Radionuclides produced by atmospheric testing of nuclear weapons have been used as environmental tracers for the past 30 years. 185W and 90Sr have proved valuable in monitoring atmospheric circulation1, 14C has been applied to oceanic mixing2, and 3H has been routinely used as a tracer in the hydrologic system. Tandem accelerator mass spectrometry4 has made possible measurements of several long-lived radionuclides that were previously virtually undetectable. One of the potentially most valuable of these for studying the movement and mixing of groundwater and surface water is 36Cl. 36Cl is produced continuously in the terrestrial environment in very small amounts by spallation of argon by cosmic radiation and by the capture of naturally produced neutrons by 35Cl. It was also produced in much larger amounts by neutron activation of seawater and released into the environment during atmospheric thermonuclear tests. This 36Cl pulse has many potential applications as a tracer in natural water systems. Here, numerical modelling and analyses of water samples indicate that in the mid-latitudes the fallout peak was 3 orders of magnitude above the natural background, and that the period of enhanced 36Cl fallout was 1953 to about 1964.
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Bentley, H., Phillips, F., Davis, S. et al. Thermonuclear 36Cl pulse in natural water. Nature 300, 737–740 (1982). https://doi.org/10.1038/300737a0
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