First measurements of ²³⁶U concentrations and ²³⁶U/²³⁹Pu isotopic ratios in a Southern Hemisphere soil far from nuclear test or reactor sites

The variation of the ²³⁶U and ²³⁹Pu concentrations as a function of depth has been studied in a soil profile at a site in the Southern Hemisphere well removed from nuclear weapon test sites. Total inventories of ²³⁶U and ²³⁹Pu as well as the ²³⁶U/²³⁹Pu isotopic ratio were derived. For this investigation a soil core from an undisturbed forest area in the Herbert River catchment (17°30' - 19°S) which is located in north-eastern Queensland (Australia) was chosen. The chemical separation of U and Pu was carried out with a double column which has the advantage of the extraction of both elements from a relatively large soil sample (~20g) within a day. The samples were measured by Accelerator Mass Spectrometry using the 14UD pelletron accelerator at the Australian National University. The highest atom concentrations of both ²³⁶U and ²³⁹Pu were found at a depth of 2-3cm. The ²³⁶U/²³⁹Pu isotopic ratio in fallout at this site, as deduced from the ratio of the ²³⁶U and ²³⁹Pu inventories, is 0.085±0.003 which is clearly lower than the Northern Hemisphere value of ~0.2. The ²³⁶U inventory of (8.4±0.3)×10¹¹ at/m² was more than an order of magnitude lower than values reported for the Northern Hemisphere. The ²³⁹Pu activity concentrations are in excellent agreement with a previous study and the ^239+240Pu inventory was (13.85±0.29)Bq/m². The weighted mean ²⁴⁰Pu/²³⁹Pu isotopic ratio of 0.142±0.005 is slightly lower than the value for global fallout, but our results are consistent with the average ratio of 0.173±0.027 for the southern equatorial region (0-30°S).