Age-dating groundwater and seawater using the 39Ar/Ar ratio is an important tool to understand water mass-flow rates and mean residence time. Low-background proportional counters developed at Pacific Northwest National Laboratory use mixtures of argon and methane as counting gas. We demonstrate sensitivity to 39Ar by comparing geological (ancient) argon recovered from a carbon dioxide gas well and commercial argon. The demonstrated sensitivity to the 39Ar/Ar ratio is sufficient to date water masses as old as 1000 years.
Hall, J., Aalseth, C. E., Bonicalzi, R. M., Brandenberger, J. M., Day, A. R., Humble, P. H., … Seifert, A. (2016). 39Ar/Ar measurements using ultra-low background proportional counters. Applied Radiation and Isotopes, 107, 187–190. https://doi.org/10.1016/j.apradiso.2015.10.006