Harold W. Bentley, Stanley M. Davis, University of Arizona
Fred M. Phillips, New Mexico Institute of Mining and Technology
Handbook of Environmental Isotope Geochemistry 2 (1986) 427-480
Chlorine possesses only one unstable isotope with a half-life greater than one hour: 36Cl. The half-life of 36Cl, (3.01 +- 0.04) x 105 years, renders it suitable for a wide range of geologic applications (half-life averaged from data of Bartholomew et al. (1955) and Goldstein (1966), as revised by Endt and Van der Leun (1973). The geochemistry of chlorine is also advantageous for many types of geological and hydrological studies. Chlorine has a higher electron affinity than any element except fluorine, and thus in virtually all natural circumstances exists as the chloride anion. Chloride is, therefore, an excellent natural tracer, passing through various systems with only minimal chemical interaction. Chloride is also one of the most hydrophilic substances, rendering it especially suitable for hydrologic tracing.