Monitoring uranium contamination by drilling wells is expensive, but a new study suggests it may be possible to do the same monitoring far more cheaply by coring trees on potentially radioactive sites.
Dr. Drew Coleman, assistant professor of geologic sciences at the University of North Carolina at Chapel Hill, and his graduate student Michael Bulleri conducted the study.
‘Based on work I did earlier, we set out to determine if we could monitor near-surface water contamination around a depleted uranium weapons manufacturing site outside Concord, Massachusetts, by measuring uranium concentrations in the living parts of trees growing nearby,’ said Coleman.
‘Mike’s results have been fantastic. By testing the sapwood – the living parts of oak trees he cored close to the site – he has found a definite bull’s-eye pattern around the site where the concentration goes up the closer one gets to it.’
Bulleri took all their samples on public and private lands surrounding the facility. The scientists tested samples using a thermal ionisation mass spectrometer at UNC and a technique known as isotope dilution.
They could distinguish between natural uranium from the soil and depleted uranium contamination by measuring the ratios of uranium 238 to uranium 235 in each sample.
Natural uranium has a ratio of 137.88 atoms of 238 for every one atom of 235, Coleman said. The depleted form – what is left over after an enrichment process used for making nuclear fuels and bombs – has a ratio of about 500 to one.
Trees suck up water beneath the ground and store the radioactivity it contains for many years, he said. Comparing isotopes allows researchers to pinpoint the radioactive contamination’s source and level.
‘We found there’s not much contamination outside the Concord site, and there’s never been very much, which we know from looking at earlier water samples,’ the geologist said. ‘What’s interesting and potentially very important is that we don’t have to drill wells, which are extremely expensive, to determine what the uranium concentrations are in the ground.’
The two detected depleted uranium in tree bark several kilometres away from the site, which could only have been deposited as airborne particles, Coleman said. They cannot say whether the radiation in the bark might present a health hazard.
‘Assuming we have trees to look at, we know we can apply this method of investigation to many other contaminant sites in the United States and abroad,’ Bulleri said.
‘There are many nuclear sites and radioactive sites in the United States that need to be monitored,’ Bulleri added. ‘Instead of going out and doing a lot of expensive testing, you can just core a few trees and get the answer over a huge area very quickly. This potentially could boost safety by enhancing monitoring.’