Excess water is being lifted with deep wells to meet the demand in the capital.
As a result, arsenic contamination is spreading in the groundwater of several villages of Araihazar upazila in Narayanganj.
Researchers of Columbia University of USA and Dhaka University said they have found proof of this.
Pressure is created on the clay layer if excess water is extracted to meet demand of water in the municipality area. Then the clay layer gets changed and arsenic contamination spreads in the groundwater.
It was believed so far that clay layers protect groundwater in the underlying aquifers from the downward flow of contaminants. As a result, the water of underlying aquifers is free from arsenic.
But scientists now say carbon becomes reactive in impermeable clay layers if excess water is lifted from the deep of the soil. This causes arsenic contamination in the groundwater of underlying aquifers. In this way, the pressure of lifting water in the city may spread risk in the nearby areas.
Scientists earlier conjectured this process of arsenic contamination.
The new research says this process has been proved this time. A research team of several universities of USA including Columbia University and Dhaka University engaged in finding this proof.
Research magazine Nature’s journal Nature Communication published an article of these researchers in May.
The full report of the research has been published in the science magazine Water Resources Research Journal published by American Geophysical Union.
According to the report, such clay layers do not always protect against arsenic, and could even be a source of contamination in some wells.
Rajib Hasan Mazumder, Benjamin C. Bostick, Martin Stute and Alexander van Geen were among the researchers of Columbia University. Professors at Geology department at Dhaka University, Kazi Matin Ahmed, Imtiaz Chowdhury and Mafuz Khan were also among the team.
Researchers have written that they have come to this conclusion after working on the water of the Mekong delta of Vietnam, the Central Valley of California and Araihazar upazila of Narayanganj in Bangladesh.
The researchers have worked in Araihazar for about two decades. They have selected this upazila as there is safe and unsafe groundwater source or aquifers. The density of arsenic was found to be increasing in a tube well in a village of Araihazar after 18 months of its establishment in 2005. In the beginning, the density of arsenic was bellow 10 microgram per litre in the water of 60 meter-deep tube-well. That increased to 60 microgram.
According to World Health Organisation (WHO), water is arsenic contaminated if there are over 10 micrograms of arsenic per litre of drinking water.
Researchers started looking into why the water of those tube wells in the villages turns unsafe so quickly.
There is no different opinion about the risk that is being indicated. It is not that extracting excess water in Dhaka is the lone cause of spreading arsenic in Araihazar. There are more reasons. It takes long for the change that takes place in clay layers. The research is issuing a message to be cautious.
The research began in 2011 on whether this happened due to changes in the formation underground. After eight years, the researchers confirmed underground clay layer is the source of contamination. The layer turns predator from protector.
Speaking to Prothom Alo, researcher Alexander van Geen said a pressure is created on the clay layer if excess water is extracted to meet demand of water in the municipality area. Then the clay layer changes and arsenic contamination spreads in the groundwater.
Rajib Hasan Mazumder explained the cause of contamination and spread to distant areas. He said arsenic usually remains in sand. It does not make any trouble. But it creates trouble if it mixes with water due to reactive carbon.
The source of this reactive carbon could not be ascertained in the researches of several decades. In recent research, it was found that reactive carbon comes out from underground clay layer due to pressure of pumping excessive water in the municipality area. That carbon brings out arsenic from sand in the underlying aquifer. Arsenic mixes with water of aquifer.
Rajib is now working as environment engineer in Boston-based consulting agency Gradient in USA. He said the water level in Dhaka receded significantly to meet the demand of the citizens.
And that has created a deep ditch or ‘cone of depression’. Water of surrounding areas ranging 30 to 40 kilometers has come under this ditch. As a result, the risk of arsenic contamination is increasing in different nearby areas including Araihazar.
Deep wells in Dhaka are extracting water from aquifers of surrounding areas as the water level in Dhaka recedes deep. As a result, reactive carbon is coming out from clay layers in these areas. Arsenic from sand of aquifers is coming out easily.
Iron mixed in this layer also plays role in this process. Water of Araihazar is turning unsafe in this way. But how is the water of Dhaka remaining safe?
Rajib Hasan said the capital is able to avoid this process of arsenic contamination due to the formation of land. As the water level of Dhaka recedes deeper, it extracts water from a deep level.
Deep wells of Dhaka is extracting water from aquifers of surrounding areas under 150 metres of soil.
Arsenic contaminated aquifers of Araihazar are far above of it. There is no link between deep wells in Dhaka and aquifers of Araihazar. The contamination reaches Dhaka if all aquifers of surrounding areas are contaminated.
Although Araihazar upazila parishad chairman Mujahidur Rahman is not aware of the research findings, he knows about the spreading of arsenic in the locality.
Speaking to Prothom Alo on Tuesday, he said there are allegations that a number of deep wells are not safe. The upazila parishad has contacted with UNO office to visit and test arsenic contaminated tube wells. It has not been possible to carry out the test.
He said some 448 safe tube wells have been installed for arsenic free water under the Department of Public Health Engineering.
Around 40 million people in Bangladesh are victims of arsenic.
Speaking to Prothom Alo, professor of geology department at Dhaka University, Kazi Matin Ahmed said the level of risk of arsenic contamination depends on the amount of water lifted.
The regulatory agency like WASA should allow installment of deep wells in city areas only after more examination.
The researchers of Columbia and Dhaka University are jointly working on arsenic contamination for two decades.
Professor Matin said a model is needed to get better results as to where deep wells should be installed and how much waters should be extracted. There should be a central information centre.
Public health expert and WaterAid’s regional director Khairul Islam is aware of the research.
Speaking to Prothom Alo, he said there is no difference of opinion about the risk that is being indicated. It is not that extracting excess water in Dhaka is the lone cause of spreading arsenic in Araihazar. There are more reasons. It takes long for the change that takes place in clay layers.
Khairul Islam believes that the research is issuing a message to be cautious.
He said the risk that has been created has to be taken into consideration. In that case, the directives of the High Court should be followed. WASA should regularly test the quality of water. It is not possible to test water of all deep wells in Dhaka. Selected deep wells can be tested. Such research findings should be included in the master plan of Dhaka WASA.
There are 900 deep tube wells in Dhaka. Deep tube wells meet eighty per cent of the demand of water in Dhaka. Dhaka WASA has a plan to decrease it to 50 per cent.
Supporting this plan, Khairul Islam said this can be made more effective taking different recent researches into consideration. Simultaneously its implementation has to be taken into consideration too.
*This report, originally published in Prothom Alo print edition, has been rewritten in English by Rabiul Islam.