Toxic chemicals have been found in waterways connected to an active airbase in Sydney's northwest - including the Hawkesbury River - but it's unclear what health risks, if any, they pose.
The Department of Defence on Thursday revealed the findings of an investigation into contamination from PFAS at the Richmond RAAF base.
It identified five key source zones of the chemicals, contained in firefighting foam used at the site during training and emergency responses from about 1976 to 2004.
All major on-site drainage systems were found to contain PFAS concentrations greater than the nominated guidelines, the report found.
The chemicals were also detected at locations where surface water discharges from the site, including nearby Rickabys Creek and Bakers Lagoon.
There were also traces in the Hawkesbury River but at lower levels.
The Defence Department's Luke McLeod said while the chemical stopped being used more than a decade ago, concentrations had built up over the years.
The levels found in the Hawkesbury - which stretches 120km - were not significant, he said, but there was a risk that rain could wash the chemicals further off the base.
"If there was a large downpour, it's hard to contain the chemical on the base - it will run off," Mr McLeod, assistant secretary of the PFAS investigation and management program, told AAP.
"It's not something we have any significant concerns about."
The investigation also recorded a PFAS 'plume' in groundwater approximately two-kilometres long and five-kilometres wide with varying concentrations of the chemicals.
Mr McLeod said any risks to human health or the environment would depend on how people use groundwater in that area.
He was not aware of anyone using it as a source of drinking water, but noted some accessed it for irrigation purposes.
Authorities will carry out a human health risk assessment later this the year.
PFAS has also been detected at the Williamtown RAAF Base and nearby creeks in NSW's Hunter region.
Mr McLeod said there was no comparison to Richmond because residents didn't use groundwater as a primary drinking source like they did in Williamtown.
RAAF Base Richmond has been active since 1925 and is an important air logistics hub for the Australian Defence Force.
At least one agency in the US says some PFAS chemicals may increase risks of cancer and other health problems but the NSW government argues there is no conclusive link.
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