Tracing the history of nuclear releases: determination of 129I in tree rings.

Concentrations of the long-lived radioisotope 129 I were measured in dated tree rings in order to determine whether the distribution of this isotope reflects the history of nuclear deposition. 129 I concentrations and 129 I/ 127 I ratios were analyzed in tree rings and bark samples from four trees at West Valley, NY, and from one tree at Rochester, NY. West Valley was the site of short-lived nuclear fuel reprocessing activities (1966-1972), while Rochester, located 115 km to the northeast, provided a regional control site for the study. The selected trees reflect different 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 129 I levels in ring-porous trees, in which xylem or hydrologic tissue is localized in the outermost growth ring, are generally well correlated with the expected 129 I deposition pattern for the region. In contrast, tree rings of the more common semidiffuse to diffuse-porous wood, where xylem is disseminated throughout the trunk, show a less well developed 12 9 I signal, probably due to the transport of iodine ions across annual rings. Iodine concentrations in the tree rings range from 0.04 to 2 mg/kg, 129 I/ 127 I ratios from 6 x 10 -10 to 3.8 x 10 -6 . Tree bark and the outermost rings show significantly higher 129 I concentrations than the wood of the trunk. The 129 I/ 127 I ratios for bark are very similar to values obtained for surface soil and water at the two localities, while inner rings have ratios similar to those in deeper layers of the soil, reflecting different pathways for 129 I uptake and the differences in ambient 129 I levels between the atmosphere and deep soil. Although ring porous trees preserve the depositional pattern of nuclear releases, rings older than or close to the onset of the nuclear age have 129 I/ 127 I ratios significantly above the preanthropogenic level, suggesting that even in these trees some redistribution of 129 I occurs throughout the trunk. Our results indicate that growth rings from ring-porous wood are useful in time-series analyses of regional 129 I deposition, yielding reliable information on relative changes in 129 I concentrations but requiring caution in the reconstruction of absolute ambient concentrations during any given time.