DISTRIBUTION AND BEHAVIOR OF RADIOCESIUM IN SCOTTISH FRESH-WATER LOCHSEDIMENTS

The distribution and behaviour of radiocaesium have been studied in th e sediments of two contrasting freshwater lochs: Round Loch of Glenhea d, an acidified loch in south-west Scotland, with organic-rich sedimen ts (approximately 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 fre shwater lochs, Cs-137 half life (t1/2) = 30.23 yr! originates from fa llout from nuclear weapons' testing (1950s and 1960s) and from the Che rnobyl reactor accident in 1986, which is also the source of the short er-lived Cs-134 half life (t1/2) = 2.05 yr!. Use of the characteristi c Cs-134/Cs-137 activity ratio of radiocaesium emitted from Chernobyl enables resolution of sedimentary radiocaesium profiles into the two c omponent 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 m axima, are clearly discernible, although an influence of partial mixin g is apparent. While the derived Chernobyl fallout inventory of radioc aesium in Round Loch sediments is broadly comparable with that for Loc h Lomond, the corresponding weapons testing inventory is an order of m agnitude 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 si gnificant loss of radiocaesium from this loch. The weapons testing inv entory 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 radiocaesi um is much less efficiently bound and is correspondingly much more mob ile in the organic sediments of Round Loch of Glenhead than in the mor e clay-rich sediments of Loch Lomond.