Officials said the hull of the vessel, which sank carrying more than 1,200 metric tonnes of coal and is yet to be salvaged, was cracked.
"The sunken coal could pose grave threat to the aquatic biodiversity of the Sundarbans," forest conservator Zahir Uddin Ahmed told AFP.
"If the coal contains too much sulphur and if it dissolves into the water, then it is a dire concern," Ahmed said.
"The effect of oil spillage from the ship could also be damaging."
Spread over 10,000 square kilometres (3,900 square miles), the Sundarbans is the world's largest mangrove forest and the core part of it is a UNESCO-listed World Heritage Site.
The forest is home to scores of endangered Bengal tigers, spotted deers, fresh-water crocodiles and rare dolphins.
The pristine mangrove forest, said to be the South Asian nation's largest protection against tsunamis and cyclones, is already facing unprecedented human and industrial encroachment and poaching by gangs of sophisticated pirates.
This month thousands of Bangladeshi environmental activists joined a 250-kilometre (155-mile) long-march to the country's southwest to protest the construction of two coal-fired power plants near the Sundarbans.