Yes, it's mostly a matter of inference that some of the compost probably has low levels of the herbicide. It is stated, though, that some of the tests found traces (meaning low levels), while others didn't - but that some of the testing labs can't detect traces below a certain level.
❝Both Suez and the Victorian Environment Protection Authority have been investigating the cluster of complaints, and have commissioned soil tests by the independent lab SESL.
Some samples have found traces of the herbicide, while others have not.
Clare's garden centre in central Victoria has written to its customers saying soil samples have come back clear. Suez also says testing shows its organic compounds are in line with EPA standards.
But it's not that simple.
SESL's senior soil scientist in Victoria Declan McDonald said the herbicide can cause problems in concentrations as low as a few parts per billion, which is too low for some labs to detect.❞
(Rubrication mine.)
Ideally, I suppose, in those samples where no traces were found, further testing in a lab capable of detecting ultra-low concentrations of a few parts per billion is called for. That would be definitive evidence.
