Browsing by Author "Williams-Hoffman, M"

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    Anthropogenic radionuclides persist in marine sediment at the Montebello Islands nuclear legacy site in Western Australia
    (South Pacific Environmental Radioactivity Association, 2022-11-28) Williams-Hoffman, M; Johansen, MP; Lavery, P; Thiruvoth, S; Serrano, O; Masqué, P
    In 1952 and 1956 the UK government conducted three atmospheric nuclear fission tests in the Montebello Islands, an archipelago approximately 100 km off the coast of northern Western Australia. Radioactive contamination from the tests was deposited locally across the archipelago in both terrestrial and marine environments but was investigated minimally in the decades that followed. This is particularly true for the marine environment which is composed of various distinct ecosystems – from mangroves to open marine channels and is home to a diverse trophic web of marine organisms. To understand the extent and persistence of anthropogenic radionuclides introduced by the tests into this region, we aim to establish the distribution of anthropogenic radionuclides in the marine surface sediments throughout the archipelago. Here we present the results on activity concentrations and distributions of 90Sr, 137Cs, 238 Pu, 239&240Pu and 241Am due to their long half lives and their potential to impart radiological dose to organisms existing at the sediment-water interface. Quantification of anthropogenic and naturally occurring radionuclides in surface and core samples was completed by alpha spectrometry, Cherenkov counting and gamma spectrometry. Activity concentrations of 90Sr, 137Cs, 238 Pu, 239&240Pu and 241Am were highest around the detonation locations in the north of the archipelago in surface sediment samples. Observable activity concentrations also persist in the north-west of the detonation locations, reflecting the direction of the original fallout plumes from the weapons. One additional location showed higher activity concentrations of 90Sr, 137Cs, 238 Pu, 239&240Pu and 241Am in comparison to other locations within the archipelago (121 Bq/kg, 1.6 Bq/kg, 80 Bq/kg, 402 Bq/kg and 28 Bq/kg respectively). This sample was collected 26 km south of the nearest detonation site and was originally selected as an environmental control outside of the state marine park boundaries, suggesting migration of anthropogenic radionuclides has occurred in the 70 years since the detonations took place. Sediment cores were also taken from different marine environments of the region, including at the bottom of the crater created by Operation Hurricane, which involved the detonation of a naval ship in shallow water in 1952. 210Pb was analysed to assess the sedimentation rates over the last century across the cores. In most instances, mixing dominated throughout the sediment core profiles. This reflects the highly dynamic and turbulent nature of the region, subject to seasonal cyclones and fast-moving tides within the archipelago. The exception to this observed mixing was found in a single core, the ‘Hurricane Core’, taken in the bottom of the Operation Hurricane crater. 137Cs and 241Am activity concentrations of this core showed a clear peak in activity at 50 cm depth; an order of magnitude above those found in similar parts of the region within surface sediments. The maximum activity concentrations of 180 Bq/kg and 66 Bq/kg for 137Cs and 241Am respectively were established in the core at 50 cm depth, with the closest surface sediment activity concentrations established at 56 Bq/kg and 20 Bq/kg.
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    Decoding an unexpected mystery at the Montebello Islands
    (South Pacific Environmental Radioactivity Association, 2022-11-30) Williams-Hoffman, M; Johansen, MP; Lavery, PL; Thiruvoth, S; Serrano, O; Masqué, P
    During the 1950’s three nuclear tests were conducted in the Montebello Islands in Western Australia by the UK government. The tests introduced a radioactive legacy into the surrounding marine environment but were investigated minimally immediately after and in the decades that followed. Work is currently underway to quantify the persistence and migration of anthropogenic radionuclides within the island archipelago’s marine sediments. Activity concentrations of studied radionuclides (137Cs, 238Pu, 239&240Pu and 241Am) in surface sediments so far have concentrated within a 5 km radius of the three ground zero locations and in the north-west of the islands inline with the original fallout plume trajectories. However, one sampling location does not fit within this originally hypothesised pattern of distribution. This single site, called the Lowendal Islands site, was 26 km south of the detonation ground zero locations and originally sampled as an environmental background. Activity concentrations of studied radionuclides were the second highest of all surface sediments collected (1.6 Bq kg-1, 80 Bq kg-1, 402 Bq kg-1 and 28 Bq kg-1 respectively). Four initial theories are proposed to explain this mystery: currents, air movement, rafting or human involvement. Current analysis indicates that either rafting or human involvement are most likely, based on reviews of historical data, knowledge of regional water movement and 241Am/240Pu/239Pu ratios.