Organically bound tritium in sediments from the Severn Estuary, UK
Amersham plc, now GE Healthcare, has discharged both organically bound tritium (OBT) and tritiated water (HTO) into the Severn estuary since 1981. The OBT component of these discharges results in elevated tritium (3H) activities in the sediments and biota of the estuary. A monthly sampling programme, covering February 2000 to May 2004, has provided the first detailed description of the spatial and temporal distribution of 3H activities in surface sediments from the estuary. Four sediment cores were also collected from salt marshes on the northern shore, to obtain longer term records of tritium accumulation. The spatial distribution of sediment 3H activities correlates well with predicted patterns of suspended sediment circulation, with higher activities (up to 3 Bq/g dry weight) in sediments from sites within 10 km of the Amersham plc discharge point and at a greater distance to the east. Temporal variations in the 3H activities of both surface and core sediments are predominantly controlled by the magnitude, composition and pre-discharge treatment of organic 3H discharges from Amersham plc, with secondary sediment composition effects; lower 3H activities are generally measured in sandy/gravelly sediments than in muddy sediments. The dated sediment 3H activity profile in one of the salt marsh cores corresponds to the Amersham plc OBT discharge record, indicating that 3H is persistent in sediments over a period of 25 years. Up to 60 % of the OBT in sediments could only be extracted with strong acids and bases, indicating that it may be composed of large, complex and hydrophobic compounds that are only extractable when lysed, such as humic compounds and/or large biomolecules. The sediment-bound fraction of OBT is predicted to be less than 2 % of the total organic tritium discharged from Amersham plc, however, these molecules do have the potential to remain in salt marsh and subtidal mud patch sediments for decades, unless the sediments are eroded and resuspended, until tritium activities decline by radioactive decay.