US Environmental Protection Agency (EPA) workers cleaning up the West Lake Landfill in St Louis County detected contamination in nearby groundwater, obliging EPA to investigate whether radium might have migrated from the site, the Missouri Independent reported.

St Louis was part of a geographically scattered national effort to build the nuclear bomb. Much of the work in the area involved uranium, where Mallinckrodt Chemical Co was a major processor of uranium concentrate near downtown St Louis. In 1946, the government bought land near the airport and began trucking nuclear waste there from the Mallinckrodt facility. Mallinckrodt stored barrels of K-65, a radioactive residue at the St Louis airport in deteriorating steel drums.

In 1966, the Atomic Energy Commission (AEC) demolished and buried buildings at the St Louis airport site. Continental Mining & Milling Co moved the waste to nearby Bridgeton. Radioactive barrels lay outside the fence. Storage was so haphazard that even the path to the site was contaminated by trucks that spread waste on their hauls from 1966 to 1969. At this site, uranium processor Cotter Corp dried the waste and shipped it to its facility in Colorado. The site remained contaminated for decades. In 1973, Cotter Corp took hazardous leached barium sulphate from the site and illegally dumped it in the West Lake Landfill, also in Bridgeton. The material contained uranium residue.

Both the St Louis airport and Bridgeton sites are bordered by Coldwater Creek, which runs through the heart of what are now busy suburban neighbourhoods. Tonnes of the waste flowed into Coldwater Creek, contaminating the often-flooding waterway and adjacent gardens for 14 miles, state and federal investigators determined.

In the late 1970s, the Nuclear Regulatory Commission (NRC), which replaced the AEC, flew a helicopter over the West Lake Landfill. The test correctly identified two contaminated areas but missed huge areas of the landfill. Despite warnings from the Missouri Department of Natural Resources and activists that the contamination was likely more widespread, NRC’s conclusion stood for more than 40 years. However, in 2023, EPA announced that contamination at the site was more widespread than previously thought.

In a periodic update to nearby communities in May, EPA said it would add groundwater monitoring wells around the site, which sits in Bridgeton, about a mile from the banks of the Missouri River. The expansion, which came after contamination was detected at the edge of the landfill, will help determine whether contamination may be migrating from the site and whether it could reach the river. Radium has been detected near the site at slightly above drinking water limits, EPA said in a statement, but radium also occurs naturally in rock formations and aquifers.

Initially, EPA had anticipated all necessary groundwater wells would be installed by August 2022, the project manager for the groundwater remediation at West Lake, Snehal Bhagat, said in a briefing in December 2023. “But the detections in offsite locations required a significant expansion of the network in order to delineate exactly where the impacts are found so a lot more wells were put in. We’re still putting them in as we chase the edges of the impacts,” he noted.

“To date, no conclusions have been made about the source(s) of the radium in off-site groundwater because data collection is ongoing,” Kellen Ashford, a spokesman for EPA’s regional office said in an email to the Missouri Independent.

Dawn Chapman, a co-founder of Just Moms STL, a nonprofit organisation formed to support communities near contaminated sites around St Louis, expressed concern that EPA had not yet identified the edges of the contamination. Taking into account both the radioactive waste and the other chemical contaminants in the landfill, she feared it could be “one hell of a nasty plume”.

She noted that the parties responsible for the site – the landfill owner, the company that dumped the waste and the US Department of Energy – are nearing the end of the process to plan the cleanup at West Lake. “I really would have hoped that by now they would have found the edge,” she said.