Stable Noble Gas Isotope Analyses

Overview

Noble gas isotopes are the most versatile natural tracers for studying surface and subsurface waters. They reveal the infiltration conditions (temperature, pressure, salinity) of a groundwater sample at the time of recharge. They can quantify excess air that is trapped during infiltration at amounts indicative of the infiltration process (e.g. flood infiltration, infiltration through an unsaturated soil zone or riverbank infiltration).

At the Noble Gas Isotope Facility in Adelaide, we measure all stable noble gas isotopes in environmental samples (primarily groundwater) to underpin studies of the terrestrial water cycle: groundwater flow through aquifers, groundwater – surface water interaction, recharge processes, infiltration conditions and flow velocities and source water for springs. Other applications of noble gas isotope analyses include coastal and marine water samples to characterise water masses in oceanography. Natural gas samples from very low permeability sediments can be used to determine natural tracer profiles in aquitards and from hard rocks using mineral fluid inclusions.

The Noble Gas Isotope Facility provides stable noble gas isotope analyses for hydrological and environmental purposes. We have an analytical facility for all stable noble gases at the CSIRO Waite Campus and collaborate with the University of Adelaide ATTA facility to deliver radioactive noble gas isotopes. We provide expertise in multi-tracer interpretation to support groundwater hydrology studies.

The stable noble gases helium (He), neon (Ne), argon (Ar), krypton (Kr) and xenon (Xe) have unique properties in that they don’t involve in mineral interactions and don’t show chemical alterations. As a result, they reliably allow reconstruction of infiltration or recharge conditions such as the soil temperature during infiltration thousands of years ago. This makes their use superior to the traditional tracers. The isotope ⁴He has been the workhorse for many groundwater studies indicating the admixture of old water from deeper formations. Increased demand for such analysis, and the need for greater accuracy, required us to develop a new noble gas facility in 2019 with greater throughput, better efficiency and improved accuracy based on a HELIX-MC plus high-resolution noble gas mass spectrometer coupled with an automated sample preparation line. 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 expand our measurement capability.

Case studies using noble gas isotopes

Noble gas isotopes together with other environmental tracers have been instrumental in better understanding groundwater processes, which have resulted in greater confidence around opportunities for sustainable groundwater resource developments and better understanding the risks to aquifers. On the basis of several case studies we illustrate how tracers have provided novel insights in complex groundwater processes and how such findings have been translated into practical applications.

Reconciling contradictory environmental tracer ages in multi-tracer studies to characterize aquifers and quantify deep groundwater flow

The effective deep recharge to the Hutton Sandstone, a major confined aquifer of the Surat Basin, Australia, has been quantified for the first time on the basis of environmental tracers. A factor of ten discrepancy in groundwater flow velocities was found when using either _¹⁴C (25 cm/year) or ³⁶Cl (2.5 cm/year). It was possible to reconcile these contradictory velocities by describing the Hutton Sandstone as a dual porosity system, in which a significant part of the tracer is not only lost by radioactive decay, but also by diffusion into stagnant zones of the aquifer. The conceptual and mathematical description of this process allowed for quantification of the effective deep recharge into this aquifer. The resulting recharge value is only a small percentage (~3%) of previous estimates using the chloride mass balance method. The chloride mass balance may give a correct shallow infiltration rate but most of that infiltration is diverted to springs and surface water nearby (the so called “rejected recharge”). Only a small fraction of recharge finally reaches the deeper system. These results are significant for water resource quantification from groundwater in deep confined systems.

Multi-isotope studies investigating recharge and inter-aquifer connectivity in coal seam gas areas (Qld, NSW) and shale gas areas (NT)

Large sedimentary basins with multiple aquifer systems like the Great Artesian Basin and the Beetaloo Sub-Basin are associated with large time and spatial scales for regional groundwater flow and mixing effects from inter-aquifer exchange. These resources have become increasingly stressed because of growing demand and use of groundwater by multiple industries (e.g. stock, irrigation, mining, oil and gas). Using multiple tracers, we investigated recharge to surficial karst and deep confined aquifer systems before industrial extraction on time scales of decades up to one million years and aquifer inter-connectivity at the formation scale. A systematic and consistent methodology was applied for the different case study areas aimed at building robust conceptual hydrogeological models that inform groundwater management and groundwater modelling. The tracer studies provided (i) increased confidence around recharge estimates, (ii) evidence for a dual-porosity flow system in the Hutton Sandstone (Queensland) and (iii) new insights into the connectivity, or lack thereof, of flow systems.

Large sedimentary basins with multiple aquifer systems like the Great Artesian Basin and the Beetaloo Sub-Basin are associated with large time and spatial scales for regional groundwater flow and mixing effects from inter-aquifer exchange. These resources have become increasingly stressed because of growing demand and use of groundwater by multiple industries (e.g. stock, irrigation, mining, oil and gas). Using multiple tracers, we investigated recharge to surficial karst and deep confined aquifer systems before industrial extraction on time scales of decades up to one million years and aquifer inter-connectivity at the formation scale. A systematic and consistent methodology was applied for the different case study areas aimed at building robust conceptual hydrogeological models that inform groundwater management and groundwater modelling. The tracer studies provided (i) increased confidence around recharge estimates, (ii) evidence for a dual-porosity flow system in the Hutton Sandstone (Queensland) and (iii) new insights into the connectivity, or lack thereof, of flow systems.

