It is impossible to determine with certainty because so many cities in the region are posted as "under review" by the EPA so it is impossible to triangulate.
On the one hand, the radiation there could be from the jet stream, which has been overhead.
On the other hand, what is strange is that cities on similar latitudes in adjoining states, like Duluth MN, don't have these high levels.
I've read that the EPA has outsourced its data collection to a company with a less than impeccable record.
The blogger Portblog has demonstrated how the EPA can deliberately manipulate the radiation readings, making them look much lower than they are http://www.potrblog.com/
So, I know that one cannot trust the EPA's data with confidence.
Still, I don't understand why Pierre SD has such high levels as compared to other places in the region.
This is a wild guess but could the National Underground Science Laboratory at Homestake (NUSL), "the world’s deepest and most extensive underground science laboratory" be the source of Pierre's high levels?
This lab, including 375 miles of tunnels, was previously a mine and is known for flooding.
Water had to be removed when the mine was converted into a lab.
A large room was recently constructed out of the mine but before it was built studies had to be conducted to make sure the room would not collapse.
Is it possible parts of the lab flooded and nuclear experiments at the lab went awry?
This is a wild guess but it is the only thing that popped up when I tried to determine the source of Pierre's high readings.
BACKGROUND ON LAB (PROPOSAL)
Interim Funding for Underground Science for the 21st Century: TheNational Underground Science Laboratory at Homestake (2001)
We propose to establish the National Underground Science Laboratory at Homestake (NUSL), the world’s deepest and most extensive underground science laboratory, through the renovation of the Homestake Gold Mine in Lead, Lawrence County, South Dakota.
The topics described in these letters of intent include ββ decay, solar neutrinos, dark matter searches, long baseline neutrinos, nuclear astrophysics, low background counting, neutrinos from supernovae, atmospheric neutrinos, advanced detector development, neutrino factory studies, geoscience, proton decay, new species of microorganisms, weakly interacting massive particles (WIMPs), gravitational radiation and ultra-low background nuclear isotope measurements. Document created in 2001 by Marvin L. Marshak http://www.int.washington.edu/DUSEL/ProjectDescription.pdf
Mine timeline http://sanfordundergroundlaboratoryathomestake.org/index.php?option=com_content&view=article&id=11&Itemid=16
research paper on stability of mine for building a large room http://www.cdc.gov/niosh/mining/pubs/pdfs/issol.pdf