Improving tritium exposure reconstructions from tree rings using accelerator mass spectrometry.

摘要

Public concerns over the health effects associated with low-level and long-term exposure to tritium released from industrial point sources have generated the demand for better methods to evaluate historical tritium exposure levels for these communities. The cellulose of trees accurately reflects the tritiated water concentration in the local environment and often contains the only historical record of tritium exposure. Techniques used previously for tritium exposure reconstructions using tree rings required tens-of-grams to kilograms of sample material, which made sample collection and preparation laborious and time-consuming.; Accelerator mass spectrometry (AMS) can rapidly measure tritium from milligrams of sample material. This permits the rapid collection and preparation of sample material, as well as a 100–1000 fold decrease in analysis time for milligram-sized samples. Because current quantification limit of this method (1500 TU) is greater than the quantification limit for other analytical methods (0.1–3 TU), AMS is only applicable for dose reconstruction studies at locations near anthropogenic tritium releases. Verification of the tritium exposure methodology was conducted on a tree sample from the Nevada Test Site. A Tamarix ramosissima (salt cedar) from the Cambric Ditch was used since it had a single source of tritiated water that was well characterized as it varied over time. Organically bound tritium measurements using AMS demonstrated that the tritium activity in the wood was comparable to the tritium activity in the source water.; Following validation, the technique was used at the Lawrence Berkeley National Laboratory's (LBNL) National Tritium Labeling Facility (NTLF). Although most of tritium in the labeling process was captured or recycled, some of the unused tritium from labeling was discharged into the surrounding environment. Small cores from a grove of Eucalyptus globulus surrounding the NTLF emission stack were analyzed for tritium and carbon-14. Since the annual nature of the eucalyptus tree rings was uncertain, the age of tritium measurements was assigned by matching measured carbon-14 at points in the core to known atmospheric carbon-14 levels over the last 40 years. The tritium measurements provided a mean to estimate the total annual releases and the chronic exposure level over the past 40 years. Since the accuracy of the older reported tritium release records has come into question, this reconstruction is the only means of assessing historical exposure. In addition, this tritium exposure reconstruction methodology is feasible, fast, sufficiently accurate, and easily explainable to the general public.