underwhich immobilization alone is a desirable goal. Hanford would be one such place.
The situation at Hanford derives from a time when much of the chemistry of fission products was poorly known, where secrecy and the demand for speed in the arms race, prevailed over good record keeping and environmental concerns. Much of the waste in the tanks is poorly characterized, and many of the tanks have been leaking for some time. Many of the waste products are already insoluble, but in many cases there are soluble forms of fission products and even some actinides.
As these move further from the leaking tanks at Hanford, they undoubtably become more dilute, making recovery more and more expensive. There are some techniques that can concentrate elements like Technetium by "pump and treat" technologies. Among the more promising are those that use fine iron dust as a reductant.
However, from what I understand, the pertechnate ion is one of the more mobile of the radioactive products leaking from the tank, and a plume of it is now leaching into the Columbia River. No huge health impacts or ecological impacts have been seen as a result, but I think for public peace of mind, immobilizing the technetium, at least near the river's interface with the groundwater plume, might be a good idea.
Fortunately technetium exhibits low toxicity (which is why the element is frequently injected into people for imaging purposes during scans) and a very short biological half-life, and about 75% is excreted in under two days.
The highest concentration of Tc-99 recorded in the groundwater outside the leaking tanks is 34,000 picocuries per liter. Since the specific activity of Tc-99 is about 17 millicuries per gram, this means there is about 2 millionths of a gram of technetium in each liter of the most contaminated water thus far found. Undoubtably, as the most contaminated water leaches into the water table and then into the Columbia River it will be further diluted. The estimated increased cancer risk from a picocurie of Tc is 2.3X10^-12. This means that by drinking a liter of Tc contaminated water from around the tank (ignoring, of course, the many other toxic substances found in it), one would raise one's risk of getting cancer from the Tc by 1 in 12 million.
Some discussion on the biological properties of Tc.The Hanford tanks are a very special case, since their construction, use, and contents were poorly recorded, materials being added to them on a more or less ad hoc basis, dating in some cases back to the early 1940's. (Incredibly much low level waste was simply dumped on the ground at Hanford without regard to the nature of the chemical contaminants like carbon tetrachloride.) Much of the concern about the leachates focuses not only on their radioactive constituents but on their chemical constituents as well. In the latter case, there is hexavalent Chromium in many of the tanks and bioimmobilization should do quite a bit with this contamination as well.
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