The Japanese and US governments are concerned about a large amount of Pacific Ocean debris, set afloat by last year's tsunami caused by the Great East Japan Earthquake, that is expected to reach the US West Coast starting in autumn.
The trouble stems from the lack of an international agreement on who is responsible for disposing of debris released into the sea.
Experts have also said harmful substances mixed with debris will likely damage the marine environment.
According to the Environment Ministry, the total amount of debris resulting from the March 11, 2011, disaster in the three stricken prefectures of Iwate, Miyagi and Fukushima is estimated at 18.11 million tons.
About 4.8 million tons of additional debris is further believed to have been washed out to sea.
About 70 per cent of drifting debris consists of such things as cars and cargo containers. Most debris is believed to have sunk off the coast of the Tohoku region.
However, the remaining 30 per cent, or about 1.54 million tons, became floating debris, including collapsed houses and wood from disaster-prevention forests. These articles have been drifting in the Pacific Ocean since the disaster.
A large amount of debris is expected to wash ashore on the US West Coast in October or later, according to the Environment Ministry's calculation based on data collected from satellite images and ocean currents.
Drifting debris such as a fishing boat and motorcycle have already landed on the US coast.
It became a local news phenomenon when a large floating pier from Aomori Prefecture washed ashore in Oregon.