The US Marine Corps Futenma Air Station in Okinawa announced it has begun releasing treated wastewater that contains organofluorine compounds into a local sewerage system.
Okinawa Prefecture responded by asking the military to stop the release immediately. Officials noted that their investigation into how to deal with the water is ongoing.
The US forces announced on Thursday that they have started to release the water being stored at the base. It contains perfluorooctane sulfonate and perfluorooctanoic acid.
US officials had previously asked Japan to allow the release, after treating the contaminated water to meet the provisional reference level set by Japan.
The US military stressed that the treated water is safe enough, noting that levels of PFOS and PFOA were reduced to 2.7 nanograms per liter or lower. It added that the levels are far lower than the reference level of 50 nanograms per liter.
The military also said it will transfer and incinerate the extracted substances outside the prefecture.
The prefecture says the US military started the operation unilaterally. It demanded on Thursday that the military immediately stop the release. It also asked Japan's central government to make an official request to the US side and have them suspend the operation.