General Seminars

39Ar dating of groundwater: still hardly available

by László Palcsu (Institute for Nuclear Research, Hungarian Academy of Sciences, Hungary)

Monday, 7 October 2019 from to (Europe/Bucharest)
Numerous methods are available to study the age distribution of karst water bodies, if they are characterised by longer residence times. To investigate the past few decades, tritium, 3H/3He, SF6, CFC’s, 85Kr
age determination methods are frequently used to calculate the time elapsed since recharge. 14C seems to be the only tool for studying water bodies having residence times of thousands to tens of thousands
of years. However, piston flow movement of groundwater occurs very rarely in aquifers longer than a few months. Hence, based on isotope analyses there are cases when young (<70 years), not too old (few hundred years) and old (thousands of years) components are present in a groundwater system. While there are tools for 0 to 70 years, and for thousands of years, for the time scale of 100-2000 years 39Ar dating is the
only available method for the time being. 39Ar is a cosmogenic isotope with a half-life of 269 (±3) years.

The activity concentration of atmospheric argon is about 1.5 mBq/dm3
ArSTP, it dissolves into groundwater during recharge. Along the flow line, its activity is decreasing due to the radioactive decay. Knowing the
activity of the atmospheric and the dissolved argon, the “age” can be calculated. Nevertheless, due to the tremendously complicated analytical requirements it is limited only to a few laboratories in the world.
In our presentation, the details of the analytical procedure to determine 39Ar/Ar will be shown. The technique consists of three steps:
1. extraction of at least 100 litres of dissolved gas in the field from the groundwater;
2. separation and purification of the argon from the gas; 
3. determination of the 39Ar activity concentration by a proportional counter system in an underground laboratory.