Inventory and vertical distribution of 137Cs, 239+240Pu and 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 137Cs, 239+240Pu and 238Pu 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 238Pu, SNAP-9A, contributed to the inventory of 238Pu deposited on the ground. In order to estimate the deposition densities of 137Cs, 239+240Pu and 238Pu 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 137Cs 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 137Cs, 12.1 ± 1.5 Bq.m−2 and 22.1 ± 1.7 Bq.m−2 for 239+240Pu, and 1.23 ± 0.46 Bq.m−2 and 1.58 ± 0.60 Bq.m−2 for 238Pu, in Hiva Oa and Raivavae, respectively. The 238Pu/239+240Pu 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 238Pu fallout from SNAP-9A, which is latitude dependent. The 137Cs/239+240Pu 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.