SREL Researcher Uses Treerings to Unravel Contaminant
History
There is considerable disagreement within the scientific community
on whether analysis of the annual rings of trees growing in contaminated
areas provides reliable information about the history of the contamination
the tree has experienced. Researchers based at the University of Georgia's
Savannah River Ecology Laboratory (SREL)
and the National Synchrotron Light Source in
Upton, N.Y., used novel analytical techniques on extracted cores from black
willow trees to find out why there are discrepancies in the data.
SREL researcher Tracy Punshon has drawn some conclusions based on her work
done on the U.S. Department of Energy's Savannah River Site near Aiken, S.C. "The
disagreement we have seen in the past is due in part to an inability to accurately
measure spatial distribution of metals within environmental samples," said
Punshon. "Scientists from SREL have access to a powerful analytical technique
known as synchrotron x-ray microanalysis - a technique which allows us to study
the distribution and concentration of metals in environmental samples, and
to determine the binding environment of particular contaminants. This is by
far a more informative technique than traditional wet-chemistry methods, which
involve drying, grinding and digestion of bulked samples in concentrated acids."
In this nondestructive technique, the sample requires very little preparation.
In the past, synchrotron x-ray fluorescence (SXRF) has been applied to mineralogical
or geological samples, although it is increasingly being used in investigations
of biological samples and will be invaluable in the future in understanding bioavailability.
In the study of bioavailability it is becoming increasingly important to know
where the metals are being accumulated, as well as merely knowing the concentration.
X-ray intensity maps of the concentrations of Fe, Mg, Zn, and Ni in a 0.5 x 0.5 mm area of a tree core showing an anomalous "hot-spot" of Ni associated with Zn. Annual banding is clearly visible in the images of Fe and Mg concentrations.
Punshon is the first author of a recent paper in Environmental Science & Technology on this topic and presented her work this month at the International Conference on the Biogeochemistry of Trace Elements in Sweden. This conference is attended by more than 600 scientists from all over the world. Punshon's work shows that trees collected from contaminated areas contain a "signature" of the metals within their annual rings, the timing of which corresponds well with historic information on the timing of contaminant input. However the study revealed an important caveat: a tree may be used as an indicator of its contaminant history only if it has not experienced excessive toxicity as a result, that is if it has received lower, chronic doses of contamination. A tree growing in the middle of a contaminated settling pond - where levels of nickel (the main bioavailable contaminant in Punshon's study) were over 1,000 mg/kg in places - showed a dramatic peak in metal concentration in one particular year even though soil contaminant levels did not change. What is most surprising however, is that this particular tree had for the majority of its life been able to avoid taking up excess nickel, especially considering the extent to which the soil around it is contaminated. In this case, changes in nickel levels must have been due to the tree itself. Firstly, the lack of nickel uptake in all but one year of its life suggests that it successfully avoided soil contaminants. This has been observed before in trees growing in patchily contaminated soils; their roots proliferate in clean areas and are inhibited in contaminated areas. The sudden increase in nickel uptake observed in this study suggests that a branch of the tree's root system grew into a grossly contaminated "hot spot," taking up a potentially toxic level of nickel; one which would have almost certainly killed the root system in which it was in contact. Given the length of time trees live, an ability to adapt to a changing environment is vital - and this adaptation often involves an avoidance of contaminants and a reduction in the amount of it they take up. Knowing this may help scientists to more accurately interpret tree ring data in the future.
Back to ERSD News
6/30/03
X-ray intensity maps of the concentrations of Fe, Mg, Zn, and Ni in a 0.5 x 0.5 mm area of a tree core showing an anomalous "hot-spot" of Ni associated with Zn. Annual banding is clearly visible in the images of Fe and Mg concentrations.
Punshon is the first author of a recent paper in Environmental Science & Technology on this topic and presented her work this month at the International Conference on the Biogeochemistry of Trace Elements in Sweden. This conference is attended by more than 600 scientists from all over the world. Punshon's work shows that trees collected from contaminated areas contain a "signature" of the metals within their annual rings, the timing of which corresponds well with historic information on the timing of contaminant input. However the study revealed an important caveat: a tree may be used as an indicator of its contaminant history only if it has not experienced excessive toxicity as a result, that is if it has received lower, chronic doses of contamination. A tree growing in the middle of a contaminated settling pond - where levels of nickel (the main bioavailable contaminant in Punshon's study) were over 1,000 mg/kg in places - showed a dramatic peak in metal concentration in one particular year even though soil contaminant levels did not change. What is most surprising however, is that this particular tree had for the majority of its life been able to avoid taking up excess nickel, especially considering the extent to which the soil around it is contaminated. In this case, changes in nickel levels must have been due to the tree itself. Firstly, the lack of nickel uptake in all but one year of its life suggests that it successfully avoided soil contaminants. This has been observed before in trees growing in patchily contaminated soils; their roots proliferate in clean areas and are inhibited in contaminated areas. The sudden increase in nickel uptake observed in this study suggests that a branch of the tree's root system grew into a grossly contaminated "hot spot," taking up a potentially toxic level of nickel; one which would have almost certainly killed the root system in which it was in contact. Given the length of time trees live, an ability to adapt to a changing environment is vital - and this adaptation often involves an avoidance of contaminants and a reduction in the amount of it they take up. Knowing this may help scientists to more accurately interpret tree ring data in the future.
Back to ERSD News
6/30/03