Inventory and vertical distribution of 137Cs, 239+240Pu an a 238Pu in soil from Raivavae and Hiva Oa, two French Polynesian islands in the southern hemisphere

Atmospheric nuclear weapons tests carried out by the United States, the former Soviet Union, the United Kingdom, France and China between 1945 and 1980 resulted in radioactive fallout over the earth's surface of long-lived radionuclides, such as Cs-137, Pu239+240 and Pu-238 that could be detected more than 50 years after their production. In addition, the burnup in the upper atmosphere of a thermoelectric generator fueled by Pu-238, SNAP-9A, contributed to the inventory of Pu-238 deposited on the ground. In order to estimate the deposition densities of Cs-137, Pu239 + 240 and Pu-238 in French Polynesia, we collected undisturbed soil samples up to 30 cm deep at eight sites in two islands (Hiva Oa, 139 degrees W- 10 degrees S and Raivavae, 148 degrees W - 24 degrees S) in 2015-2016. The top 0-10 cm of the soil cores were sliced into five 2-cm layers and the bottom 10-30 cm into four 5-cm layers for gamma spectrometry and alpha spectrometry measurements. We found that more than 50% of the radioactive inventories are still contained within the first 10 cm and that the average vertical migration velocities of Cs-137 and Pu are less than 0.2 cm y(-1). The average accumulated depositions, deduced from the profile measurements, are 236 +/- 11 Bq.m(-2) and 313 +/- 39 Bq.m(-2) for (CS)-C-137, 12.1 +/- 1.5 Bq.m(-2) and 22.1 +/- 1.7 Bq.m(-2) for Pu239 + 240, and 1.23 +/- 0.46 Bq.m(-2) and 1.58 +/- 0.60 Bq.m(-2) for Pu-238, in Hiva Oa and Raivavae, respectively. The Pu-238/Pu233+248 ratios are 0.102 +/- 0.050 at Hiva Oa and 0.072 +/- 0.033 at Raivavae. Both values are higher than the ratio in nuclear weapons tests fallout estimated to be 0.016 in 2016 (Hardy et al., 1973), because of the contribution of Pu-238 fallout from SNAP-9A, which is latitude dependent. The Cs-137/Pu238+240 ratios, 19.5 +/- 3.2 at Hiva Oa and 14.2 +/- 2.8 at Raivavae are in the lower part of the range of values observed in other regions of the world.