Further studies of the distribution of technetium-99 and caesium-137 in UK and European coastal waters

Data on the temporal and spatial variations of 99Tc and 137Cs in UK coastal waters are provided from surveys carried out over the period 1993–1998; prior to and post the discharge of elevated quantities of 99Tc from the nuclear fuel reprocessing plant at Sellafield (Cumbria, UK). 99Tc seawater concentrations in the Irish Sea increased by more than an order of magnitude within the study period concomitant with the increase in discharges. 137Cs discharges were relatively low (∼0.3% of their peak values in the 1970s). Simplistically, the 137Cs distribution reflected varying hydrographic conditions superimposed upon the residual concentrations of a system approaching steady state. Differences between the 137Cs/ 99Tc ratio in Sellafield discharges and seawater indicate that 137Cs remobilisation, from sediments contaminated by large discharges in the 1970s, is presently the predominant (∼90%) source term to the water column. The 137Cs/ 99Tc ratio in the Irish Sea decreased significantly within the period of these surveys (e.g. reduced from ∼14 in 1993 to 0.4 in 1996, within the vicinity of Sellafield). However, it is difficult to extrapolate this information to assess the contribution from Sellafield in distant waters because the low levels of 137Cs are continuously perturbed by additional inputs during transit (e.g. remobilisation from sediments outside the Irish Sea). Examination of 99Tc data for the North Sea indicates that the leading edge of the first EARP pulse, entering via the Scottish coastal current, may have migrated to the limit of the current flowing south along the British coastal margin within 9 months. The estimated transfer factor (TF) for Sellafield discharges in this current is 52 Bq m −3/PBq annum −1 with transit times from Sellafield to the Pentland Firth and Lowestoft of ∼9 and ∼24 months, respectively. These transit times, derived from the 99Tc data, are significantly shorter than previous estimates based on 137Cs and 90Sr data from the 1970s and 1980s.