The luxury liner crashed off the Tuscan island of Giglio in 2012, killing 32 people and sparking an unprecedented salvage operation set to conclude this month with the floated wreck being towed more than 200 nautical miles north to the port of Genoa to be scrapped.
Greenpeace and Italy's main environmental group, Legambiente, have voiced concerns that the hull of the damaged ship may not withstand the stress of the four-day journey and could rupture, spilling a noxious brew of heavy metals, oils, plastics and sewage chemicals into the sea.
What is more likely is that it will remain intact but shed debris and leak some of the estimated 263,000 cubic metres (over 69 million gallons) of polluted water inside it, or the 100 tonnes or so of fuel left behind when the tanks were emptied.
"The Concordia will cross a protected area home to dolphins and sperm whales, as well as fin whales who bring their young here at this time of year to feed in the rich waters off Genoa," Greenpeace's Giorgia Monti said.
"We are very worried about the effect spills or debris could have on them," she said, with possible flotsam such as cables, varnished furniture or electrical appliances releasing substances such as phthalates and
alkylphenols, which harm the reproductive system in mammals.
Costa Crociere, the ship's owner and Europe's biggest cruise operator, insists the amount of leakage will be comparable to that discharged by any vessel crossing the area — one of the most trafficked in the Mediterranean.
'No tourist attraction'
Ten boats will accompany the Concordia up the Corsica Channel alongside teams tasked with collecting any debris, testing the water for toxins and spotting any approaching dolphins or whales to prevent any collisions.
Emergency equipment to be used in case of toxic leaks from the ship will include 800 metres of oil booms – a kind of temporary floating barrier that helps to contain a spill – and infrared sensors to detect oil on water at night.
While the convoy will travel at just two knots an hour for safety reasons, a nasty turn in the weather would leave the liner at the mercy of the waves. Once it reaches Genoa, scrapyard workers will drain the water left inside, divide the ship into three parts and dismantle it over a two-year period.
Meanwhile, Costa is expected immediately to begin efforts to clean up the shipwreck site off Giglio and return the island to its original condition. Vast steel platforms erected underwater to support the ship when it was pulled upright in September will have to be removed, as well the 21 metre-thick pillars and 16,000 tonnes of cement-filled sandbags used to support it.
Legambiente said the huge shadow cast by the toppled Concordia has killed a large swath of sea grass beneath it, but around 200 giant Pinna Nobilis mussels which were rescued and transplanted elsewhere may be returned to the site.
The group's tourism representative Angelo Gentili said there had been "worrying suggestions about leaving the underwater platforms in place as an environment for fish," and possible attraction for snorkelers and scuba divers.
But on Monday Italy's environment minister Gian Luca Galletti insisted "Giglio must be left pristine", adding that Costa had promised to monitor the seabed and environment around the site for the next five years.
Giampiero Sammuri, head of the Tuscan Archipelago national park, ruled out using the site – or the Concordia wreck should it sink during the journey – for tourism purposes.
"Talk of creating some sort of underwater tourist attraction is extremely bad taste. This tragedy cost dozens of people their lives," he said.