Writing in the August edition of Environmental Science and Technology Letters, Jason Nolan and Karrie A. Weber of the University of Nebraska report unsafe levels of uranium in groundwater from California's San Joaquin Valley [wikipedia.org] and from the Ogallala Aquifer [wikipedia.org] underlying Texas, New Mexico, Oklahoma, Colorado, Kansas, Wyoming and South Dakota.
In Natural Uranium Contamination in Major U.S. Aquifers Linked to Nitrate [acs.org] they note a correlation between concentrations of uranium and nitrate ions in the groundwater samples they tested. They theorize that the nitrate, a major component of fertilizer, can oxidize uranium from U(IV) to U(VI), making it water-soluble. They found that in the San Joaquin Valley, uranium reached as much as 180 times the maximum contaminant level (MCL) set by the Environmental Protection Agency, and nitrate was as much as 34 times the MCL. Samples from the Ogallala Aquifer had as much as 89 times the MCL of uranium and 189 times the MCL of nitrate.
Water from these aquifers is used for drinking and for irrigation. Soluble uranium is bioaccumulated by certain food crops; uranium in the human body can result in cancer and kidney damage.
The Associated Press also reported on the story [ap.org].