Photo/Illutration Authorities in Ginowan, Okinawa Prefecture, collect water from the sewage system outside the city's U.S. Marine Corps Air Station Futenma. (Provided by Ginowan city government)

GINOWAN, Okinawa Prefecture--Extremely high levels of a suspected carcinogen detected in the city's sewage system apparently resulted from dumping of contaminated water by the U.S. Marine Corps Air Station Futenma, local officials said.

Ginowan municipal government authorities announced the finding on Sept. 10 and said further studies would be carried out to clarify if the base in the center of the city was indeed responsible for the high levels of perfluorooctane sulfonic acid (PFOS).

According to city officials, the U.S. base began releasing the water into the Ginowan sewage system around 9:30 a.m. on Aug. 26. At 11:10 a.m. or so, city officials collected water samples at a site outside the base where sewage passes through.

A check of water quality found 670 nanograms of PFOS per liter of water. Water quality guidelines for drinking and river water restrict the combined presence of PFOS and other substances to under 50 nanograms per liter of water.

City officials said they were unable to determine a direct causal effect because sewage at the site where the water was collected also contains wastewater released from the base.

But in checks of water quality levels conducted at the same site in 2020 and 2019, the level of PFOS detected was 13 nanograms per liter and 25 nanograms per liter, respectively.

Before the water was released into the sewage system, U.S. military officials said it had been processed so that the level of PFOS was brought under 2.7 nanograms per liter.

“I am just staggered at the extremely high concentration level,” Ginowan Mayor Masanori Matsugawa told a Sept. 10 news conference after the latest finding. “At the present time, we cannot definitively say the results were due to the release by the base, but because the Okinawa prefectural government is also conducting its own study, we want to assess the problem after their results are released.”

Another check of water quality will be done on a sample collected from the same location on Sept. 9.

The Okinawa prefectural assembly convened a plenary session on Sept. 10 and unanimously passed a resolution of protest over the release of water by the U.S. military as well as an opinion paper addressed to Prime Minister Yoshihide Suga.

The document called for an explanation of the U.S. military's decision to go ahead with the water release while discussions on the issue were continuing between Japan and the United States. It also called on the central government to conduct a study within the premises of the Futenma base.

The protest resolution said the assembly found the situation intolerable at a time when local residents had still not overcome their concerns about the leakage of firefighting foam containing PFOS from the Futenma base into a nearby river in April 2020.

Another opinion paper was also passed that unanimously called on the U.S. military to suspend flights of MV-22 Osprey transport aircraft until measures are taken by the Japanese and U.S. governments to prevent a recurrence of an Aug. 12 incident in which parts of an Osprey based at Futenma fell off in flight.

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This article was corrected on Sept. 16, 2021.

It originally described PFOS as perfluorooctane sulfonate.