The behaviour of uranium isotopes with salinity change in three U.K. estuaries

Abstract

The behaviour of natural uranium isotopes during estuarine mixing has been studied in three British estuaries. Uranium exhibits conservative behaviour in the Clyde and Tamar estuaries but there is evidence of removal in the Forth estuary at salinities <10‰, where high particulate loads of up to 180 mg l −1 occur. Phosphate removal is also observed in the Forth in this salinity range. The uranium removal rate in the upper part of this estuary is estimated at 44 kg y −1. The uranium concentrations and corresponding 234U 238U activity ratios measured for the freshwater end-members of the three rivers are: 0·15 μg l −1 and 1·65 for the Clyde 0·04 g −1 and 1·44 for the Tamar and 0·09 μg l −1 and 1·50 for the Forth. Regression analysis gives an overall U S ‰ ratio of (9·53 ± 0·84) × 10 −8 g g −1 over the salinity range 5·0–33·2‰ for all three estuaries, which is in excellent agreement with values obtained by other workers.