ON THE BACKGROUND LEVEL OF TC-99, SR-90 AND CS-137 IN THE NORTH-ATLANTIC

Mean concentrations of the anthropogenic radioactive oceanographic tra cers Tc-99, Sr-90 and Cs-137 have been measured as 0.005, 1.6 and 2.5 Bq m(-3) in oceanic Northeast Atlantic surface water, east and northea st of the Azores, in 1992. This is, apparently, the first published va lue for fallout ''background'' Tc-99 in oceanic Atlantic water. Compar ison with older data indicates an observed half life for Sr-90 and Cs- 137 in the northeast Atlantic surface water of 20 yr corresponding to a mean residence time of 80-100 yr for the stable elements. The observ ed Tc-99/Sr-90 ratio (3 X 10(-3)) in the Azores samples is 10 times hi gher than the theoretical fission yield decay corrected to 1992. This is in agreement with published data on rain water samples and may be c haracteristic for 1960's global fallout. Furthermore, the measured Cs- 137/Sr-90 ratio is not significantly different from that observed for global fallout. There do not appear to be any additional significant s ources of artificial radionuclides in this region.