Fred M. Phillips, New Mexico Tech

Harold W. Bentley University of Arizona

David Elmore, Purdue University

Most ground water in deep sedimentary basins is too old to be dated by ¹⁴ C. Ananionic, conservative radionuclide produced mainly in the atmosphere, with a half-life much longer than ¹⁴ C is necessary in order to date such waters. Chlorine-36 meets these requirements. It has a half-life of 301,000 years, is produced by spallation of atmospheric argon, and is not typically adsorbed on, or reactive with sedimentary rocks. We have successfully tested the ³⁶ Cl dating techniques on ground water from the Great Artesian Basin of Australia and the Milk River Aquifer in Alberta. Waters from both basins were shown to have residence times exceeding one million years. Chlorine-36 appears to have widespread applicability to dating old ground water in sedimentary basins.