Quantifying permeability of aquitards at the formation scale

Noble gases have also provide information on residence time of water in the subsurface (groundwater and aquitard pore water) by the accumulation of helium (⁴He) from the decay of naturally occurring uranium and thorium in the rock matrix or ⁴⁰Ar from decay of natural potassium. Our work has shown that ⁴He extracted from quartz grains within aquitard sediments allows determination of the dominant transport mechanism across the aquitard and its permeability at the scale of the aquitard formation. This study showed that helium concentrations in quartz can constrain fluid flow rates in many deep aquitards when temperatures and boundary conditions are suitable.

Groundwater recharge processes in the Beetaloo region, Northern Territory

The Cambrian Limestone Aquifer (CLA) provides key water resources for regional areas of the Northern Territory and dry-season baseflow to several iconic rivers. Irrigated agriculture and the resources industry are potential additional users of these water resources. The CLA has a complex hydrogeology: strong north-south rainfall gradient, extensive karst features (such as sinkholes and caves), a thick unsaturated zone, and partial confinement of the aquifer. Understanding recharge processes in the CLA is key to quantify groundwater recharge and assess potential risks to groundwater from development (e.g. surface spills). To address this, we sampled water for environmental tracers including major and minor ions, ¹⁸O and ²H, ¹⁴C, tritium, CFCs, SF₆, Halon-1013, dissolved stable noble gases, and the radioactive noble gas isotopes ⁸⁵Kr and ³⁹Ar. We improved the understanding of recharge processes by modelling tracer concentrations. To investigate the role of sinkholes, we evaluated tracer patterns with respect to the distance to sinkholes.

Key results include:

• Radioactive noble gas tracers: The new tracers ⁸⁵Kr and ³⁹Ar agree better with tritium than other gas tracers because they are less affected by the exchange of gas between soil air and the water.
• Water-air gas exchange modelling: Multiple air entrapment events after groundwater recharge are required to explain observed concentrations of stable noble gases, SF₆, and tritium with a single consistent model.
• Role of sinkholes: The higher sinkhole density north of Daly Waters correlates with higher tritium concentrations.

Implications for groundwater management:

• Recharge: The tritium spatial pattern indicates that recharge increases from south to north as expected from the rainfall gradient. The residence time of water in the unsaturated zone is longer in the south due to less recharge, a thicker unsaturated zone, and less preferential recharge through sinkholes. It is difficult to isolate the effects of sinkhole density and rainfall gradient as their patterns of occurrence are highly correlated.
• Water-air gas exchange: Tritium and gas tracers suggest gas exchange after recharge. Preferential recharge through sinkholes causes large water level fluctuations that entrap and dissolve air. Therefore, pollutants, such as from a spill may quickly bypass the unsaturated zone and be mixed into groundwater.
• Future research: Tritium and ³⁹Ar are the best tracers to better quantify recharge. Detailed 3D mapping with tritium, including the unsaturated zone, could quantify the relative importance of the identified processes and reveal the spatial variability of recharge to improve groundwater management and protection.

Fluid inclusion studies

To support prospectivity studies of natural hydrogen (aka gold hydrogen) in the Earth’s subsurface, the genesis of ore deposits, paleoclimate reconstruction or deep groundwater circulation, noble gas isotopes can be measured when released from rock minerals using high-pressure crushing. We offer specialist mineral preparation services combined with high-precision noble gas isotope analysis (typically ³He, ⁴He, ²⁰Ne, ²¹Ne, ²²Ne, ¹³²Xe, and ¹³⁶Xe).

New challenges for Groundwater

• Hydrogen production and underground storage enabled by environmental tracers. Production of hydrogen requires (ground)water. Environmental tracers are key to improving our understanding of the impacts from groundwater extraction on a particular water resource. Tracers are also used to characterise the suitability of deep rocks for underground hydrogen storage. [factsheet]
• Understanding the impacts of climate change on water resources using environmental tracers. A new facility for sensitive tritium measurements (TRIFIN) is being established at the CSIRO Waite Campus. Tritium and noble gas isotopes are essential tools to investigate climate variability and resulting impacts on groundwater systems. We investigate past impacts as well as the potential future impacts of climate change on groundwater and groundwater-dependent systems. [factsheet]
• Noble gas tracers define the capacity of geological formations to dispose of radioactive waste. Stable and radioactive noble gases are essential tools to investigate the capacity of deep rocks to provide long-term isolation and safe geological disposal of intermediate-level and high-level radioactive waste. [factsheet]
• Monitoring coastal water systems with the aid of noble gases. An adequate understanding of the complex interactions between coastal groundwaters and unique ecosystems like mangrove forests or the Great Barrier Reef is needed to secure their survival. Environmental tracers can monitor changes in interactions between coastal groundwater and coastal ecosystems caused by natural and anthropogenic factors. [factsheet]

The major strengths of our services include:

• the provision of quality tracer analyses for over 30 years
• backed by Australia’s National Science Agency
• ongoing research and development to extract and measure noble gas isotopes in new environmental media (soil gas, rock minerals, …).
• only facility for noble gas measurements in water samples in the southern hemisphere
• reliable turnaround and support
• one-stop-shop for the coordination of other environmental tracers not available in-house
• collaboration with universities and Australian federal and state government departments.