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Chlorine-36 and Very Old Groundwaters

Physics and Chemistry of Water

  1. Gholam A. Kazemi

Published Online: 15 APR 2005

DOI: 10.1002/047147844X.pc111

Water Encyclopedia

Water Encyclopedia

How to Cite

Kazemi, G. A. 2005. Chlorine-36 and Very Old Groundwaters. Water Encyclopedia. 4:416–420.

Author Information

  1. Shahrood University of Technology, Shahrood, Iran

Publication History

  1. Published Online: 15 APR 2005


Chlorine has nine isotopes whose mass numbers range from 32 to 40, but only three of these occur naturally. The remaining six have half-lives shorter than 1 hour, and this is why they do not occur in nature. The three abundant isotopes of chlorine include two stable isotopes, chlorine-35 and chlorine-37, with 75.53% and 24.47% abundances, respectively, and one radioactive isotope, chlorine-36, with a half-life of 301,000 ± 2000 years (some references ± 4000 years). The natural mixture of these three isotopes makes up environmental chlorine.

Weapon testing of fusion devices starting in late 1952 and extending through mid-1958 has led to the production of up to 70,000 atoms m−2 s−1. Chlorine-36 produced by this phenomenon is used as a marker to age-date young groundwaters that are less than 50 years old.


  • Great Artesian Basin (GAB) of Australia;
  • production of chlorine-36;
  • accelerator mass spectometer (AMS);
  • initial value problem;
  • Murray Group Limestone of Australia;
  • groundwater in the United States;
  • chlorine-36 isotopes;
  • atoms per liter