Inventory and vertical distribution of 137 Cs, 239+240 Pu and 238 Pu 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 137 Cs, 239+240 Pu and 238 Pu 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 238 Pu, SNAP-9A, contributed to the inventory of 238 Pu deposited on the ground. In order to estimate the deposition densities of 137 Cs, 239+240 Pu and 238 Pu in French Polynesia, we collected undisturbed soil samples up to 30 cm deep at eight sites in two islands (Hiva Oa, 139°W - 10°S and Raivavae, 148°W - 24°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 137 Cs 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 137 Cs, 12.1 ± 1.5 Bq.m-2 and 22.1 ± 1.7 Bq.m-2 for 239+240 Pu, and 1.23 ± 0.46 Bq.m-2 and 1.58 ± 0.60 Bq.m-2 for 238 Pu, in Hiva Oa and Raivavae, respectively. The 238 Pu/239+240 Pu 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 238 Pu fallout from SNAP-9A, which is latitude dependent. The 137 Cs/239+240 Pu 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.