Distribution and behaviour of radiocaesium in Scottish freshwater loch sediments

The distribution and behaviour of radiocaesium have been studied in the sediments of two contrasting freshwater lochs: Round Loch of Glenhead, an acidified loch in south-west Scotland, with organic-rich sediments (≈20%C) and Loch Lomond, 35 km north-west of Glasgow, where sediments are low in organic matter (1–6%C, southern basin), but with a relatively high clay content. In the sediments of Scottish freshwater lochs,¹³⁷Cs [half life (t_1/2) = 30.23 yr] originates from fallout from nuclear weapons’ testing (1950s and 1960s) and from the Chernobyl reactor accident in 1986, which is also the source of the shorter-lived¹³⁴Cs [half life (t_1/2) = 2.05 yr]. Use of the characteristic¹³⁴Cs/¹³⁷Cs activity ratio of radiocaesium emitted from Chernobyl enables resolution of sedimentary radiocaesium profiles into the two component sources. In the organic-rich sediment of Round Loch, downward diffusion of radiocaesium in porewaters obscures its pattern of input to the loch. In the more clay-rich sediments of Loch Lomond, separate radiocaesium concentration peaks, related to atmospheric deposition maxima, are clearly discernible, although an influence of partial mixing is apparent. While the derived Chernobyl fallout inventory of radiocaesium in Round Loch sediments is broadly comparable with that for Loch Lomond, the corresponding weapons testing inventory is an order of magnitude lower than in Loch Lomond. Although Round Loch is situated in an area of known elevated Chernobyl deposition, the inventory is much lower than literature values of atmospheric deposition, indicating significant loss of radiocaesium from this loch. The weapons testing inventory in Round Loch is also lower than reported estimates, whereas in Loch Lomond the established inventories from both sources are similar to, or greater than, fallout deposition. The differences between the distribution and inventories in the two lakes confirms that radiocaesium is much less efficiently bound and is correspondingly much more mobile in the organic sediments of Round Loch of Glenhead than in the more clay-rich sediments of Loch Lomond.