OMAHA, Neb. (AP) - A new study suggests that nitrates may play a key role in increasing uranium contamination in groundwater.

The researchers from the University of Nebraska-Lincoln estimate that nearly 2 million people in California and the Great Plains live over groundwater that has been contaminated with uranium, which can cause health problems.

The study found that 78 percent of the groundwater samples that showed unsafe levels of uranium were from areas with high levels of nitrates, which typically come from nitrogen fertilizers and animal waste.

"If the problem is this widespread, more research needs to be done," said UNL assistant professor Karrie Weber, who led the research.

Environmental Protection Agency rules say that uranium shouldn't exceed 30 micrograms per liter in drinking water. Weber's research found examples in California well above that threshold.

Prolonged exposure to high levels of uranium in water has been linked to kidney problems and increased cancer risk.

Some studies have also suggested that uranium might accumulate in certain crops if they are irrigated with contaminated water.

Weber said groundwater samples aren't always tested for uranium making it harder to study. She said the lack of testing also raises safety concerns because smaller communities and rural families often use well water without treating it.

Weber said the nitrates moving through the soil can convert uranium from a solid state to a soluble form that can contaminate groundwater.

"As nitrates come into the system, they are increasing the concentration of uranium in the water," Weber said.

Data from roughly 275,000 samples from two of the nation's largest aquifers - the High Plains aquifer and the Central Valley aquifer in California - were examined for the study. Those two underground stockpiles supply water for irrigation and many communities rely on the aquifers for drinking water.

The High Plains Aquifer stretches underneath some 174,000 square miles in parts of South Dakota, Nebraska, Wyoming, Colorado, Kansas, Oklahoma, New Mexico and Texas.

The study was published in the August edition of the journal Environmental Science and Technology Letters.