NT's vast Cambrian Limestone Aquifer 'getting drier', new study finds
The vast Cambrian Limestone Aquifer stretches from near Katherine in the Top End to almost as far south as Alice Springs.
It feeds the iconic Mataranka thermal springs and charges the Roper River, which winds through a series of Aboriginal communities on its way to the Gulf of Carpentaria.
The study, published in the journal Ecological Informatics, looked at the state of the aquifer from 2002 to 2022, using a combination of satellite data and direct observations from groundwater monitoring bores.
It found that water storage slowly increased through the first decade, despite some major droughts. But in the more recent decade, "significant depletion" of groundwater storage was observed.
"The Cambrian Limestone Aquifer appears to be getting drier," the study authors wrote.
Between 2011 and 2022, the amount of groundwater stored in the system shrank by about 3.88 cubic kilometres per year.
While it is normal for groundwater storage to fluctuate between the Territory's wet and dry seasons, the recovery of the system has started to get out of sync.
"We started to see that the recovery of the system from dry season conditions became slower and slower," said Matt Currell, an author of the study and Griffith University hydrogeologist.
Extraction for irrigated agriculture, including mangoes and melons, has ramped up over the past decade. Water licences have already been granted that would allow the take from the productive area around Mataranka and Larrimah to increase four-fold.
"There's an implication there that we might be, through that water extraction happening in the aquifer, lessening the resilience of the aquifer and all its connected surface water systems to those drought periods in the future and making it more vulnerable," Dr Currell said.
Surface water coverage has also been declining, according to the study.
"In the past 10 years, some of the surface water did not just shrink. Some were actually permanently lost," co-author Christopher Ndehedehe said.
The findings align with what Aboriginal people in the region have been saying about the state of their springs and creeks.
The community of Jilkminggan sits on the Roper River.
"The Roper River is our life because we fish in it, we hunt in it," Mangarrayi woman Karlisha Ponto said.
"The water is our life."
The 25-year-old said there were fishing spots she visited often in her childhood that her family no longer went to, including one called Barramundi Crossing.
"It's nice and wonderful country to fish," Ms Ponto said.
"Throughout the years, we have been losing a lot of water down that area, and it has been dropping since then. That's why we don't go out more often because of our water changes."
A spokesperson for the NT's Department of Lands, Planning and Environment told the ABC that any suggestion of a link between reduced groundwater storage and extraction licences was "false", and that groundwater storage remained strong.
Water allocation plans in place for various parts of the aquifer all paint a rosy picture of the resilience of the aquifer and its capacity to handle more extraction.
The department spokesperson said estimates of how much water could be sustainably extracted were "determined based on long-term annual averages".
"Announced annual allocations can be reduced if and when necessary, and water plans are reviewed every five years."
In the Mataranka plan area, water licence holders are currently extracting about 10,000 megalitres a year, according to the department.
That's about one-quarter of the amount they are entitled to under existing licences, and about one-sixth of the total NT government planning documents say can be sustainably taken.
A major new 10,000 megalitre licence for a cotton, melon and mango operation in the fields around Larrimah was granted in recent months.
Dr Currell said the Larrimah licence was a "step change" in the intensity of water extraction.
"We need to look really carefully at the impact of increasing extraction in that area before allowing further licences to go ahead and potentially really stressing the aquifer system," he said.
Ms Ponto said she would also like to see the development of the water resource slow down.
"Maybe they can stop taking more water out of our country, and maybe they can come and see what they are damaging and destroying," she said.
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