Flux of certain radionuclides in the blood-clam Anadara granosa Linneaus under environmental conditions

Abstract

The flux of fission product nuclides, caesium-137. cerium-144. and ruthenium-106, in the Bombay harbour ecosystem has been measured over a period of six years (1970–1976). during which the input of controlled low level radioactive waste was gradually reduced by more than 20 times. Of the various pelagic and benthic species studied, the arcid clam Anadara granosa Linn, showed a specific affinity for all three radionuclides; sequence of the degree of bio-accumulation was Ce ⩾ Ru > Cs. Furthermore, the concentration of these nuclides in the tissues varied with the levels in the sedimentary material, which in turn was related to the inputs, maintaining some kind of equilibrium. On reaching maximum accumulation the levels of all three nuclides in the clam population decreased exponentially with time although the radioactivity was still available especially in the sediment. This decrease of radioactivity is defined as an ‘ecological loss rate’. The ecological half-time for cerium-144 and ruthenium-106 was about 3.6 times longer, and that of caesium-137 significantly shorter, than their respective physical half-lives. The ecological half-times of all three radionuclides were predominantly influenced by the environment rather than by the biological half-lives. This indicates that the biological half-lives of these nuclides were much shorter than their respective ecological half-lives. The biological half-time of caesium-137 was biphasic; the short-lived component had a half-time of about three days and the long-lived component had one of about 15 days. The bio-accumulation of an activation product nuclide cobalt-60 in A. granosa increased with the time of exposure and reached a level significantly higher than that of caesium-137, but was independent of the ambient concentrations and temperature. Thus the species was found to maintain the tissue concentration of fission product nuclides, cerium-144, ruthenium-106, and caesium-137 through adjusting physiological flux rates, in equilibrium with the changing levels of radionuclides in the environmental matrices, but not of the activation product nuclide cobalt-60.