The slew of dangerous toxins have built up on the harbour floor over more than 150 years of industrial activity and storm water overflows flushing pollutants into the harbour. Past studies have shown the tracts of sediment are among the most contaminated in the world - worse than Tokyo Bay or New York Harbour. The chemicals include dioxins - carcinogens that can cause birth defects - along with hydrocarbons, tributylin, polychlorinated biphenyls, pesticides, heavy metals and per- and poly-fluoroalkyl chemicals (PFAS). Under the plans on exhibition in an environmental impact statement (EIS), the contaminated sludge will be dredged to allow tunnels to be laid along the bed of the harbour.

Dr Ryall fears the plans may not adequately protect the public and the environment, including fishermen and families who swim in harbour pools across the foreshore. Transport for NSW released the draft plans in January, which will see the construction of twin tunnels linking Rozelle with the Warringah Freeway in North Sydney. The harbour would be crossed using two pre-fabricated tube tunnels, lowered onto the sea floor between Birchgrove and Waverton. Trenches would secure the tunnels in place, meaning workers would have to dig up a heavily polluted layer of sediment 1.5 metres deep. About 140,000 cubic metres of the poisonous sludge would be taken by barge to White Bay for treatment, removing its moisture content. Once solidified, the material would likely be trucked to a landfill for disposal.

An excavator with a closed "clamshell" style bucket. Credit:NSW Department of Planning It is anticipated the rest of the sediment - about 760,000 cubic metres - would be unaffected by contamination. Transport for NSW has applied to the federal government for a permit to dump it at a disposal site about 10 kilometres offshore. A risk assessment found the buried contaminants posed a high risk, but would become low risk with management actions. An excavator with a sealed "clamshell" bucket would be used to scoop up sediment, while shallow silt curtains would be installed to prevent its spread in the top two to three metres of water. Most of the toxic chemicals would stay bound to the sediment and less than two per cent of the sediment would escape into the water, it was found.

Escaped sediment was "not likely" to significantly impact water quality or human health but a plume around Balls Head would be monitored as a precaution. A Transport for NSW spokesperson said the works would be carried out in strict compliance with EPA guidelines, which was being regularly consulted. The proposed method was selected after "extensive" modelling and was a "proven approach ... used on various completed projects in Sydney Harbour" But Dr Ryall - who has worked for 30 years as a contamination adviser to government agencies and private industry, including around Sydney Harbour - remained unconvinced. He did not believe regulators would find it acceptable that up to 1300 cubic metres of contaminated sediment particles would be lost to the harbour waters under the plan - likely to be ingested by fish, crustaceans and oysters.

Dr Ryall said the government appeared to ignore its own legislation requiring developers to report contamination to the EPA, carry out a detailed investigation and have remediation action plans signed off before releasing an EIS to the public. Dr Bill Ryall has concerns over contamination. Credit:Dean Sewell "I am very disappointed with the lack of detail in the EIS relating to measures to protect the quality of water in Sydney Harbour," Dr Ryall said. He held serious concerns the "clamshell" bucket would malfunction if it encountered rocks or other objects on the sea floor. The plans did not account for wind gusts of more than 30 kilometres an hour, which occurred every month in the past year, Dr Ryall said.

These could sweep the sediment out from within the shallow silt curtain, and it was "preferable" to use a full depth silt barrier anchored to the sea floor. According to Dr Ryall, industry best practice usually saw contaminated sediment stabilised with grout before being disturbed and it was likely the amount that would be generated had been "grossly underestimated". "The EIS provides no details of the methods used to treat the sediments at White Bay and dispose of the contaminated wastewater," Dr Ryall said. "The costs may be in excess of $100 million". Dr Ryall was also dubious about assurances odours - including "rotten egg gas" from acid sulfate soils - would not affect communities because they would remain wet.

Member for Balmain Jamie Parker pressed Transport for NSW to produce the specific concentrations of the contaminants, after it would only confirm they have been found "above guideline criteria". In an email seen by the Herald, he was told in response the figures formed "part of a commercial in confidence procurement process" and could not be released "for probity reasons". Mr Parker was incensed. "This is unprecedented in scale," he said. "The proposed mitigation measures are an embarrassment, they are totally inadequate." Each summer Sydney Harbour is used for a string of open water swimming and paddling events, while families flock to harbour pools including the Greenwich Baths and the Dawn Fraser Baths at Balmain.

In 2006 commercial fishing in the harbour was banned when elevated levels of dioxins were discovered in fish, traced to the notorious Union Carbide site at Homebush Bay. Another pollution source has been the Cockatoo Island ship building yard, where toxic tributyltin was used to prevent barnacles sticking to the hulls of ships. In 2010 authorities told the Herald the fishing ban could be in place for decades and the only solution was to wait for clean sediments to cover the contaminated layer so fish could not absorb the poison. The harbour remains popular with recreational fishermen who frequently ignore warnings to limit consumption of their catch. The harbour crossing for the new metro rail line has seen deep tunnelling through a layer of rock underneath the sediment.