Toxic chemicals seeping into waterways in Melbourne’s west could persist for weeks, leaving locals at risk of long-term health impacts, after a blaze ripped through a chemical factory.

The inferno was sparked by a big explosion at the Derrimut factory on Wednesday, shooting chemical drums into the air.

No one was injured and it was declared under control hours later after more than 180 firefighters fought the biggest industrial fire in years.

Several hotspots were still smouldering on Thursday afternoon, with collapsed walls and parts of the roof caved in preventing the fire from being extinguished.

Fire crews will wait for the site to be demolished before they can snuff out the blaze with about 60 personnel and 12 trucks remaining on scene overnight.

More than three million litres of water and 40,000 litres of foam were used to fight the flames, with fire water runoff causing concern for local waterways as attention turns to water safety.

Residents have been told to keep themselves and their pets away from waterways in the area including Cherry Creek, Anderson’s Swamp and Kayes Drain until further notice.

Contamination from chemical storage sites includes a diverse range of toxic substances such as per and polyfluoroalkyl substances (PFAS) and heavy metals, Monash University chemical engineer Sally El Meragawi said.

“These chemicals can have serious long-term impacts on health and the environment,” she said.

A fire in a Footscray-Tottenham chemical factory in 2018 resulted in levels of PFAS chemicals being 16 times higher than the safe recreational water quality levels downstream from the site for two weeks after the incident.

PFAS are often called “forever chemicals” because they persist in the environment and the human body for over 35 years and have been linked to health issues such as cancers and thyroid diseases.

“Once in the environment, PFAS do not break down and can accumulate in the food chain,” Dr El Meragawi said.

Waterways are being tested to understand the scale of pollution, the Environmental Protection Agency’s Steve Lansdell said.

“We know what’s been coming out of the firewater will have contaminants, so it’s really important to avoid contact,” he said.

Wastewater has been pumped away from the site and local stormwater drains were blocked off to stop polluted water from escaping.

One worker died and two were injured in a fire explosion at the same building in October, which houses chemical blending company ACB Group and fuel producer Powerplus.

Drums containing kerosene, methylated spirits, methanol and other substances were stored at the site, which fuelled the blaze.

The EPA has issued the site with two notices and inspected it nine times since that first explosion, finding it compliant.

While WorkSafe conducted 24 visits, issuing seven compliance notices.

EPA and WorkSafe investigations into the 2023 fire continue, while a Worksafe spokesperson said the authority would determine further action over the most recent blaze at an appropriate time.

Fire authorities will remain onsite until the incident is expected to be handed over to investigators in a couple of days.

Source: AAP