THE BEHAVIOUR OF TRITIUM IN
SOUTH WEST ESTUARIES
Andrew Turner, Geoffrey E. Millward & Martin Stemp
School of Earth, Ocean and Environmental Sciences, University of Plymouth, Drake
Circus, Plymouth PL4 8AA, UK.
Tritium is a cosmogenic and anthropogenic radionuclide of hydrogen with a half-life
of 12.3 years. The magnitude of liquid tritium discharges from anthropogenic sources,
which exceed 1016 Bq per annum in the UK, have been based on the premise that
tritium is a radioisotope of relatively low toxicity that is not concentrated in aquatic
organisms or sediments, but there exists very little experimental evidence to support
this assertion. To this end, we studied the speciation and sorption of tritium which was
added (as tritiated water) to water samples from the Dart and Plym estuaries. The
results showed that tritium rapidly equilibrates with dissolved organic ligands that are
retained by a reverse-phase C18 column, and with suspended estuarine sediment
particles. Significantly, a substantial fraction of sorbed tritium combined with
proteinaceous material that is potentially available to sediment-feeding organisms.
The extent of association of tritium with organic matter and suspended sediment far
exceeds that predicted from its isotopic exchange with hydrogen, indicating that these
two isotopes behave differently in aquatic environments. Tritium-hydrogen
fractionation could arise because the heavy isotope preferentially enters weaker
hydrogen bridges, which are characteristic of biopolymers (including natural organic
substances), rather than the strong hydrogen bridges which exist between molecules
of water. These characteristics have not been reported previously, but are in
qualitative agreement with available measurements of tritium in estuarine and coastal
waters where the principal discharge is as tritiated water. Radiological and
environmental models for the transport, fate and impacts of tritium may, therefore,
require revision.
Corresponding author: aturner@plymouth.ac.uk