The Mataranka Springs Complex is a unique water resource, one of the most popular tourist attractions in the Northern Territory and holds great cultural significance for Aboriginal communities in the surrounding area.

An environmental tracer study, conducted by the CSIRO and funded by the Australian Government's Geological and Bioregional Assessment (GBA) program, has provided clues about the origin of the water in the springs, including how and when it entered the groundwater system.

The GBA Program is a $35 million project to assess the potential impacts from unconventional gas resource development and explore ways to mitigate those impacts. The Beetaloo assessment found that pathways for impacts on spring flow, baseflow to rivers and groundwater-dependent ecosystems were of very low concern.

The resulting paper, Groundwater Sources for the Mataranka Springs, was published in late 2021 in Scientific Reports - an online, open access journal from the publishers of Nature. The findings will help ensure informed decision making by government, industry and communities in a region that is highly prospective for onshore gas production.

The Mataranka Springs Complex is the headwater of the Roper River, which flows into the Gulf of Carpentaria in northern Australia. The Springs Complex is adjacent to the Beetaloo Sub-basin, which lies southeast of Katherine in the NT and spans an area of about 30,000 square kilometres.

One of the most promising areas for shale gas production in Australia, the Beetaloo contains an estimated resource of 178,200 petajoules (PJ) of gas. This equates to more than 30 years of current gas production in Australia.

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Stakeholder engagement during the GBA Program revealed that a key community concern around gas resource development in the Beetaloo is the potential impact on the main water supply for both domestic use and agriculture in the region, the Cambrian Limestone Aquifer (CLA). The CLA provides essential dry season baseflow to valuable environmental assets like Mataranka Springs.

CSIRO's Dr Sebastien Lamontagne is a hydrogeologist who led the Mataranka Springs research.

"The CLA is one of the largest aquifers in Australia - it's about 1.5 times the size of Italy," said Dr Lamontagne.

"If you imagine a cake with many layers, the geological formations with the unconventional gas resources are near the bottom - many kilometres deep. And the CLA and its water resources is just the icing on the top."

For the CSIRO study at Mataranka Springs, scientists collected spring and groundwater samples in October 2019. This was the end of the dry season. The samples were then tested for tracers that included dissolved salts, strontium, tritium, carbon-14, noble gases, and radiogenic helium-4.

"We looked at the chemicals and the properties of the water. This provided us with clues about where it had come from, and how long it had been travelling underground," said Dr Lamontagne.

The research uses some of the most sophisticated technology available in Australia for environmental tracer measurements. Dr Lamontagne explained that the study was also unique in the level of community and stakeholder input it involved.

"We were very grateful for the support that the Traditional Owners provided for accessing the springs, and to the Northern Land Council for facilitating that collaboration," he said.

"We were also lucky to work with rangers at Elsey National Park. Additional research at the springs is also underway through our colleagues at Charles Darwin University. A lot of time and effort from many people went into the research."

Previous studies have indicated that the Mataranka Springs Complex is fed by two CLA groundwater flow paths. One is a longer flow path from the south, known as Georgina. The other is a shorter flow path from the north, known as Daly.

The water samples analysed support the findings of those previous studies. Tracers reveal that the Daly flow path of the CLA is a major source of water for Rainbow Spring, while the Georgina flow path is a major source for Bitter Spring.

"We found very little evidence of 'young water'," said Dr Lamontagne.

"That's the term used for water that has been travelling for, at most, around fifty years. And we can identify it [young water] by the levels of tritium that are present in the samples. With the exception of one smaller spring, there was nothing to suggest a significant local recharge. Most of the groundwater originated from further away in the CLA, from what we call regional sources."

In addition to the water from the two regional flow systems, sampling at Rainbow Spring and Bitter Spring revealed a contribution from an older, deeper groundwater source. This older groundwater source could be hundreds of thousands of years old. This is a common occurrence at springs and rivers associated with regional aquifers in Australia. Scientists will need to undertake further research to characterise that contribution.

"Radiogenic helium is only found in very old groundwater," said Dr Lamontagne.

"It accumulates over hundreds of thousands of years. Some of our samples had high levels of helium, showing a deeper groundwater source contributes to the spring. But we can't yet say exactly where that is coming from or how much."

According to Dr Lamontagne, the findings are significant. They indicate some degree of vertical connectivity - an exchange of water between deeper and shallower geological layers. But scientists need to undertake further research to characterise this exchange and to quantify the contribution that older groundwater sources make to the Mataranka Springs.

"Any work to investigate deeper groundwater sources will need to be done collaboratively with industry stakeholders. These stakeholders have the technology and instrumentation that can be used for deeper exploration," said Dr Lamontagne.

"The CLA is itself hundreds of metres deep. And when you start looking at what's happening several kilometres underground, that becomes very complex and expensive. It provides an important opportunity to address key knowledge gaps and manage potential impacts, such as 'where does this deeper groundwater come from and over what time scale?'

"To answer this, we need to drill wells into the underlying formations such as the Bukalara, Jamison and Moroak Sandstone (the deeper layers of the cake) and investigate them with the same technology."

This article was first published on CSIRO website

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