Although the isotope is naturally-occurring, the concentration in the many of the wells was higher than the maximum allowed by the Environmental Protection Agency. Polonium-210 is known to cause cancer in humans.
Officials from Sorensen's Dairy and Oasis Dairy agreed to stop selling milk until test results from the Food and Drug Administration could be completed. On Monday, the FDA results show no evidence of the isotope in the milk.
The USGS study was conducted as part of an investigation into a possible link between groundwater contamination and the high number of leukemia cases around