Abstract
A method has been developed for the determination of the 238U/235U isotope abundance ratio in sequential extracts of freshwater sediment by means of ICP-MS. Instrument parameters were optimised, and sample pre-treatment procedures developed, to improve the quantification of 235U. Extracts prepared according to the three-stage BCR sequential extraction protocol were passed through a 2 cm column of anion exchange resin (BioRad AG1-X8), and uranium eluted with 8 mol l−1 HNO3 prior to analysis. The precision obtained for measurement of the 238U/235U ratio in solutions containing 20 ng ml−1238U and 0.2 ng ml−1235U was 0.18%. Analysis of stream sediments from the vicinity of a nuclear fuel enrichment plant revealed an enhancement in uranium levels (up to 27 µg g−1 dry weight) which decreased with distance from the effluent discharge point. The 238U/235U ratios measured in sequential extracts of the stream sediments were significantly less than 100 (cf. the natural value of 137.9), confirming that Rivacre Brook is contaminated with 235U-enriched uranium. Results of the sequential extraction also suggested that uranium is discharged from the plant in, or rapidly converted to, a relatively refractory form.