Residents living in a forever-chemical hotspot are being asked about their home-grown produce and backyard chickens after high levels of the toxic substances were detected.

The doorknocking campaign by environmental authorities followed new testing by Western Sydney University water scientist Ian Wright near contaminated dams and creeks in the Blue Mountains.

The water expert found the concentration of man-made PFAS (per- and polyfluoroalkyl substances) in sediment was about 100 times that of the water.

"It's a very small, swampy drainage that has the highest contamination," Associate Professor Wright told AAP.

"That is very close to people's backyards.

"It could mean that the groundwater and perhaps the soil in their backyard is highly contaminated."

About 15 homes in the Medlow Bat

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