Abstract: Fukushima nuclear accident has caused serious pollution, including water body pollution. The water contamination and radiation from contaminated sediments have raised great concern among Japanese government and the public. A survey conducted by Ministry of Environment of Japan shows that most radioactive contaminants in water bodies are accumulated in sediments, and the water contamination is negligible under most circumstances. It has no need to deal with the contaminated sediments that constantly lie under several meters of water, for water shields \gamma -rays. However, the ponds in rural area with a dry period may have the necessity of disposal. Various construction companies in Japan have developed a variety of disposal methods according to the local conditions. This paper presents the investigations and researches of water body pollution initiated by Japanese Government, and the process of decision-making and operation of pollution treatment, with some representative new methods and their application cases. An introduction to the unique example of in-situ coverage for fluid mud within the harbor of Fukushima Daiichi Nuclear Power Plant is presented as well.