Groundwater in two of the nation’s major aquifers is contaminated with natural uranium that may pose a health risk to millions of people in the Great Plains and California, according to a new study.

University of Nebraska-Lincoln researchers, who examined data from groundwater samples in the High Plains and Central Valley aquifers, found that nearly two million Americans live less than two-thirds of a mile from wells that exceed–often by a lot–the uranium limits set by the U.S. Environmental Protection Agency.

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The study, published in the journal Environmental Science and Technology Letters, also found that human activity is largely to blame. In 78 percent of the sampled sites, the high uranium levels were linked to the presence of nitrate, a common groundwater contaminant that results from use of chemical fertilizers and animal waste in agriculture.

Through a series of bacterial and chemical reactions, the nitrate combines the uranium and makes it soluble in groundwater.

In the High Plains aquifer, some water samples contained as much as 89 times the EPA’s acceptable level of uranium. In the Central Valley, the highest readings were 34 times the EPA limit.

Though we’re used to thinking of uranium as a radioactive fuel for nuclear reactors or raw material for bombs, in its natural, unprocessed state, it is commonly found in tiny amounts in rock, soil, plants and water and has only a low level of radioactivity. Nevertheless, when safe levels are exceeded, its chemical properties pose a health risk to humans, particularly when it is ingested in drinking water.

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According to the Centers for Disease Control, elevated uranium has been linked to kidney damage and cancer. (Here’s a fact sheet on uranium from the National Institutes of Health.

“It needs to be recognized that uranium is a widespread contaminant,” said UNL researcher Karrie Weber, an assistant professor of biological, Earth and atmospheric sciences, who co-authored the study with graduate student Jason Nolan.

“And we are creating this problem by producing a primary contaminant that leads to a secondary one.”