Govt to Allow Ocean Disposal of Volcanic Ash in Event of Mt. Fuji’s Eruption

Yomiuri Shimbun file photo
A massive amount of ash is expected to accumulate in the Tokyo metropolitan area in the event of Mt. Fuji’s eruption.

The government plans to allow ocean disposal of volcanic ash if the Tokyo metropolitan area is hit by a massive ashfall following an eruption of Mt. Fuji, according to government sources.

In the event Mt. Fuji erupts on a scale similar to the 1707 Hoei eruption, volcanic ash that would need cleanup would be 10 times greater than the amount of debris cleaned up after the 2011 Great East Japan Earthquake, according to projections by the government’s Central Disaster Management Council.

The government plans to include the policy in its guidelines on handling volcanic ash, to be compiled in spring 2024 or shortly thereafter.

Mt. Fuji has not erupted for over 300 years since the Hoei eruption and is believed to be in its longest dormant period in the past 5,000 years.

When the Hoei eruption started on Dec. 16, 1707, the ash from Mt. Fuji fell for 16 days and reached what is now central Tokyo. The eruption is estimated to have spewed out about 1.7 billion cubic meters of ash and other substances.

In April 2020, the council presented an analysis of impacts of ashfall in the event of an eruption on the scale of the Hoei eruption.

In the worst-case scenario, rail networks in the Tokyo metropolitan area will be paralyzed three hours after the eruption. If the eruption continues for about two weeks, about 10 centimeters of ashfall is projected in central Tokyo, while more than 30 centimeters is predicted in Kanagawa and Yamanashi prefectures.

Up to 490 million cubic meters of ash would need to be removed, according to the analysis. It also said it will be necessary to deploy about 1,000 units of heavy machinery and remove ash daily in cooperation with local governments concerned.

The envisaged guidelines will ask local governments to reach prior agreements with business operators handling heavy machinery and secure sites for temporarily storing and disposing volcanic ash.

Parks and playgrounds are anticipated as temporary ash storage sites while soil disposal sites are expected to be used for dumping. However, such sites are likely to prove insufficient, so officials plan to include ocean disposal in the guidelines.

The marine pollution prevention law bans ocean dumping of waste in principle. However, it will be allowed if the environment minister deems it necessary as an emergency measure.

According to related sources, while experts said ocean dumping of volcanic ash likely has little impact on the environment as it originates naturally, the government is considering testing in preparation for dumping to analyze environmental impacts and then carry out any necessary procedures.

The government plans to start full-fledged discussions with experts as early as January.