SIOUX FALLS, S.D. (AP) — Sioux Falls officials are grappling with well shutdowns as the extent of the city's water contamination from decades of firefighting foam use remains unclear.

Sioux Falls currently has 19 municipal wells sitting dormant in the aftermath of innumerable gallons of toxic firefighting foam that contaminated the grounds of the city airport nearly 50 years ago, the Rapid City Journal reported. Chemicals linked to cancer and other health issues were found to have contaminated 15 municipal wells, including 10 that have concentrations above what the Environmental Protection Agency deems safe.

About 28 percent of the city's water production from the Big Sioux aquifer is shut down.

The South Dakota Air National Guard and the Sioux Falls Fire Department both used the toxic firefighting foam for many years near the airport, which led to the contamination of the city's drinking water. But the scope of the issue is still unknown.

"We really haven't determined the extent of release yet," said Capt. Jessica Bak, a public affairs officer with the Air Guard at the Sioux Falls Regional Airport.

In 2013, the city's water purification plant found chemicals from firefighting foam, known as per and polyfluoroalkyl substances (PFAS), at levels below the EPA's health advisory level. The level of exposure beneath the EPA's threshold means there aren't expected adverse health risks.

The city responded to the findings by testing all municipal wells to identify the source and shutting down every well where the chemicals were found.

City engineer Tim Stefanich, who oversees the water system, acknowledged that "there was a little bit of time between" finding the contamination, determining its source and deciding to shut off wells. But he said that there was minimal fear of an immediate health risk with the low levels of exposure.

The city tested for PFAS again in 2014 as part of an EPA-mandated water sampling program, but didn't detect any of the chemicals. The city tested again in 2016, when some low levels were found.

The city shut off more wells, leading to the 19 wells offline today. Water leaving the city's purification plant is now sampled monthly, and no water samples have contained the chemicals since 2016.

Stefanich and Trent Lubbers, the city's utilities operation administrator, believe the contaminated water situation is under control.

The city has been purchasing water from the Lewis and Clark Regional Water system, a nonprofit, wholesale provider of treated water. But Sioux Falls will likely need a more sustainable option.

"They have the short term kind of covered," said Mark Meyer, drinking water program administrator for the state's Department of Environment & Natural Resources. "But as we march into the future, having 28 percent of their well capacity offline . the future is going to come sooner than later."

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Information from: Rapid City Journal, http://www.rapidcityjournal.com

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