Groundwater around the site of a 1996 Skyhawk crash is being tested to see if the firefighting foam used could have contaminated the Bulls water supply.

The Royal New Zealand Air Force plane crashed at Kakariki, near Halcombe and north of Bulls, in 1996. The pilot ejected safely and no one was hurt.

But firefighting foam used at the scene more than 20 years ago may be the source of PFAS substances contaminating the Bulls water supply. If not, contamination could have come from landfills, wastewater or industrial activity.

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The contamination is at levels well below the Ministry of Health's interim guidelines for drinking water. How it has happened is a mystery.

PFAS substances in firefighting foams have also been found in water around the Ohakea Air Force Base, but the direction of water flow would not take them to the Bulls water supply.

The New Zealand Defence Force (NZDF) is to pay specialists to test groundwater in existing wells near Kakariki, a spokesperson said.

The results will be provided to the landowners, local councils and to the Government group responding to instances of PFAS contamination.

The testing will determine whether a one-off event like using foam in the crash could be affecting groundwater 20 years later.

In spring the NZDF will also undertake a fourth round of testing for PFAS compounds around Ohakea.

The poly-fluoroalkyl (PFAS) substances are toxic and long-lasting in the environment.

They are found in firefighting foams and other products. The NZDF says the foams it uses have contained only trace amounts since 2002.