ST. LOUIS — The Army Corps of Engineers has found radioactive nuclear weapon waste in residential lots along a St. Louis County creek.

Coldwater Creek runs some 20 miles through St. Louis’ northern suburbs and was contaminated in the decades after the Manhattan Project’s activities in the region. The creek has flooded residential areas many times over the past seven decades, but the floodplain had never been tested for radioactive waste until now.

The corps announced last week that it has found thorium 230 — a radioactive isotope — in yards and public parks exposed to Coldwater Creek’s floodwaters. Officials say there is no immediate risk to residents. The positive results so far have been from samples taken 6 to 12 inches in the ground, in some cases showing levels two to four times the threshold for required cleanup.

“Literally in people’s backyards,” said Jenell Wright, a former area resident who said she has watched dozens of childhood friends die of cancer over the years. “That’s big. It doesn’t get bigger than having nuclear weapons waste in your backyard.”

In a three-part series published in April, Al Jazeera reported on the presence of radioactive waste in the St. Louis metropolitan area. For years, residents near contaminated sites have reported strings of rare cancers and autoimmune diseases, and many areas potentially exposed to contamination remain untested and unmarked.

Long-term exposure to low-level radiation is linked to an increased risk of cancer, according to the EPA.

The confirmation that the creek deposited radioactive waste did not surprise residents who have been pushing for cleanup. After an outcry from locals, the Hazelwood municipal government closed a park on Monday where the corps found waste.

The corps will continue to test the area in the coming months.

“Utility and road crews frequently dig into these soils. Children play on and in them,” wrote the directors of the Missouri Department of Natural Resources and Missouri Department of Health and Senior Services in a joint letter to the Pentagon in September 2014. “We believe that priority funding to allow rapid and complete remediation is needed to address this concern.”

Since Al Jazeera’s series was published, testing has begun at another St. Louis County contaminated site, the West Lake Landfill, in Bridgeton.

According to government documents and researchers, the extent and location of the contamination at that landfill complex are unclear, and an underground fire smoldering nearby compounds residents’ concerns.

A company potentially liable for the cleanup has said that there may be sources of contamination that have not been acknowledged, The St. Louis Post-Dispatch reported.

As of now, there is no lining between the landfill’s known radioactive waste and groundwater or a cover. There is a higher-than-normal rate of childhood brain cancers, among others, in nearby ZIP codes, according to Missouri Department of Health and Senior Services data.

Local activists got a boost in July when Missouri congressional delegates sent a letter to the Department of Energy pushing that the landfill be moved from EPA jurisdiction to the Former Utilized Sites Remedial Action Program (FUSRAP), which was created by the Department of Energy but is now executed by the Army Corps of Engineers.

The testing and cleanup of Coldwater Creek and other contaminated sites in the St. Louis metropolitan area is carried out by the corps under FUSRAP.

“I take that as a good step, because we’ve convinced the elected officials that this needs to be a FUSRAP site,” said Dawn Chapman, a resident who, for years, has been pushing for the situation at West Lake to be addressed.

In the meantime, the Missouri Department of Natural Resources released a report on Aug. 10 suggesting that the underground fire continues to move toward the known radioactive waste despite efforts to contain it.

I keep waiting for the day where I’m not going to have to fight it, where I can go back to life as normal,” she said. “I’m starting to forget what that looked like before this.”

In the coming weeks, additional data are expected to shed further light on the extent of the contamination.

“The Manhattan Project casts a shadow across this entire area,” Chapman said. “But even if you clean it up completely, you can’t take away the fact that people were exposed. You can’t erase that shadow. It will always be there.”