The water has been used to cool three melted reactors since the nuclear disaster in March 2011. According to BBC, "most of the radioactive isotopes have been removed using a complex filtration process. But one isotope, tritium, cannot be removed so the water has been stored in huge tanks which will fill up by 2022."
Environment Minister Yoshiaki Harada supports the plan of dumping the water into the Pacific, where it would be diluted, posing a low risk to human and animal health. The Japanese Government, however, said that no decision was made yet.
Fishermen’s groups are opposed to the plan, as it affects their activities and reputation directly. The South Korea government also has its concerns due to the geographical proximity and the possible impacts the dumping may cause to the health and safety of people and ocean.
Further sources: CNN, CBC (Video)