Uranium occurs naturally in soils. It is uranium that is responsible for the risk of radon in your house because radon is produced as uranium decays.
But "natural" does not make uranium good. Uranium emits gamma radiation as it decays, which increases the risk of cancer. But the EPA indicates that natural background radiation from uranium is of less concern than uranium in drinking water. This is because uranium can also be toxic to kidneys (which process the dissolved uranium as it is eliminated from the body) and bones (where some of the uranium that is not eliminated remains stored in the body).
A study by researchers at the University of Nebraska has found that high levels of uranium in groundwater strongly correlate with elevated levels of nitrates, a contaminant related to nitrate run-off from factory farming.
The paper describes several mechanisms by which nitrates increase uranium solubility, explaining how nitrates contribute to uranium levels in aquifers as high as 180 times the EPA "maximum contaminant level" for uranium in drinking water.
The Nebraska Department of Health and Human Services has responded to the study by confirming that "it is not seeing widespread problems with uranium at this time".
That may in part be due to the fact that standard drinking water treatment systems remove dissolved uranium. This can create a secondary problem: the waste sludge from water treatment plants concentrates the uranium and becomes Technologically Enhanced Naturally Occurring Radioactive Material (TENORM) waste, increasing the costs for managing public utilities.
The High Plains (HP) and Central Valley (CV) aquifers studied provide drinking water to 6 million people and irrigation for 1/6th of American agricultural production (based on revenue). The authors have made their study, Natural Uranium Contamination in Major U.S. Aquifers Linked to Nitrate, open access in order to raise awareness of the issue.