Dhaka sits on big hollow:Groundwater level drops 3 metres a year; collapse in surface feared as aquifers not recharged adequately
Groundwater table in the capital has dropped down to 61.18 metres below the surface, 35 metres in the last eleven years, due to lack of recharging which has put the sprawling metropolis at great risk.
In 1996 the groundwater table was only 26.6 metres bellow the surface. But it went 61.18 metres below in 2007, declining at the rate of over three metres on average every year.
Experts say as an increasing number of city people cannot use surface water due to consequent pollution, pressures on groundwater have increased, resulting in declination of water table.
"If water table continues to fall then a vacuum will be created in the aquifer which could cause a sudden collapse in the surface," said Khandakar Fazal Hasan, chief geologist of Bangladesh Agricultural Development Corporation (BADC). "In Bangkok city it happened due to over extraction of groundwater," he warned.
A study undertaken by BADC recently has pointed this sharp drop in groundwater level in Dhaka city.According to the study, Dhaka's groundwater level was 59.72 metres below in 2006, 57.42 metres in 2005, 50.6 metres in 2004, and 46.24 metres in 2003.
The water table in Dhaka was on average 42 metres below the surface in 2002, 37.78 metres in 2001, 34.18 metres in 2000, 31.86 metres in 1999, 30.45 metres in 1998, 28.15 metres in 1997 and 26.6 metres in 1996.The study was carried out with a vertical automatic water level recorder, which is now the most accepted method of verifying water level.
Quoting a Wasa report, officials said underground water table was on average 11.3 metres below the surface in the 1970s and 20 metres in the 1980s.
The experts say the decline is occurring fast side by side with the increase of population. Demand for water in the capital, fifth largest city in the world with a population of over 12 million, is now more than 2,000 million litres per day (mld) which was 1,500 mld in 1998, Wasa sources said.
The three water treatment plants provide with uneven amounts -- Sayedabad supplies 225 million litres, Chadnighat 39 million litres and Narayanganj 46 million litres. The remaining has to be extracted from groundwater sources.
The experts say withdrawal of groundwater by deep tube-wells is a major reason for fall in the table. In 1998, the number of deep tube-wells was 234, which now stands over 400.
The number is actually much higher as the owners of almost all the high-rises have installed deep tube-wells, a top official of Wasa (Dhaka Water and Sewerage Authority) said seeking not be named. Sources say Wasa's two treatment plants can meet only 20 percent of the city's demand by treating the surface water. The rest is met by deep tube-wells.
Based on their statistics, Wasa officials say water level has drastically fallen since 1996 in almost all parts of the city including Mirpur, Dhaka Cantonment, Dhanmondi, Mohammadpur and Old Dhaka.
Department of Environment sources say concentration of heavy metal including cadmium, lead, mercury, chromium and aluminium in rivers around Dhaka is increasing day by day. Improper application of pesticides and disposal of industrial waste are also adding to the pollution.
Pollution has already made the surface water of Dhanmondi, Gulshan-Baridhara and Ramna lakes unusable."Declining groundwater level will greatly increase the risks during earthquakes. It could lead to subsidence of the clay soil plate Dhaka is situated on," said Khandakar Fazal Hasan.
Asked about possible remedial actions, he said wetlands and water bodies should be preserved to recharge the groundwater level. "It's not possible for us to decrease withdrawal of groundwater as treating surface water in full volume is not possible due to contamination. So we should go for groundwater recharging," he noted.
Over the years, different real estate businesses have encroached on open space, lakes and rivers, barring full recharging of groundwater in the monsoons.
In 1996 the groundwater table was only 26.6 metres bellow the surface. But it went 61.18 metres below in 2007, declining at the rate of over three metres on average every year.
Experts say as an increasing number of city people cannot use surface water due to consequent pollution, pressures on groundwater have increased, resulting in declination of water table.
"If water table continues to fall then a vacuum will be created in the aquifer which could cause a sudden collapse in the surface," said Khandakar Fazal Hasan, chief geologist of Bangladesh Agricultural Development Corporation (BADC). "In Bangkok city it happened due to over extraction of groundwater," he warned.
A study undertaken by BADC recently has pointed this sharp drop in groundwater level in Dhaka city.According to the study, Dhaka's groundwater level was 59.72 metres below in 2006, 57.42 metres in 2005, 50.6 metres in 2004, and 46.24 metres in 2003.
The water table in Dhaka was on average 42 metres below the surface in 2002, 37.78 metres in 2001, 34.18 metres in 2000, 31.86 metres in 1999, 30.45 metres in 1998, 28.15 metres in 1997 and 26.6 metres in 1996.The study was carried out with a vertical automatic water level recorder, which is now the most accepted method of verifying water level.
Quoting a Wasa report, officials said underground water table was on average 11.3 metres below the surface in the 1970s and 20 metres in the 1980s.
The experts say the decline is occurring fast side by side with the increase of population. Demand for water in the capital, fifth largest city in the world with a population of over 12 million, is now more than 2,000 million litres per day (mld) which was 1,500 mld in 1998, Wasa sources said.
The three water treatment plants provide with uneven amounts -- Sayedabad supplies 225 million litres, Chadnighat 39 million litres and Narayanganj 46 million litres. The remaining has to be extracted from groundwater sources.
The experts say withdrawal of groundwater by deep tube-wells is a major reason for fall in the table. In 1998, the number of deep tube-wells was 234, which now stands over 400.
The number is actually much higher as the owners of almost all the high-rises have installed deep tube-wells, a top official of Wasa (Dhaka Water and Sewerage Authority) said seeking not be named. Sources say Wasa's two treatment plants can meet only 20 percent of the city's demand by treating the surface water. The rest is met by deep tube-wells.
Based on their statistics, Wasa officials say water level has drastically fallen since 1996 in almost all parts of the city including Mirpur, Dhaka Cantonment, Dhanmondi, Mohammadpur and Old Dhaka.
Department of Environment sources say concentration of heavy metal including cadmium, lead, mercury, chromium and aluminium in rivers around Dhaka is increasing day by day. Improper application of pesticides and disposal of industrial waste are also adding to the pollution.
Pollution has already made the surface water of Dhanmondi, Gulshan-Baridhara and Ramna lakes unusable."Declining groundwater level will greatly increase the risks during earthquakes. It could lead to subsidence of the clay soil plate Dhaka is situated on," said Khandakar Fazal Hasan.
Asked about possible remedial actions, he said wetlands and water bodies should be preserved to recharge the groundwater level. "It's not possible for us to decrease withdrawal of groundwater as treating surface water in full volume is not possible due to contamination. So we should go for groundwater recharging," he noted.
Over the years, different real estate businesses have encroached on open space, lakes and rivers, barring full recharging of groundwater in the monsoons.
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