Thermonuclear ³⁶Cl pulse in natural water

Abstract

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.