239Pu fallout across continental Australia: Implications on 239Pu use as a soil tracer

At present there is a need for the development of new radioisotopes for soil erosion and sediment tracing especially as fallout ¹³⁷Cs levels become depleted. Recent studies have shown that ²³⁹Pu can be a useful new soil erosion and sediment radioisotope tracer. ²³⁹Pu was released in the major atmospheric nuclear weapons tests of 1950's and 1960's. However ²³⁹Pu has a half-life of 24110 years and more than 99% of this isotope is still present in the environment today. In contrast ¹³⁷Cs with a half-life of 30.07 year has decayed to <35% of initially deposited activities and this isotope will become increasingly difficult to measure in the coming decades especially in the southern hemisphere, which received only about a third of the total global fallout from the atmospheric tests (UNSCEAR, 2000).In this study an assessment of the ²³⁹Pu fallout in Australia was carried out from comparison of measured ²³⁹Pu inventories with expected ²³⁹Pu inventories from fallout models. ²³⁹Pu inventories were also compared with rainfall and measured ²⁴⁰Pu/²³⁹Pu ratios across Australia.²³⁹Pu fallout inventories ranged from 430 to 1461 μB/cm². Central Australia, with fallout 107% in excess of expected values, seems to be strongly impacted by local fallout deposition. In comparison other sites typically show 5–40% variation between expected and measured fallout values.The fallout inventories were found to weakly correlate (using power functions, y = axᵇ) with rainfall with r² = 0.50 across the southern catchments (25–40°S latitude band). Across the northern catchments (10–25°S latitude band) fallout showed greater variability with rainfall with r² = 0.24. Central Australia and Alice Springs which seem to be strongly impacted by local fallout are excluded from the rainfall correlation data (with these sites included r² = 0.08 and r² < 0.01 respectively).²⁴⁰Pu/²³⁹Pu atom ratios range from 0.045 to 0.197, with averages of 0.139(0.017), 0.111(0.052) and 0.160(0.027) in the 10–20°S, 20–30°S and 30–40°S latitude bands respectively. The ²⁴⁰Pu/²³⁹Pu atom ratios in Central Australia (0.069) likely represent fallout from the Australian tests which also have low ²⁴⁰Pu/²³⁹Pu atom ratios i.e., Maralinga (0.113) and Montebello (0.045). The average ratios in the 20–30°S and 30–40° bands are closer to the global average (0.139 and 0.177 respectively when not including the close-in fallout data from the nuclear test sites) if the Australian test sites and Central Australian sites are neglected as they clearly represent the effects of close in fallout.