It was the Case of the Upwardly Mobile Radioactivity, and scientists-turned-sleuths led by a Colorado State University hydrogeologist think they’ve gotten to the root of the matter.
Radioactivity leaks don’t amuse William Sanford, Colorado State assistant professor of earth resources, but the one he describes as "a sort of detective story" could be considered unique. The root of an unidentified plant probably brought radioactive isotopes to the surface at a buried-waste test site at Oak Ridge National Laboratory in Tennessee. Sanford and his colleagues call it a worst-case scenario that nonetheless merits further investigation.
In 1994 Sanford, Ingvar Larsen of Oak Ridge National Laboratory and John McConnell Jr. and Robert Rogers of the Idaho National Engineering Laboratory investigated an Oak Ridge test site containing five devices that leach water through soil and collect it for analysis.
The stainless-steel cylinders, 3-by-10 feet, consist of an upper chamber full of soil, instruments and radioactive wastes, the last sealed in 2-by-3-inch cylinders of Portland cement. A lower chamber collects leachate. Ironically, the waste consisted of resins made radioactive by water during decontamination from the Three Mile Island nuclear plant, where America’s worst nuclear accident occurred in 1979. The devices, buried vertically in 1985, were uncovered a few months later, exposing them to precipitation for the duration of the project. Four contained soil; a fifth held medium-fine sand. It was this test device that drew the attention of Sanford and his colleagues.
"A routine survey for radioactive contamination at the (Oak Ridge) site using a hand-held dosimeter in December 1990 revealed that the surface of the sand-filled lysimeter (test device) had (radioactivity) significantly above ambient background levels," the authors wrote in the current issue of the Journal of Environmental Radioactivity.
Samples revealed significant quantities of cesium-134 and cesium-137. Comparing ratios of the two isotops at the surface and in the waste containers and accounting for their different half-lives indicated the surface cesium came from the buried waste forms. Strontium-90 also was present.
While all three isotopes have relatively short half lives, they are biological threats because when present in the environment, they can readily enter the food chain.
In late January 1994, cores were taken at the center and near the wall. The center core contained a fine root present throughout the 30-plus inch column of sand, which was discolored for about one centimeter around the root. The plant could not be identified because maintenance crews had been instructed to keep the site clear of vegetation to permit rainwater to enter.
Tests on all three isotopes showed peak levels of radioactivity at distances beginning approximately eight, 14 and 26 inches below ground level. Those peak levels cracked the case.
Sanford and his collaborators at first puzzled over whether the concentrations meant increased releases of radioactivity, periodic upward transport of the isotopes or both. What they deduced is that during the eight-and-a-half years prior to 1994, there were four occasions of rainfall so intense that the volume of leachate collected was "significantly greater" than the capacity of the test device’s lower compartment, where the leachate collects.
In other words, leachate flowed back up into the upper chamber and eventually subsided, leaving behind a concentrated, peak layer of radioactive material. Radioactivity reached the surface, the scientists concluded, via the root (cesium and strontium are chemically related to potassium and calcium, respectively; the latter are readily absorbed by plants.)
"The upward transport of cesium and strontium is an unusual phenomenon not commonly reported," the authors wrote. It’s assumed that buried cesium and strontium compounds move downward if flushed by rainfall, rasing concerns about radioactivity leaching into groundwater supplies. On the other hand, the sand lacked clay and organic materials that absorb radioactivity.
"Cesium and strontium normally don’t move around much," Sanford said. "They are easily absorbed by soil particles, and in areas where rainfall penetrates the ground deeply those radioisotopes would normally move downward.
"What we found still needs further investigation, because if radioactive materials can move to the surface of a low-level, buried waste site, they could still expose personnel to radiation at the facility and could be carried elsewhere by water runoff and erosion."