Tracing the History of Nuclear Releases: Determination of 129I in Tree Rings

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• 作者：Usha Rao ; Udo Fehn ; Yasuyuki Muramatsu ; Heather McNeil ; Pankaj Sharma ; and David Elmore
• 刊名：Environmental Science & Technology
• 出版年：2002
• 出版时间：March 15, 2002
• 年：2002
• 卷：36
• 期：6
• 页码：1271 - 1275
• 全文大小：70K
• 年卷期：v.36,no.6(March 15, 2002)
• ISSN：1520-5851

文摘

Concentrations of the long-lived radioisotope ¹²⁹I weremeasured in dated tree rings in order to determinewhether the distribution of this isotope reflects the historyof nuclear deposition. ¹²⁹I concentrations and ¹²⁹I/¹²⁷Iratios were analyzed in tree rings and bark samples fromfour trees at West Valley, NY, and from one tree atRochester, NY. West Valley was the site of short-livednuclear fuel reprocessing activities (1966-1972), whileRochester, located 115 km to the northeast, provided aregional control site for the study. The selected trees reflectdifferent modes of fluid and nutrient transport in trees,with three species of ring-porous trees (elm, oak, and locust),one semidiffuse (cherry), and one diffuse-porous tree(maple). The results show that ¹²⁹I levels in ring-poroustrees, in which xylem or hydrologic tissue is localized in theoutermost growth ring, are generally well correlatedwith the expected ¹²⁹I deposition pattern for the region. Incontrast, tree rings of the more common semidiffuse todiffuse-porous wood, where xylem is disseminated throughoutthe trunk, show a less well developed ¹²⁹I signal, probablydue to the transport of iodine ions across annual rings.Iodine concentrations in the tree rings range from 0.04 to2 mg/kg, ¹²⁹I/¹²⁷I ratios from 6 × 10^-10 to 3.8 × 10^-6.Tree bark and the outermost rings show significantly higher¹²⁹I concentrations than the wood of the trunk. The ¹²⁹I/¹²⁷I ratios for bark are very similar to values obtained forsurface soil and water at the two localities, while inner ringshave ratios similar to those in deeper layers of the soil,reflecting different pathways for ¹²⁹I uptake and the differencesin ambient ¹²⁹I levels between the atmosphere and deepsoil. Although ring porous trees preserve the depositionalpattern of nuclear releases, rings older than or close tothe onset of the nuclear age have ¹²⁹I/¹²⁷I ratios significantlyabove the preanthropogenic level, suggesting that evenin these trees some redistribution of ¹²⁹I occurs throughoutthe trunk. Our results indicate that growth rings from ring-porous wood are useful in time-series analyses ofregional ¹²⁹I deposition, yielding reliable information onrelative changes in ¹²⁹I concentrations but requiring cautionin the reconstruction of absolute ambient concentrationsduring any given time.