High levels of carcinogens have been detected in rivers and water treatment plants around U.S. military bases in Okinawa Prefecture, but local authorities seeking the source have been denied access to the installations.

The refusal represents the latest example of the Japan-U.S. Status of Forces Agreement (SOFA) hindering Japanese officials trying to address problems facing local residents.

The SOFA, which stipulates the jurisdiction and legal status of U.S. military forces in Japan, states, “Within the facilities and areas, the United States may take all the measures necessary for their establishment, operation, safeguarding and control.”

High concentrations of the compounds perfluorooctanesulfonic acid (PFOS) and perfluorooctanoic acid (PFOA), which has a structure similar to PFOS, were found at test sites near the U.S. Air Force’s Kadena Air Base and the U.S. Marine Corps Air Station Futenma in Ginowan.

PFOS and PFOA are typically used in firefighting foam and other chemicals.

According to the prefectural government, the U.S. military announced in July 2016 that there was the possibility that the foam was used on four occasions to extinguish fires at Kadena since 1994. It also said the leak of the compounds was confirmed on nine occasions since 2001.

The Japanese government banned PFOS in April this year after it had restricted the import, production and use of the compound from 2010 except for in the manufacturing of semiconductor devices, among other exceptions.

No maximum levels for PFOS and PFOA are set for tap water in Japan.

But in 2016, the U.S. Environmental Protection Agency set the limit for PFOS and PFOA in tap water to a maximum of 70 nanograms together per liter. A nanogram is equal to one-billionth of a gram.

Experts say it is not supposed to pose health problems to humans even if they drank 2 liters of water containing that level of PFOS and PFOA every day for 70 years.

The prefectural government studied the water quality of rivers and water treatment facilities at 55 sites in the prefecture between fiscal 2013 and fiscal 2018.

Of these, 15 located near U.S. bases showed the two compounds exceeding the combined limit of 70 nanograms.

But the levels in the remaining 40 locations, including those near Nago and around Naha Airport in the prefectural capital of Naha, were at or below 70 nanograms.

Of the 15 sites showing a high concentration of the toxins, six were located near Kadena Air Base, which straddles Kadena, Okinawa and Chatan.

Up to 1,379 nanograms of the two organofluoric pollutants were recorded in samples from the Dakujakugawa river that runs through the base.

The figure reached 120 nanograms at the Chatan water treatment facility in Chatan during a study in fiscal 2015. The facility draws water from the Hijagawa river, with which the Dakujakugawa river joins.

A maximum 1,300 nanograms of the compounds were discovered at nine locations, including the site of spring water that is used for irrigation, in the vicinity of U.S. Marine Corps Air Station Futenma in Ginowan.

On Nov. 22, prefectural officials announced that 2,000 nanograms were detected at a site near the Futenma facility in a study they conducted this summer.

With PFOS and PFOA figures higher around the two bases, the prefectural government believes that the contamination sources are located within the installations.

In January 2016, the prefectural government requested an on-site inspection of the bases via the Defense Ministry’s Okinawa Defense Bureau, but U.S. forces declined.

The bureau, too, planned to conduct a study of the Kadena and Futenma bases in fiscal 2017, but the U.S. side again said no.

Officials familiar with the matter say the U.S. side notified local authorities that a meeting of the Japan-U.S. Committee should be called to consider giving a green light to their request for an on-site inspection.

The committee is formed primarily of representatives of foreign and defense ministries of Japan, as well as U.S. forces, to discuss how to interpret and operate the 1960 SOFA.

Attendees at the committee meeting rejected the Okinawa Defense Bureau’s request for access to the U.S. bases.

An official at the bureau declined to elaborate on the specifics of the meeting.

“We are taking the concerns of Okinawans seriously and are determined to work closely with the prefectural government, the U.S. side and relevant entities,” the official said.

Prefectural officials tried to make a breakthrough on the contamination issue by proposing a meeting with representatives from the U.S. military and the defense bureau.

Although the three parties met four times from October 2016, the U.S. side has not disclosed the results of its study concerning the suspected contamination.

In efforts to control the pollution, the prefectural government spent about 170 million yen ($1.5 million) in fiscal 2016 to install new activated charcoal filters at the water purifying facilities.

It is also calling on Okinawans living around the U.S. bases not to drink spring water.

In reply to an inquiry by The Asahi Shimbun this month, an official at the Media Section of Kadena Air Base said in an e-mail, “Access for on-the-spot observations and sampling would have to be requested through the Joint Committee.”

It added: “We are committed to protecting the health of our airmen, families and Okinawan neighbors.”