Like footprints in the sand reveal the passage of animals, environmental tracers characterise the speed at which water moves underground.
Groundwater is the main water source of inland Australia. The challenge is to manage this resource sustainably under ever increasing pressures.
Groundwater provides up to 30 per cent of Australia's annual water consumption, of which 50 per cent is used for irrigated agriculture.
In inland Australia groundwater is often the only source of freshwater. Quantitative information on these groundwater resources is pivotal for water planning, operational management, and policy. Groundwater serves competing interests, from mining, agriculture to ecosystem services. Groundwater is also fundamental to food security, energy, industry development and biodiversity.
Communities, industries and governments need confidence that developments and management actions will avoid undue ecosystem disruption and sustain existing industries that have developed around groundwater resources.
Environmental tracers are key tools to build better understanding of groundwater systems
Environmental tracers are chemical substances that exist within the natural environment, either produced naturally from rocks or infiltration of rainwater, or released into groundwater through decades of human activity (e.g. fertiliser application or industrial processes releasing trace gases). Like footsteps in the sand they allow us to trace the water cycle, its underground movement and its speed. The information obtained from studying many different tracers together is necessary to understand any groundwater system: where and how groundwater recharges, how different aquifers are connected and how they feed surface water, springs and ecosystems.
Our tracer-based research played a key role in improving the understanding of groundwater systems with more robust conceptual models being developed in most states and territories. Some environmental tracers (such as 2H, 3H, 18O, 13C, 14C, 36Cl) are routine tools that have been used for decades in groundwater science. More recently noble gases (helium, neon, argon, krypton, and xenon) have become available as a very reliable and versatile tool to complement the above traditional tracers.
Applications with the radioactive noble gases radiokrypton and radioargon have become feasible since the ATTA facility at The University of Adelaide become operational in 2019.
Management decisions underpinned by robust scientific evidence
In Queensland GISERA-funded tracer studies [pdf · 6.5mb] quantified for the first time regional scale recharge to the deep flow system of the Hutton and Precipice Sandstone aquifers and characterized the Hutton as dual porosity system (i.e. pore space separated in a fast and slow moving fraction). This decreased the uncertainty about the water balance of these key regional aquifers, and provided also fundamental data for modelling the cumulative impact of water users.
In New South Wales another GISERA-funded tracer study quantified recharge to the Pilliga sandstone and detected two different flow paths highlighting the large-scale heterogeneity of the aquifer. The southern flow path was about five times faster than the northern flow path, resulting in greater groundwater recharge in the former compared to the latter. This study is one of the first in Australia where the radioactive noble gas isotope tracer 81Kr was used to calibrate the conventional tracers 14C and 36Cl.
Helium, measured in quartz grains as proxy for deducing helium in pore waters of sealing layers, demonstrated its suitability for determining very low aquitard transmissivities in Queensland and New South Wales at the scale of the entire formation. This method was shown to reliably quantify the degree of connectivity (or lack thereof) between aquifers separated by sealing layers (aquitards or caprocks). This robust technique can be used across many aquitards across Australia and provides greater confidence for modelling the impact of dewatering.
In northern Western Australia a multi-tracer study quantified recharge to the Grant Group and Poole Sandstone in the Fitzroy catchment. Another study demonstrated the inter-connectivity and provided improved estimates of recharge components of the aquifer system Leederville-Yarragadee-Cattamarra in the Peel region south of Perth.
Our multi-tracer isotope capability has been instrumental in improving the understanding of interactions between groundwater from the Limestone Aquifer and coastal environments (South Australia). Several multi-tracer studies quantified recharge to the valley infills of northern Eyre Peninsula and the APY lands (G-FLOWS).
In the Northern Territories multi-environmental tracer studies improved existing groundwater system conceptualisations and provided improved estimates of recharge to the Cambrian Limestone Aquifer in the Beetaloo Sub-Basin. In doing so we generated indispensable baseline knowledge before the start of shale gas developments.