By Greg Macvicar

SYDNEY - A Sierra Club consultant says a proposed plan to stabilize and solidify Sydney tar ponds sludge is flawed.

"The reason they (stabilization and solidification) are used is they're cheaper or they're cheaper than proper excavation and removal of the sediments, and treating the sediments or soils to truly destroy or immobilize the pollutants," said Fred Lee, a retired environmental engineering professor from California hired as a Sierra Club consultant.

"Does it (stabilization and solidification) really immobilize materials? It's not appropriate for high organic waste and the tar ponds sediments are something like 50 per cent total organic carbon. You could have slow release."

The Sydney Tar Ponds Agency is proposing to stabilize 580,000 tonnes of tar ponds sludge on site, using hardening agents such as cement powder, and shipping 580,000 tonnes of sludge contaminated with polychlorinated biphenyls and polycyclic aromatic hydrocarbons to Victoria Junction for incineration.

Don Shosky, a geohydrologist with tar ponds agency partner firm Earth Tech, said the agency's plan is safe and that the sludge stabilized on site, known as the "monolith," will not leach unacceptable levels of toxins.

He said features of the plan that Mr. Lee said will break down over time, such as plastic sheeting proposed to keep water away from the monolith and groundwater redirection drains, are redundant. "If we were to lose some of those safety features in 20, or 25, or 50, or 100 years from now, the mass would still be, in our opinion, intact and non-leachable," he said.

"We could have groundwater come in contact with the monolith and not have an impact."

Public hearings into the cleanup will wrap up on Friday.