Groundwater loss puts city at risk
Dhaka is sinking over half an inch a year on average because of excessive extraction of groundwater and inadequate recharging of the vacuum it creates below the surface, found a recent study.
Prof Syed Humayun Akhter, a geology teacher at Dhaka University, conducted the study by analysing data from global positioning system (GPS) at the DU Earth Observatory Centre.
Examining the city's vertical velocity in 2004-2008, he found that the land mass here is going down quite faster now.
“When we extract groundwater, the empty spaces in the sponge like soil are filled with air until they get recharged with rainwater again,” said Prof Humayun.
Experts also blame movements of tectonic plates (large rigid blocks) for land subsidence.
Bangladesh is on the junction of Indian plate and Burma sub-plate. The Burma sub-plate is overriding the Indian plate, causing subsidence of Dhaka land mass.
The subsidence due to motions of tectonic plates is much slower; it should not be more than 3 to 5 mm a year. But Dhaka's land mass is dropping by 13.91 millimetres a year, as the plate motion coincides with water extraction, says Prof Humayun.
The geologist, who is also an earthquake expert, said that since the city is located in a quake-prone area, the land subsidence puts it in even greater danger.
Other geologists, too, have warned that the declining water table poses extreme hazards during earthquakes. It could lead to subsidence of the clay soil plate the capital is standing on.
There are chances that the vacuum created underground might be filled with brackish seawater, turning the area into a saline land, according to expert analysis.
Recently, a Bangladesh Agriculture Development Corporation research pointed to a fast depletion of groundwater level -- six metres in the past seven years. The water table has already dropped to 52 metres below mean sea level (MSL).
MSL is a measure of the average height of the ocean's surface (such as the halfway point between the mean high tide and the mean low tide); used as a standard in reckoning land elevation.
Geologists and environmentalists on several occasions warned that an inadequate natural recharge and too much groundwater withdrawal might cause land subsidence what many cities in the world experienced earlier.
Against the backdrop of surface water pollution by industrial and household waste, 83 percent of the water supplied to about 15 million city dwellers comes from underground.
Experts also say the pressure on groundwater is increasing every year as water from rivers, canals, lakes and other water bodies have become polluted.
“Nowhere in the world authorities depend fully on groundwater,” said urban expert Professor Nazrul Islam and. “If we want to live in this city, we have to stop groundwater abuse.”
Water Supply Authorities in Dhaka supply around 1,560 million litres a day against a required 2,000 million litres. Of that, they lift around 1,250 million litres from underground.
Some new commercial water supply companies have mushroomed in the cities and they extract groundwater to make profit. Environmentalists condemn the act but nobody cares.
A study of Bangladesh Agricultural Research Council shows with most of the city land rapidly coming under concrete cover, rainwater is being obstructed to enter the aquifer. The declination of groundwater occurs less in green and spacious areas.
Asked about remedies, Prof Syed Humayun Akhter said the wetlands and water bodies should be preserved for groundwater recharge.
"It is not possible for us to limit groundwater withdrawal as a full-scale treatment of surface water is not possible. So we should go for groundwater recharging," he said.
He feared that the continued fall of water table could eventually lead to desertification in and around Dhaka.