Study sees excessive withdrawal of water as the reason; saline water will make inroads if trend continues
The groundwater level in the capital has dropped by six metres in the last seven years.
The stunning slide is due to excessive withdrawal of groundwater to meet the needs of about 15 million city residents, and inadequate recharging of the underground vacuum it creates.
Such a fast depletion of the water table will result in intrusion of southern saline water into the groundwater reservoir, depriving this mega city of pure drinking water.
These are the findings of two groundwater zoning maps prepared by Bangladesh Agricultural Development Corporation.
BADC’s Minor Irrigation Information Services Unit (MIISU) prepared the first groundwater zoning map in 2005 with data of 2004 and the second one through analysing the 2010 data.
Talking to The Daily Star on Saturday, the chief of the unit, Eftekharul Alam, said the capital’s groundwater level had now dropped to 52 metres below mean sea level (MSL).
Explaining, Eftekhar said the water table was 51 metres below MSL in 2010 against 46 metres in 2004. And it had further fallen by one metre since last year, he added, citing Thursday’s data from one of the 255 auto water-level recorders across the country.
MSL is a measure of the average height of the ocean’s surface — the halfway point between the mean high tide and the mean low tide.
In the past, the underground vacuum in the city used to be recharged by the water flowing from the north — from the aquifer of Gazipur district and its adjoining areas, Eftekhar said.
“But, at present, these areas too are suffering from severe depletion of groundwater level,” said Eftekhar, an agriculture, water and environment engineer. Over the past seven years, the water table of Gazipur district has dropped from four metres below MSL to 16 metres.
He attributed the depletion to excessive mining of groundwater for irrigation, industrialisation and household use.
Except for the rainy season, the only way for Dhaka city to get its underground vacuum recharged is water flowing from the south saline sea, Eftekhar noted.
According to Dhaka Water and Sewerage Authority (Wasa) sources, 87 percent of its pipelined water supply in the capital is from groundwater sources and only 13 percent from surface water treatment plants.
Dhaka Wasa has recently set a target to draw water from the city’s suburbs — Singair in Manikganj and Bhakurta in Savar. This move would help Wasa lift a daily 300 million litres of water from the two well fields and save the city’s Mirpur area from danger of groundwater over-pumping.
Dhaka Wasa supplies 2,100 million litres of water daily against the current demand for 2,250 million litres.
Asked to comment about Wasa’s heavy dependency on groundwater, its Managing Director Taqsem A Khan told this correspondent on Saturday that the organisation was in the process of reversing the trend.
“I’ve not seen the groundwater zoning map, but from our records we know that Dhaka’s water table is depleting fast. That’s why we are planning to increase the surface water generation to 70 percent [from 13 percent now] and reduce the groundwater generation to 30 percent [from 87 percent now] by 2021,” he said.
The Wasa MD referred to some of the surface water plants in the pipeline — second phase at Sayedabad, one at Pagla and another at Khilkhet.
Sources speaking on condition of anonymity, however, expressed doubt over such an overturn in 10 years given that the focus is still very biased towards lifting water from groundwater aquifers.
Dhaka Wasa raised the number of its deep tube-wells to lift groundwater from less than 440 in 2004 to 560 now, a source said. But the number of its surface water treatment plants has remained static at only four.
Besides, owners of big residential apartment blocks and industrial units in and around the city are also installing deep tube-wells to lift water from below the ground.
There are also concerns about the level of pollutions in the rivers surrounding Dhaka, which are the source of the raw water for treatment in Wasa’s four surface plants for pipeline supply.
KM Tanvir Ahmmed, teacher of chemical engineering at Bangladesh University of Engineering and Technology (Buet), told The Daily Star on Saturday that unless the industries were compelled to treat liquid waste before releasing it into rivers, Wasa would not eventually be able to treat river water up to the standard of human consumption.
A recent study carried out jointly by Tanvir and Dil Afroz Begum, Buet’s chemical engineering department head, raised alarm about the water quality and asked the government to regulate industries to set up effluent treatment plants (ETPs) so that treating river water does not become extremely expensive for Wasa.
“It would not be right to say ‘river water treatment’, rather we need to say that Wasa would have to treat the ‘wastewater’ from the polluted rivers,” commented Tanvir Ahmmed, noting that river water is getting too polluted to be treated.
-With The Daily Star input