in whole animals including fish were 5 urprisingly unff. factorof 10, 1.2-9.9% of mean sediment concentration) Theat ° ated to gut loading of sediments and/or surface contamination. The Quite uptake sett. by rooted «0.03 macrop tonna h ge not exposed to surfac e contamination was Sorpt ion of plutonium to plant surf. aces, on gut walls, and o of Pu behaviow nate in Submerged components of aquatic systems. Recv iten potential for teressenscusse used d | in terms of ecological signi i ficance C and INTRODUCTION There are three important reasons why we have used the plutonium-237 isotope so extensively in our laboratory work involving aquatic organisms. Firstly, plutonium-237 is predominantly a photon emitter-190 Kev-complex x-rays accompany 45% of all decays while a-emisstons occur in only 3 out of every 100,000 transitions (Lederer et al,,1967). Thus, one is able to do repeat counting of individually-tagged organisms in an uptake experiment. Since destructive analytical techniques are not required, one can follow uptake and elimination patterns in some organisms over the entire lifetime. Even where destructive sampling is required (as in tissue distribution studies, for example), analytical procedures are both simple and inexpensive. One is also able to reduce the sample size required for a given end point since individual variability as a function of time is determined in uptake and elimination work. Secondly, plutonium-237 has a high specific activity - approximately 1 x 10" Ci/g in our product. Thus, uC concentrations of plutonium-237 contain the same concentrations of plutonium atoms as pCi levels of plutonium-239, Therefore, we are able to work with reasonably high activity levels of plutonium-237 to reduce radioanalvtical error and at the same time maintain plutonium atom concentrations which closely mimic a "real-world" situation. This is an important requirement in terms of the environmental chemistry of the element. Thirdly, plutonium-237 has a relatively short physical half-life of 45.6 days; and, as noted previously, negligible a-activity. Thus, the investi- gator can avoid the enormous costs associated with use of double-containment facilities required for reasonable quantities of plutonium-239 or plutonium-238, (ne has essentially the same freedom in experimental design as if the radionuclides were Zn-65, or Cs-137, or Ce-144, for example. PRODUCTION AND CHARACTERISTICS Plutonium-237 is produced by helium ion bombardment of 735U. Trace quantities of plutonium-236 provided the only significant contamination of the material we received. Table 1 illustrates the physical charac- teristics of plutonium-237 for comparison with a-emitting isotopes (Eyman et al,,1976). The characteristics for plutonium-246 are included since it represents an alternative photon emitter for use in plutonium research, Its disadvantages are its very short physical half-life and low specific activity (as a consequence of contamination with its parent isotope, ?""Pu}. Although we did undertake some early work involving plutonium-246, we would not recommend it for future aquatic environmental studies because its specific activity fs unrealistically low. 490 491