Natural Uranium Contamination in Central Valley Aquifer Linked to Nitrates

September 18, 2015

The presence of Nitrates in Central Valley groundwater is well known as documented by (Harter).  And the fact that uranium is also present has been the subject of studies, including a recent report by USGS on July 16, 2015.  But quite recently in a study reported on August 16, 2015 University of Nebraska scientists studying data on the Ogallala and California Central Valley aquifers linked the presence of nitrates and uranium stating that “these results indicate that nitrate, a primary contaminant, should be considered a factor leading to secondary groundwater uranium contamination,” and that’s new.

Dr. Thomas Harter documented the extent of nitrate contamination in his report entitled “Addressing Nitrate In California Drinking Water.”  While nitrates are naturally occurring in groundwater, the use of nitrogen based fertilizers and the operation of dairies are pointed to in this study as major contributors to high levels of nitrates well above drinking water levels (MCL – 30 ug/L),  particularly in the Central Valley.  Complicating matters is that the nitrate sources continue to move through the soil column into the groundwater and will do so for many years.

Source remediation such as pump and treat technologies are out of the question financially.  However nitrates can be treated by removal and/or dilution at the well head/water treatment plant.   But for smaller water companies, and for consumers relying on well water, these removal/treatment options may prove financially burdensome, impractical, or not available.  In some rural and smaller communities there have been reports of potential health issues for those drinking water with levels of nitrates exceeding drinking water standards.

The presence of uranium in the natural environment is also well known, and has been reported in groundwater to exceed federal and state drinking water standards in certain areas of the San Joaquin Valley.  In a 2010 USGS Study entitled "Effects of Groundwater Development on Uranium:  Central Valley, California USA” uranium and arsenic were found at high concentrations in groundwater in several parts of the Eastern San Joaquin Valley. The authors stated that these levels were due to the presence of bicarbonate which oxidized and made soluble uranium otherwise held in the soil and sediment.

The USGS study certainly raised concern about uranium, but what the University of Nebraska added to the equation was to find high concentrations of uranium in the groundwater where there are also high concentrations of nitrates, which are considerably more present than bicarbonate in many areas of the Central Valley.   High concentrations of nitrates, as the authors indicate, are generally associated with the application of fertilizers, animal manure, and septic systems, and can be found not only in agricultural areas but also areas of past agricultural use that have since urbanized. 

The Nebraska study found that approximately 78% of the areas studied where uranium levels were above the MCL were correlated to the presence of nitrates.  The authors state that the results of their study indicate that nitrates should be considered a factor leading to secondary groundwater contamination.  And this is because nitrates act as an oxidant that increases uranium solubility.  In other words, the presence of nitrates renders otherwise insoluble uranium in the soil soluble, and that as a consequence as nitrate levels in groundwater increase so will the level of soluble uranium contamination.

The study includes excellent maps of the Central Valley showing the extent of the presence of uranium in the groundwater.

As Harter’s study indicates there are not only significant nitrate levels in ground water in the Central Valley, but there is a good deal more in the soil column that over time will migrate into the groundwater.  And that would suggest that uranium contamination in the groundwater may increase over time.

There are, of course, limitations to this study.  It’s a mega study based on data gathered from many sources, and its conclusions are based on finding correlations.  We can expect more studies going forward.  But what this study suggests is that in addition to nitrates in the groundwater we are likely to see more of a focus on uranium contamination of groundwater supplies in the Central Valley.

The major issue that this may present to water supply companies and those who rely on well water is that the cost of dealing with the treating water with uranium over the MCL is likely to be considerably more expensive and difficult in comparison to simply dealing with nitrates.