Plutonium activity ratios in plankton: New evidence of hold-up time in Irish Sea sediments

The determination of activity ratios for radioisotopes of different half-li ves can be used to estimate transit times from a point source to locations further away. For conservative elements, this time is approximately equival ent to the net hydrological transport. However, for non-conservative elemen ts such as plutonium, the additional influence of biogeochemical processes decreases the net transport time. In this study, Pu-241 and Pu-239,Pu-240 c oncentrations in Irish Sea plankton samples, collected in May 1994, were de termined acid the Pu-241/Pu-239,Pu-240 ratios calculated. Plutonium-239,240 was measured using a standard method by ion exchange chromatography and al pha counting, and Pu-241 was determined by liquid scintillation counting us ing the disk-supported technique. The latter showed some methodological pro blems, which are briefly discussed. The Pu-241/Pu-239,Pu-240 ratios gave an estimate of the "transit time" from Sellafield to the different sampling p oints. In fact, this time represents the age of plutonium in plankton, i.e. , the time lag between release from Sellafield and detection at the differe nt sampling stations. The mean plutonium age was 17+/-2 years (n=10) and 18 .6+/-0.8 years (n=13) in phytoplankton and zooplankton, respectively. The s patial distribution was reasonably homogeneous over the Irish Sea. The assi milation-elimination processes of plutonium in plankton are rather rapid. T herefore, it may be assumed that, in this time scale, the plutonium concent rations were in equilibrium with surrounding waters. Thus, it is concluded that plutonium was rather old because resuspension-sedimentation processes had occurred that delayed its transport within the Irish Sea. Therefore, th e age of plutonium in plankton represented the hold-up time of plutonium in the sediments from the Irish Sea.