Groundwater sampling for radiokrypton and radioargon made easy

Sampling made easy with standard 20 litre propane gas bottles.

The noble gas radioisotopes radiokrypton (Kr-85 and Kr-81) and radioargon (Ar-39) are ideal tracers of groundwater movement: they are chemically inert and provide the most reliable assessment of groundwater residence time. Krypton-85 (half-life 10.7 years) is most suitable for short-term hydrological process of decades; argon-39 (half-life 269 years) is the only tracer for groundwater residence times on the century timescale; and krypton-81 (half-life 229,000 years) is a near-ideal tracer to characterize ancient groundwater. The isotopes Krypton-85 and Krypton-81 can now be measured at The University of Adelaide using the state-of-the-art technique Atom Trap Trace Analysis (ATTA).

Traditionally, large volumes of groundwater had to be field degassed from water using a custom built membrane contactor. This sampling method is based on the principle that when water is pumped through the membrane contactor, dissolved gases diffuse through the walls to the gas side of the contactor where they can be collected. This requires that the gas side is kept at low pressure using a vacuum pump; as a result this sampling method is relatively complex with very few field-based degassing systems available in Australia. The extracting of purified krypton and argon gas happens in the laboratory.

A new method was developed by a collaborative effort from The University of Adelaide and CSIRO involving the use of evacuated standard 20 L propane gas bottles, with a three-way valve or T-piece connected to the inlet. The bottles are filled with groundwater that can be extracted from the well with a submersible pump. Samples are then sent to the CSIRO isotope lab for automated gas extraction and purification followed by isotope measurement using ATTA.