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Chlorine-36 is an unstable isotope of chlorine produced by cosmic-ray bombardment of the atmosphere and the Earth's surface as well as by absorption of cosmic-ray or natural radioactivity-derived neutrons in stable chlorine. Nuclear weapons detonations, nuclear reactors and the use of neutron probes in environmental studies add to the inventory of Cl-36. Radiochlorine is strongly associated with water in the atmosphere, environment, soils and groundwater. Chlorine-36 undergoes radioactive decay with a half-life of 301 ka. These properties make radiochlorine a valuable tracer in the study of aqueous and saline systems and a useful tool for dating closed systems (e.g. confined aquifers).

Recent measurements of Cl-36/Cl, using the 14UD tandem accelerator at ANU, show that a variety of processes is important in groundwaters of the Murray-Darling Basin and in the Great Artesian Basin. The results also provide useful information on the levels of Cl-36 in soils, salt lakes and other parts of the Australian environment. The results confirm that estimates of Cl-36 precipitation rates are reasonable for southeastern Australia but that more information is needed for other regions. Further modelling of chloride movement is also needed, particularly for arid Australia.