Poisons from one of Perth's most polluted sites are likely to be leaching into the Swan River because a barrier that was supposed to protect the waterway is faulty.
A report, prepared by the Department of Water for the Swan River Trust, found a "containment cell" built over the top of Mardalup Park in East Perth had become compromised.
The cell, consisting mainly of clay, was constructed in the 1990s as part of efforts to remediate the old East Perth gasworks site.
It was designed to prevent rainfall from topping up groundwater under Mardalup Park and overflowing into the Swan Estuary, which borders the park.
The groundwater is some of the most contaminated in Perth, containing toxins including hydrocarbons and heavy metals.
But the report found the clay barrier may have failed in places and become more "permeable" than the original requirements specified, meaning toxic groundwater would have leached into the Swan.
This was compounded by the fact the northern section of the site was not capped properly during the original remediation works, while too few trees had been planted in the park to keep groundwater levels suppressed.
The report comes on the heels of revelations that the Swan River adjacent to the $2 billion, State-backed riverside development in East Perth was far more polluted than previously known.
"The direction of groundwater flow suggests offshore migration of this contaminated groundwater, despite attempts to contain it," the report notes.
Environment Minister Albert Jacob said the Health Department told him the report did not "reveal any risk to public health and safety".
He said work was under way to investigate further any issues with the containment cell, while planting more trees at Mardalup Park could compromise it even more.