39Ar/Ar measurements using ultra-low background proportional counters

•39Ar/Ar age dating is important for understanding environmental water migration.•Ultra low background proportional counters have been developed.•39Ar is detected in atmospheric argon at a rate of 70.3 counts per day. The demonstrated background is 166 counts per day.•Age dating is possible for water with underground residence time of up to 1000 years.

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.