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Chlorine 36 has many advantages as a dating tool for very old groundwater. These advantages include a suitable half‐life (3.01 × 10 5 years), simple geochemistry, conservative behavior in groundwater, and a general absence of subsurface sources at levels comparable to the atmospheric input. Recent advances in tandem accelerator mass spectrometry have permitted the analysis of 36 Cl at the low abundance expected following residence in the subsurface for 10 6 years or more. In order to test the suitability of 36 Cl for dating very old groundwater, the 36 Cl/Cl ratios of 26 groundwater samples from the Great Artesian Basin of Australia have been measured. Groundwater ages calculated from the 36 Cl data compare favorably with ages computed independently from hydrodynamic simulations.
Bentley et al. (Mon,) studied this question.