"I visited the marine work areas of the Hong Kong-Zhuhai-Macau bridge and saw plumes of [suspended solids] surrounding the sand barges," he said. "I suspect there will be a lot of vessels, which might not just disturb the seabed but also become a source of [pollution] leakages."

Fellow member Gary Ades shared Hau's concern. He asked what lessons the authority had learnt from the bridge project.

Eric Ching Ming-kam, a consultant for the authority, admitted it did not assess the likely impact on water quality because the vessels would be subject to a speed limit of 10 knots per hour.

He said there were guidelines for contractors to control leakages from their vessels and a response plan for any spills.

Another member, Jonathan Wong Woon-chung, suggested the authority return with an estimate of the number of vessels used and their likely impact.

The authority was also grilled over its planned deployment of silt curtains for the reclamation of 650 hectares of sea.

Ching said it had never assessed the effectiveness of having the whole work area sealed by silt curtains - a barrier to confine pollutants within certain areas. He insisted the most effective way of protecting the area was to deploy the curtains only at a number of "strategic and active" areas.

Meanwhile, a poll commissioned by WWF Hong Kong found that 47 per cent of 1,000 respondents supported the runway project despite the environmental impact - much less than the 73 per cent found in a 2011 poll conducted by the authority.

"It is obvious people are less supportive of the project after learning more about the environmental impacts," said Samantha Lee Mei-wah, a marine conservation officer with the group.

About 71 per cent of respondents in the latest poll did not believe or had doubts about the authority's claims that dolphins would return to the affected waters after the runway was built.

About 58 per cent did not believe the government would strike a good balance between development and conservation.