R01034: Organically-bound tritium (OBT) dispersion and accumulation in Severn Estuary sediments

Study Duration: January 2002 to April 2005

Contractor: National Oceanography Centre, University of Southampton

Discharges of tritium from UK nuclear and non-nuclear sites are usually assumed to be in the form of tritiated water (HTO). However, a proportion of HTO may be converted to organically bound tritium (OBT). Discharges from Nycomed Amersham (Cardiff) are unusual in that they contain tritium specifically as OBT, but marine dispersion modelling, and dose assessments, are based on tritiated water. The behaviour of OBT in the environment would be different from HTO. Little is known about the proportion of OBT present in foodstuffs, but the committed effective dose from OBT is higher than that from a comparable activity of HTO. The project will enable the assessment of the specific impact of OBT in critical group dose assessments.

Laboratory-based studies
The design of analytical protocols to better distinguish and quantify the various forms of tritium in environmental samples. Identification and asessment of routes of interconversion between tritiated water and non-aqueous forms of tritium. Sequential extraction and size separation of intertidal mudflat and saltmarsh sediment samples from Cardiff Bay and the Cumbrian coast. Batch experiments to examine the impact of variation in salinity, PH and redox state on the release of tritium from sediment samples.

Field-based studies
Collation of data from environmental tritium studies, to aid site selection and sample collection. Determination of spatial distribution of tritium species in sediments from Cardiff Bay and the Cumbrian coast. Extraction and analysis of biological material

Analyses to be done using sample combustion followed by low-level liquid scintillation counting. A 3He mass spectrometer to be used, as required, for ultra low-level tritium measurements.

Following the completion of the three-year phase I study on the dispersion and accumulation of OBT in Severn Estuary sediments, the Food Standards Agency commissioned a second, two-year study (from January 2002 to December 2003) to investigate the impact of changes in Amersham plc (now GE Healthcare) discharges and their treatment on OBT activity concentrations in sediments. The study focused on the continued monitoring of previously established sites and additional monitoring of sites distant from the discharge point, to identify any changes in the temporal and spatial distribution of OBT in sediments following the changes to discharges.

In addition, the thermal stability of tritiated species in sediments collected prior to, and following the commissioning of the new sewage treatment works, was investigated to identify any gross changes in the form of OBT present in the sediments. Finally, a second saltmarsh core collected from Peterstone Marsh was characterised to corroborate previous observations that the Amersham discharge record was preserved in the saltmarsh sediments, indicating long-term persistence of tritiated species in intertidal sediments.

Spatial distribution of tritium activities in intertidal sediments did not vary significantly during the survey period, with the highest activities being measured in areas closest to the discharge point. This distribution is consistent with those observed during the initial survey from January 2000 to December 2001. An extended survey covering the northern coastline of the Severn Estuary between Swansea and Gloucester confirmed that the levels of tritium distant from the discharge point were low. In addition, levels of tritium in sediments collected on the southern side of the estuary were negligible, confirming that there was limited transport of tritium labelled sediments across the estuary.

Levels of tritium generally declined at all routine sites with time although a very small increase in tritium activity concentrations was observed at the sites nearest to the discharge point following the commissioning of the sewage treatment works. Tritium evolution profiles for samples collected prior to, and following the start of waste water treatment showed similar characteristics, although there was an apparent increase in the proportion of tritium released at higher temperature for samples taken after commencement of sewage treatment. This possibly indicates a small change in the resilience of tritiated species present in the sediment.

Characterisation of the second saltmarsh core provided confirmation of the first study's findings that past tritium discharges are apparently preserved at depths in the marsh. The similarity between the tritium profiles in the two cores and with the discharge profile suggests that tritiated species are not readily biodegraded and as such the mechanism for the decrease in tritium activities in sediments is most likely linked to sediment mixing, redistribution and radioactive decay of tritium.

This study, together with others, has confirmed that tritium will persist for several decades to come in the Severn Estuary. Consequently, seafood caught or harvested from the estuary in the vicinity of Cardiff will be contaminated with tritium to a greater or lesser extent, depending on the feeding habits of the species concerned. Routine monitoring by the Agency, as published in the Radioactivity in Food and the Environment (RIFE) series of annual reports, has shown that the radiation exposure of a high rate consumer of Cardiff area sourced seafood is a few per cent of the internationally recommended limit for members of the public.

Tritium is the name given to the radioactive isotope of hydrogen. OBT is tritium strongly bonded to organic molecules that do not easily exchange with hydrogen in water. These organic molecules, together with the tritium, can be incorporated into living tissue. In these cases, tritium is not easily passed out of a living organism as is the case with so-called tritiated water. As you progress up the food chain, these organic molecules become more concentrated in living organisms at each stage of the chain; a process called bio-accumulation. Thus OBT in food gives a greater radiation exposure to humans than if the tritium was present in food as tritiated water.

The final report is available from the Agency's Information Centre. To obtain a copy, please contact the Enquiry Desk, Information Services, Food Standards Agency (tel: 020 7276 8181/8182 or email: infocentre@foodstandards.gsi.gov.uk)

Contact: For any enquiries concerning this research project, please contact the relevant programme contact or email: science@foodstandards.gsi.gov.uk