The Environmental Tracer Laboratory (ETL) of CSIRO hosts the only mass spectrometric system in the southern hemisphere that can measure all stable noble gases dissolved in groundwater. This significantly improved system (the previous system was only able to measure the light noble gases) has been operating routinely since January 2017. The aim of this presentation is to provide an overview of the successes and challenges we have faced in applying and interpreting noble gases in groundwater in Australia.
Concentrations of 4He have been used to improve our understanding of groundwater flow patterns in various hydrogeological systems in most Australian States/Territories. These studies (622 samples) provide data to quantify the variability of 4He accumulation rates in groundwater systems across Australia by cross-calibrating 4He with other tracers such as 14C, 36Cl, and 81Kr. In addition, measurement of the heavier noble gases (Ne, Ar, Kr, Xe) allowed us to derive excess air (EA) estimates and noble gas temperatures (NGT), which reflect soil temperature during recharge for approximately 244 samples in 22 different studies around Australia.
There were two major challenges in the interpretation of these data. First, models to correct for excess air often inferred unreasonably high NGT (>40⁰C), with the exception of the very simple “unfractionated excess air” model. Second, when plotted against a “water-age” scale, the timing of changes in NGT and EA often does not coincide with changes in the stable hydrogen and oxygen isotopes. Because some of the observed changes were dated to the last glacial maximum (based on 14C), NGT and/or ¹⁸O and ²H may not be reliable indicators to identify groundwater from the last glacial maximum in Australian aquifers due to a lack of historical data to understand how other processes might also have affected either soil temperatures or or ¹⁸O and ²H.