Polonium and plutonium in an intertidal food chain

Abstract

LARGE quantities of seaweeds (kelp) are washed ashore along the coasts of California. Estimates range from 10⁴ to 10⁶ tonnes (10⁷–10⁹ kg) wet weight per yr. These algal masses provide food for insects and other invertebrates on the beaches. The plants carry with them a variety of di-, tri-, and polyvalent cations of heavy metals, largely adsorbed or chelated to superficial acidic polysac-charides and surface slimes of bacterial origin. Among the heavy metals one can detect small amounts of radioactive elements, including some that emit high-energy α particles such as the natural polonium (²¹⁰Pu, half life 138 d) and the artificial plutonium nuclides (^239,240Pu, half lives 24,360 yr and 6,580 yr respectively)¹. In surface waters of the oceans, polonium is born from the decay of ²¹⁰Pb which is continuously replenished by decay of radon released from the Earth's crust. The plutonium isotopes occur in ocean waters as a consequence of fallout from atomic test explosions some 15–20 yr ago. We have examined the extent to which these elements may be transferred from the ocean to the terrestrial biosphere by means of kelp, kelp flies and their predators. Fears have been expressed that radioactivity from the algae could be reaching man through the terrestrial food chain. Our investigations suggest that, on the basis of current radiation thresholds for human healthand safety, such fears are groundless.