Characterising groundwater flow on time scales that date back a million years requires new technology for detecting noble gas isotopes
The complexity of natural groundwater systems and the limitations of many traditional environmental tracers calls for the use of a new suite of 'ideal' tracers: the noble gases. These are the most reliable tracers to investigate groundwater history, quantify recharge processes and determine the degree of aquifer interconnectivity. Two families of noble gas tracers exist: stable and radioactive.
Stable noble gases: The noble gases helium (He), neon (Ne), argon (Ar), krypton (Kr) and xenon (Xe) don’t show chemical alterations and allow reconstruction of infiltration conditions such as the soil temperature thousands of years ago. This makes their use superior to the traditional tracers. The isotope 4He has been the workhorse for many groundwater studies. Increased demand for such analysis, and the need for greater accuracy, required us to develop a new noble gas facility with greater output, better efficiency and improved accuracy. At the same time, the need to quantify the age of much older fluids required that additional isotope ratios of the noble gases be added (for example 21Ne,20Ne, 38Ar,40Ar,136Xe,132 Xe) to further our measurement capability.
Radioactive noble gases: The noble gases argon and krypton have three radioactive isotopes (85Kr, 39Ar and (81Kr) that are amongst the most difficult to measure because of their very low concentration. They can be used to determine the origin of water or polar ice tracing back decades (85Kr), centuries (39Ar) or up to one million years (81Kr). The challenge with traditional measurement technology for radiokrypton and radioargon is that large amounts of water (several tons) need to be sampled to achieve sufficient accuracy making this technique inefficient, time consuming and hence a limited analytical capability exists worldwide.
Combining the environmental tracer capabilities of CSIRO and several universities in cross disciplinary partnerships while championing the latest technologies
We developed a state-of-the-art noble gas facility for stable noble gases and supported the development of the Atom Trap Trace Analysis Facility (ATTA) for radioactive noble gases.
Noble Gas Facility
This facility at CSIRO's Waite campus in Adelaide was designed for high sample throughput and high accuracy. It measures the stable noble gases and all their stable isotopes, is completely computer controlled with all raw data and results stored and archived in a dedicated laboratory management and database system. This management and database system also has tools for interpreting and modelling the results. In 2019 the facility was further enhanced with a high-resolution multi-collector static noble gas mass spectrometer (Helix-MC plus) . This allows measurement of the rare noble gas isotopes 3He, 21Ne or the rare xenon isotopes such as 126Xe, 128Xe, 129Xe, and 130Xe. Recent improvements include a larger multiport system for higher sample throughput and the development of a mineral crushing system to measure noble gases in fluid inclusions of minerals.
This facility at The University of Adelaide uses the latest laser technologies to drive the successful application of radioactive noble gas tracers for natural groundwater systems. CSIRO scientists along with researchers and engineers at our partnering institutions (The University of Adelaide and Griffith University) combine laser-physics based atom excitation, trapping and detection to selectively separate and individually count the targeted atoms of 85Kr, 39Ar and 81Kr. These technologies, derived from recent years of high profile applied physics, quantum information and computing, can take direct measurements of the purified Ar and Kr fractions of environmental samples. Since the technique captures single atoms in a magnetic field and crossed laser beams, it is labelled Atom Trap Trace Analysis (ATTA). This unique facility – only the second of its kind to measure both radioargon and radiokrypton - is a leap forward in groundwater research.
The new facilities are hailed a success in first case studies
The capability of the ATTA facility incorporates the latest technology for groundwater sampling, gas separation, and noble gas purification.
In addition, the facility for stable noble gases has been producing data since mid-2016 and these measurements drove scientific results in multi-tracer studies in nearly all states and territories. Since 2019 also the rare isotopes 3He and 21Ne were routinely measured and provided additional information about the origin of helium and neon in groundwaters, identifying deep primordial sources. Details of applications in NSW, NT, QLD, SA and WA can be found on the Environmental Tracers homepage.
The CSIRO tracer team developed field sampling equipment capable of extracting 40 litres of dissolved gases from approximately two tons of water in the field. Transporting such large amounts of groundwater to the lab would not be feasible, so field extraction was a necessary step in the technology. The subsequent purification of the gas (to produce purified microlitre Kr and decilitre Ar fractions) is based on a large-scale gas chromatographic system at the CSIRO Waite laboratories.
In collaboration with CSIRO and Griffith University, the ATTA facility (located at The University of Adelaide at the Institute for Photonics and Advanced Sensing) is in its testing phase. It will undergo further development to reduce the need for collection of large groundwater samples even more, i.e. down from 1-2 m3 at present to only a few tens of litres.
The application of 85Kr in groundwater system science in Australia requires a known input function (known concentrations entering groundwater via rainwater). We have developed this time series since July 2015 at CSIRO's Waite Laboratories. The measurement of this isotope 85Kr involved a collaboration with the German Federal Office for Radiation Protection (Bundesamt für Strahlenschutz).
A first pilot study using 85Kr targeted the freshwater lens in Rottnest Island (Perth). This involved purifying the samples at CSIRO while 85Kr was measured at the University of Bern, Switzerland. Rottnest Island, a national park and popular tourist destination, has a freshwater lens under it that sits atop seawater like a drop of grease on soup.
A second pilot study involved sampling the aquifers of the eastern recharge region of the Great Artesian Basin (Pilliga Sandstone, NSW). Following gas separation in the field, gas samples were processed at the CSIRO Waite laboratories and the Kr fractions were analysed at the University of Science and Technology in Hefei, China. The isotope 81Kr was used to characterise the regional groundwater system on the timescale of millennia. It revealed two flow paths whose groundwater velocities differed by an order of magnitude, each representing regions with a different recharge condition.