Initial tests of the odorous clay has ruled out chemical and radioactive contamination.

Three weeks ago, an unusual clay was uncovered on part of the Herons Creek to Stills Road section of the upgrade, causing five workers to vomit and feel nauseated.

The Port News understands the workers who became ill have since been had blood and urine tests for radioactive exposure.

Thirteen other workers were tested but did not show any symptoms.

Soil tests by the Roads and Maritime Services (RMS) were negative for pesticides, polyaromatic hydrocarbons, petroleum hydrocarbons, polychlorinated biphenyls or any of the volatile organic hydrocarbons.

Metals present were within natural levels.

The tested material was positive for “acid sulphate soil potential”.

Further investigation will determine what the odour given off by the orange clay when it oxidises in air may be.

RMS and contractor BMD Contractors created an exclusion zone around a 1980 fatal crash site where a semi-trailer carrying drums radioactive material, Chinese food including lychees and the now banned pesticide DDT. rolled and spilled its load over the road.

An RMS spokeswoman said the workers now on the site were sent to a doctor as a precaution.

“All workers have now recovered from symptoms and have reported back to work,” the spokeswoman said.

Other workers have been asked to come forward if they feel unwell.

RMS has stopped work in the area and will not start within the exclusion zone until all relevant testing is finished and approval from authorities is given.

The cause of the workers’ illness was feared to be the 1980 crash, which occurred when the semi-trailer was hit by a panel van and veered into the truck’s path.

The 23-year-old driver of the panel van was killed instantly.

A drum and small cylinder carrying radioactive materials americium and caesium were thrown onto the road upon impact.

The drum’s exterior was damaged but lead canisters housing the two radioactive substances were intact, so the possibility of leakage was ruled out.

At the time of the accident, the Port News reported that the radioactive materials were removed, but now-banned insecticide DDT (dichlorodiphenyltrichloroethane) and food preservative sodium propionate was buried on the site.

Two policemen who attended the scene later reported feeling dizzy and nauseous.

There has been a total ban on the use of DDT in Australia since 1987.

Decades on, the Environmental Impact Statement for the highway upgrade had noted some uncertainty about where the substance was buried.

Geotechnical engineering and consultancy firm Hackett Laboratory Services owner-manager Ken Hackett said before construction began on a site such as the highway upgrade, soil testing was conducted about every 100m.

“It’s impossible to test every square metre of a site, particularly one of that size,” Mr Hackett said.

The highway was “a very windy track” in 1980, with accidents frequently occurring in the area, the expert said.

“It was common practice to bury any material not considered dangerous at the time during the clean-up of an accident,” he said.

BMD Constructions refused to comment on the incident. An area of known contamination was identified earlier during the project development phase.

This material was assessed and disposed of offsite to a licensed waste facility in accordance with EPA Guidelines.