PLUTONIUM IN TREE-RINGS FROM FRANCE AND JAPAN

Plutonium, along with other radionuclide concentrations, was measured in evergreen tree rings from two different locations. This was used as an information source for the past two centuries. Tree rings are a pr oduct of annual layers and thus chronological information is clearly v isible. Three trees were harvested in 1988-1990: a French white fir (1 37 years old) and a spruce tree (177 years old) from the France-German y border near Nancy, France and a sugi (78 years old) from Nagasaki, J apan. The uniform branchless part of the trunks from the harvested tre es were immediately separated into a set of tree ring samples each of which contained 3-20 years of growth. The separated samples were mecha nically powdered, dried at 105 degrees C to obtain the dry weight, ash ed at 350 degrees C to measure K-40, Cs-134 and Cs-137 and ashed again at 600 degrees C to determine Pu-239 + 240. The highest (239 + 240) P u concentration of 30.0 mBq/kg of dry wood was obtained from the tree rings from Nagasaki, located at the centre of the local fallout of the Pu A-bomb detonated in 1945. This concentration peak was, however, ob served in tree rings of 1965-67. The concentration was only 2.9 mBq/kg for the tree rings of 1944-46. The contribution of the local fallout on the surface soils from the A-bomb was 181 mBq/cm(2) at the harveste d area of the tree, while the contribution of global fallout by many w eapons testing was 5.9 mBq/cm(2) (or 3.3% total fallout in the region) . The reason for the over 20 year time lag of Pu-239 + 240 uptake by t he tree rings is unknown because many factors influence the routes of Pu into the tree rings. Also the chemical form of Pu in surface soils may have been changed by the surrounding environment. The highest conc entration in the tree rings from France was 9.4 mBq/kg which is about 31% of Nagasaki Pu-239 + 240 concentration. The harvested area did not have any recorded Pu sources other than global fallout. An interestin g result was that the distribution of Cs-134 and Cs-137 concentrations in the French white fir was different from Nagasaki. Data suggested t hat these new radionuclide inputs were from the Chernobyl accident. Th e mobility (or diffusion coefficient) of cesium is 2-8 cm(2)/yr in the portion of heart-wood tree rings (1955-1870). Although tree rings can record chronological inputs of various trace elements, some elements cannot be used. These exceptions would be elements that: (1) are mobil e within tree rings; (2) have littlte understood entry routes to the t ree rings (via roots, leaves or barks); and (3) have unknown biogeoche mical behaviour in the surrounding environment. Further investigation is warranted to use tree rings as a tool to record past environmental history.