A new technology that acts like a giant underground filter is successfully beginning to mop up the uranium contaminating an aquifer in a remote Utah canyon. Uranium contamination in groundwater is a serious problem because the toxic metal can travel long distances in underground aquifers, which are vital sources of fresh water for people, animals and agriculture.

Installation of the apatite PRB at Fry Canyon, Utah. (Photo courtesy of John Bargar)

Recent research at SSRL showed that the filters—called PRBs for permeable reactive barriers—intercept uranium in an unexpected way. This fundamental knowledge has important implications and serves as the latest example that many environmental cleanup ideas work differently in reality than in theory.

"We knew that the barriers worked to stop uranium, now we know how they work. We can use this information to predict how long they will work and what the costs will be," said John Bargar (ESRD), molecular environmental scientist at SSRL. "This information is necessary to compare this concept to other technologies and to select and engineer new designs."

Originally, scientists thought uranium would react with a mineral—called apatite—in the filter to form an inert mineral that would effectively remove uranium from the water. This general concept has worked well for lead and cadmium contaminated soils. Apatite is also the mineral that makes up the teeth and bones in all vertebrate animals.

To verify this hypothesis, Bargar and two colleagues, Christopher Fuller and James Davis from the US Geological Survey in Menlo Park, used x-ray diffraction and EXAFS spectroscopy, both of which are synchrotron-based techniques. They were surprised to find that uranium adsorbs, or sticks, to the surfaces of apatite, rather than chemically reacting with it to make the new mineral.

The team studied samples created in a lab as well as samples from Fry Canyon, Utah, where several government agencies (USGS, EPA, DOE and BLM) are collaborating to demonstrate PRB technology in a shallow aquifer contaminated by an abandoned uranium-ore processing plant. Numerous sites throughout America, particularly in the Four Corners area and Wyoming, are contaminated with uranium and other radionuclides as a result of mining, milling and other industrial processes.

"It’s really unacceptable to have polluted watersheds. This is a clear example of how synchrotron techniques can be used to solve a very practical problem regarding the clean up of uranium contamination in aquifers," said Bargar.

The field demonstration also shows that PRBs will need to be monitored over time to ensure they are still working. Apatite was the best hope yet for encapsulating uranium through chemical reaction into a mineral, providing a way to permanently remove uranium’s threat. Still, scientists are happy that apatite does trap uranium, with the advantage that there is no new mineral precipitate that could clog up a PRB.

One key area to investigate now is how much uranium the PRBs can trap and for how long before it gets re-released under certain conditions (e.g. a change in groundwater acidity or saturation of the barrier).

"Field tests are really the only way to evaluate the useful lifetime of any PRB," said Fuller. "A number of kinds of barriers are being studied around the country. However, knowledge of the contaminant removal process is critical to designing an effective PRB with sufficient lifetime necessary for real world applications."

Monitoring the apatite PRB at Fry Canyon will continue for at least three more years.