The Defence Department is supplying bottled drinking water to 107 Bullsbrook properties because of fears over contamination to groundwater by toxic firefighting foam.

Defence has only just closed a tender to treat sites contaminated with per and polyfluoroalkyl substances found in the fire-suppressing foam, The West Australian can reveal, despite having known they posed a problem since at least 2004, when it began to phase out their use.

But, faced with growing community concern about PFAS contamination across the country, in August the Federal Government appointed law firm King & Wood Mallesons to advise on its legal liability over contamination on Commonwealth sites, including five in WA.

One of those is the RAAF Pearce air base, where more than 100 local residents have taken up an offer of bottled water rather than risk using bore water.

Initial results of a study of the contaminated area reveal fish and marron in the Ellen and Ki-it Monger brooks — which eventually flow into the Swan River — have tested positive for the substance, along with locally produced eggs.

A Defence briefing note said both tested at “low levels”, though it did not disclose the testing results. That will not occur until the final results of the investigation are released, some time in the first half of next year.

Defence began investigating contamination levels around the air base more than 18 months ago. It recently launched a new testing program at a reserve airstrip in Gingin, which sits above the environmentally sensitive aquifers that make up part of the Gnangara mound.

The initial results at Bullsbrook, presented to a community meeting late last month, found PFAS contamination in the groundwater at seven local properties surrounding the air base. Four had levels higher than recommended by health authorities.

The main townsite at Bullsbrook is not believed to be at risk because it is not supplied by bores and because surface and groundwater flows away from the townsite.

The Federal Government also faces a Senate inquiry, launched on Friday, into its response to the growing PFAS problem, which now affects at least 23 Defence sites across the country.

PFAS contamination has also been found in soil excavated as part of the Forrestfield Airport Link, as well as at former fire stations across the State.