Reconstructing Tritium Exposure Using Tree Rings at Lawrence Berkeley National Laboratory, California

Abstract

Annual tritium exposures were reconstructed using tree cores from Pinus jeffreyi and Eucalyptus globulus near a tritiated water vapor release stack. Both tritium ( 3 H) and carbon-14 ( 14 C) from the wood were measured from milligram samples using accelerator mass spectrometry. Because the annual nature of the eucalyptus tree rings was in doubt, 14 C measurements provided growth rates used to estimate the age for 3 H determinations. A 30-yr comparison of organically bound tritium (OBT) levels to reported 3 H release data is achieved using OBT measurements from three trees near the stack. The annual average 3 H, determined from atmospheric water vapor monitoring stations, is comparable to the OBT in proximal trees. For situations without adequate historical monitoring data, this measurement-based historical assessment provides the only independent means of assessing exposure as compared to fate and transport models that require prior knowledge of environmental conditions and 3 H discharge patterns.

Assimilation and transport of organic bound tritium in an irrigated pine forest

“…Differences between the assimilation between tritium and 14 C have been noted in the literature and they appear to be related to the source term of these isotopes. 17,22,23 Environmental 14 C often undergoes dilution by C-containing material that is old and depleted in 14 C (including that of anthropogenically-released 13 C-enriched fossil fuels) 24 and this makes the study of 14 C in the environment as well as comparisons between the behavior of tritium and 14 C difficult. Additional studies are clearly warranted here.…”

“…A few studies report TFWT, NE-OBT, E-OBT and total OBT levels in plant tissue and other studies focus mostly on the isolation of cellulose-based tritium in wood. 17 Le Goff et al (2014) take a different approach and employ a correction factor for NE-OBT:HTO tissue ratios in vegetation and other biological matrices which can result in 5 to 50% higher NE-OBT:HTO values. These corrections (which are supposed to account for isotopic fractionation) cannot be applied when NE-OBT is not reported, but in our study, such a factor would only increase the OBT:HTO ratios.…”

“…16 Tree rings have occupied considerable research interest due to their potential to be a metric of historic tritium releases. 17,18,19,20,21 Cellulose-based tritium is thought to be a highly stable form of OBT within the tree and comparisons between cellulose tritium and wood tritium have produced similar results. 21 The preservation of tritium release levels in wood rings can be related to tree species.…”

“…19 Measured OBT (as cellulose) in tree rings of pine and eucalyptus successfully reflected atmospheric water vapor monitoring data after the release of tritium-labeled organic compounds that were made for biomedical studies. 17 In addition to T, carbon-14 ( 14 C) is another radionuclide that is sometimes found in contaminated areas and its levels have been measured in tree rings. 22,23,24,25,26 These studies suggest tree ring 14 C levels can be fairly representative of historic 14 C releases from nuclear reactors as well as from nuclear weapons testing.…”

Improving tritium exposure reconstructions using accelerator mass spectrometry

“…The details of this exposure reconstruction are in Love et al [65]. Figure 2 compares tree measurements to the total amount of tritium released during that calendar year.…”