A vessel sank in the Sundarbans' Passur river with 775 tonnes of coal after it ran aground Saturday night.
All the seven members of the crew of MV Bilash were rescued, HM Waliullah, harbour master of Mongla port, said.
This has been the fourth cargo of coal to sink in the mangrove forest in the last three years.
"The load of the vessel was within its capacity. It seems the incident was caused by crew ignorance. The Passur river is a constantly changing terrain with submerged chars," Waliullah told The Daily Star.
He added that the vessel ran aground around 10:00pm and sank hours later.
The bridge could be seen above the water during low tides yesterday.
Waliullah said he had asked the vessel owner to recover the vessel within 15 days and claimed that the coal was covered and would be contained.
Lalon Hawlader, manager of Sahara Enterprise, which imported the coal, said the sunken vessel had loaded the coal from MV Observator, a Liberian-flagged ship that came to Mongla Port on April 13 and anchored at Harbaria anchor-6 with a freight of 24,500 tonnes of coal from Indonesia.
The cargo vessel, heading for the capital's Mirpur, sank at Harbaria anchor-5, he added.
Harbaria 6 is an outer anchorage area near the Harbaria Eco Park Station. On both sides of the river are the Sundarbans.
Visiting the area yesterday afternoon The Daily Star found that the char was not marked.
Shahjahan, a crew member of the sunken vessel, said they were going upstream during low tide.
They anchored the vessel after it had run aground. The crew later noticed that the vessel was talking in water and eventually abandoned ship.
Asked for comment, Dilip Kumar Datta, professor of environmental science at Khulna University, said, “Coal can reduce the ph of water and increase its acidity.
In the low-current areas of the river, it can cause damage to the environment.”
He added that if the coal was well covered in the hull, it would not spread rapidly.
Dilip said recurrence of such incidents in the Sundarbans should be prevented.
An oil tanker sank in the Shela river in the Sundarbans in December 2014 .