Assessment of organically-bound tritium (OBT) dispersion and accumulation in the environment

Measurements of tritium (^H) in fish in the Cardiff Bay area have consistently shown levels greater than the surrounding seawater as well as having concentrations significantly higher than those found in fish from other areas of the British Isles. The elevated concentrations of this radionuclide in the Severn Estuary arise from the authorised discharges of wastes from Hinkley point and the laboratories at Nycomed -Amersham. The wastes that these establishments discharge differ significantly in that Hinkley discharges ^H predominantly in the form of tritiated water whilst Nycomed- Amersham waste is a mixture of tritiated water and uncharacterised ^H-labelled organic compounds. A combustion method was developed for the analysis of total 'H in sediment samples, with a limit of detection of 0.01- 0.03 Bq/g. The total ^H concentrations of sediments analysed in the area were in the order of 0.2- 0.5 Bq/g, with highest activities found in areas surrounding the sewer pipeline near Nycomed-Amersham. Seasonal reductions in sediment ^H levels were found to result from extreme storm and tidal activity, linked to re-suspension of the fine surface sediments during the winter months. The tritium found in the sediments was associated with clay rich particulate matter and in the form of organically bound tritium (OBT), where OBT was measured as the difference between total tritium and water-extractable tritium. The nature and characteristics of the OBT was investigated using a series of solvent extraction systems covering a range of polarities. Failure of the solvents to extract more than -30% of the activity, with values near to the limit of detection, makes it difficult to characterise the nature of the OBT. This suggests that the OBT might be present in organisms which do not lyse during this extraction or that it is part of a complex molecule which needs more vigorous techniques to extract it from the sediment.