On the background level of 99Tc, 90Sr and 137Cs in the North Atlantic

Mean concentrations of the anthropogenic radioactive oceanographic tracers 99Tc, 90Sr and 137Cs have been measured as 0.005, 1.6 and 2.5 Bq m−3 in oceanic Northeast Atlantic surface water, east and northeast of the Azores, in 1992. This is, apparently, the first published value for fallout “background” 99Tc in oceanic Atlantic water. Comparison with older data indicates an observed half life for 90Sr and 137Cs in the northeast Atlantic surface water of 20 yr corresponding to a mean residence time of 80–100 yr for the stable elements. The observed 99Tc/90Sr ratio (3 × 10−3) in the Azores samples is 10 times higher than the theoretical fission yield decay corrected to 1992. This is in agreement with published data on rain water samples and may be characteristic for 1960ʹs global fallout. Furthermore, the measured 137Cs/90Sr ratio is not significantly different from that observed for global fallout. There do not appear to be any additional significant sources of artificial radionuclides in this region.