Researchers have linked spiking groundwater uranium levels to agricultural runoff.
Image: Lynn Betts/USDA NRCS
Uranium levels nearly 100 times that of safe limits established by the Environmental Protection Agency have been repeatedly detected in two of the United States' largest groundwater sources: the High Plains and Central Valley aquifers. This in itself isn't a big surprise—excessive groundwater uranium is a known problem—but researchers at the University of Nebraska have elucidated a correlation between uranium and nitrate levels, which is thus a correlation between radioactive water and fertilizer.
So, some two million residents now residing above or near the aquifers in the Upper Midwest and California, respectively, likely live within a mile of a uranium-contaminated well, while six million in total depend on the same groundwater sources for drinking water. This is bad, of course, and uranium-related health problems are well known, but now we have a cause, or at least a very powerful suggestion of a cause.
The University of Nebraska group looked at roughly 275,000 water samples from 62,000 different sites in nine US states. The nitrate and uranium raw data came courtesy samples collected by 12 different agencies, ranging from the Kansas Geological Survey to the National Uranium Resource Evaluation (NURE) Program to the United States Geological Survey (USGS), which also helped fund the current study.
There are a lot of human sources of uranium, including but not limited to nuclear fuel disposal, mining, and milling. But there are also natural "mineralized" uranium sources, which include volcanic rock, black shales, and granite. This naturally occurring uranium usually just sits there in an insoluble, unavailable form, but it can also be woken up by oxidizing groundwater, which renders the dormant uranium soluble and, thus, mobile.
Nitrate, among the most common groundwater contaminants, does the job, oxidizing previously insoluble uranium sources and allowing them to dissolve into groundwater. The University of Nebraska group found that in 78 percent of samples showing high levels of dissolved uranium, high levels of nitrogen were also found.
"Here we establish a link between groundwater U and nitrate contamination in the [High Plains] and [Central Valley] aquifers," the researchers report in the Environmental Science and Technology Letters. "Our data indicate that nitrate concentrations near the MCL [maximum contaminant level] are correlated to groundwater [uranium] contamination."
"Thus, nitrate-mediated U solubilization presents a threat to the quality of groundwater resources already under pressure because of population growth and global environmental change," they continue. "This has the potential to negatively impact the health of millions of residents in the United States and around the world utilizing U-contaminated drinking water. Additionally, irrigation accounts for an estimated 43 percent of global groundwater use. Food crops irrigated with contaminated water have been demonstrated to accumulate U, thus leading to an additional route of U exposure through food crops."
This picture looks even worse when you consider the increasing importance of groundwater sources given the perma-drought conditions of the past decade. In California, these sources have gone from the 40 percent of total usage seen in a "normal" precipitation year to nearly 60 percent. What's more, Central Valley groundwater is being overdrafted by nearly 12 million acre-feet per year, which would be ominous enough until you consider that the meager water supplies that do replenish the aquifer are mostly farm runoff: nitrogen.