implementation evaluation of the dry method dredging technology for cofferdam of Changdang Lake

Endogenous pollution is an important source of lake eutrophication and heavy metal risk. Based on the cofferdam dry dredging project for Changdang Lake in Taihu Basin, the variation of water quality and the content of heavy metal and nutrient of dam are discussed according to the analysis of water and sediment samples. The results showed that the implementation of the dredging project has a good effect on the removal of some nutrients in the sediment, and the average reduction of the content of organic matter in the sediment is 27.09%. In the 14 sampling sites, the average content of organic matter in the sediment after dredging is lower than that before desilting in 11 sites. The TN content of sediment after dredging is 76.95% lower than that before desilting, and the TN content of sediment was effectively reduced. The TP content of sediment after dredging is 35.16% higher than before, which may be related to the sampling season, phosphorus deposition and mechanical interference during dredging. In addition, the sediment removal project effectively reduced the risk of heavy metals in sediment, and the average removal rate of cadmium in sediment was 76.00%. After sediment removal, the ecological risk level of sediment at no point exceeded “medium risk”, and the ecological risk of sediment was significantly reduced. The dredging project did not significantly affect the water quality, and the water quality after dredging was lower class IV, according to Environmental Quality Standard for Surface Water (GB3838-2002). Although dry digging method can effectively reduce part of nutrient salt and heavy metal storage in the sediment, in actual engineering, the depth of dredging cannot be accurately controlled, and it may cause certain disturbance to the sediment. The selection of dry dredging process should be carried out according to the actual situation of the project.