On the mangrove-lined banks of Ludmilla Creek, Helen Secretary reaches into the mud and extracts a cone-shaped shell.
"We call them 'danijarra' in Larrakia — it's a delicacy," she said.
For many decades, members of the Minmarama and Kululuk Aboriginal communities in Darwin have come here to feast on long bums, periwinkles and crabs.
"We live off these foods," the Gwalwa Daraniki Association (GDA) chairwoman said.
"These are foods to our people."
But the crustaceans and molluscs are not as wholesome as they once were.
Since 2016, authorities have advised people to limit their intake of the wild-caught creatures.
That's because studies have revealed small traces of toxic chemicals which were previously used in firefighting foams.
The Defence Department began phasing out the use of PFAS in 2004.
But the latest data shows about 10 kilograms of the substance is still ending up in the creek each year from highly contaminated soils on the nearby Darwin RAAF Base.
"We were consulted a couple of years ago … where they informed us that [if] you eat a certain amount of fish and crustaceans, you won't get affected," Ms Secretary said.
"But now knowing that there is still PFAS that runs down into our property — alongside our community where our people have been living off this creek and this land — is a very big concern."
Treasured Rapid Creek also a site of 'concern'
A few kilometres away, Peter O'Hagan points out the different native species lining Rapid Creek, which weaves its way along the outer perimeter of the airport before flushing out into the Arafura Sea.
"It's not only treasured by my family, but it's treasured by a lot of people in the Darwin community," the local Landcare Group public officer said.
But PFAS also continues to leave a legacy here, with 20 kilograms still making its way into the water each year, according to Defence.
"There's been a lot of concern about PFAS, and particularly because the jury's still out to a certain extent as to what the health risks are," Mr O'Hagan said.
"Twenty kilos is [like] a bag of pool salt, but obviously PFAS is a bit different to pool salt."
While Mr O'Hagan thinks a lot of the substance probably flows out to sea, he's aware of the health advice to limit consumption of seafood caught in the waterway.
"When's it going to stop?" he said.
"Is there going to be 20 kilos [of PFAS] every year for the next 50 years? Is it going to slow down?"
Mission underway to reduce ongoing PFAS impact
The NT government's PFAS website says that while the chemical can be toxic to some animals, its potential long-term effects are not clear.
"National health authorities advise that there is currently no consistent evidence that exposure to PFAS causes adverse health effects," it states.
"However, because these compounds are persistent, human exposure should be minimised as a precaution."
To help mitigate any potential risks from the 30 kilograms of PFAS flowing into the two Darwin creeks each year, Defence has now embarked on a major logistical operation.
"It's obviously more [PFAS] than we would like," the department's first assistant secretary of infrastructure, Dan Fankhauser, said.
"We want to get to a stage where the levels of any PFAS contamination leaving the base are as close to zero as we can get."
Chemical found at 28 Defence facilities
The Defence operation includes the removal of more than 1,500 tonnes of highly contaminated soil from old firefighting training areas on the RAAF base, which will be transported to Victoria for destruction over the next year.
A further 2,100 tonnes of soil with lower concentrations of PFAS will be "immobilised and reinstated" at the Darwin RAAF base.
"We do expect to see quite a positive change in terms of the amounts flowing off the base into surrounding waterways once we've addressed those source areas where the higher concentrations are," Mr Fankhauser said.
"It will of course take time for us to be able to say with certainty that the levels have reduced to below the thresholds that currently are set in relation to the precautionary advice."
The Darwin RAAF base is just one of 28 sites around the country where Defence has been investigating the impact of PFAS and, where necessary, implementing mitigation measures.
The residential suburbs surrounding the Darwin RAAF base are also the subject of a class action being run by Shine Lawyers, which is pursuing damages for property value losses associated with PFAS.
The case is expected to go to trial in the Federal Court next year.
A similar class action in the communities of Katherine in the Northern Territory, Oakey in Queensland and Williamtown in New South Wales resulted in a $212.5 million payout in 2020.
