Friday, October 31, 2008
Groundwater decrease in Dhaka - research, the problems and counteractive steps to curb
Groundwater decrease in Dhaka
Faizul Khan Tanim gets the lowdown on the problems with groundwater extraction from researchers, Dhaka WASA and a Rajuk town planner and searches for possible solutions in the face of the water crisis
‘We are now going 200 to 300, and in some cases even 500 feet deep to raise groundwater, and in some very special cases, even 1000 feet.
This is alarming. There is a demand of around 200 crore liters of water per day but WASA’s capacity of production is 190 crore litres/day and of that we can only supply 170 crore litres/day presently,’ a senior WASA official reports.
He adds, ‘Dhaka is dependent primarily on groundwater for the urban water supply and about 85 per cent of the present municipal water supply comes from groundwater and 15 per cent is from surface water. And the rate at which the groundwater level is decreasing on an average is one metre per year. Too much use of deep tube wells is not at all good but until we have an alternative, there is no other option other than using them.’
Groundwater levels in Dhaka are decreasing at an alarming rate. Extraction by water-wells is causing a substantial aquifer — an underground layer of water-bearing permeable rock or unconsolidated materials like gravel, sand, silt, or clay from which groundwater
can be extracted with a water well — that is dewatering Dhaka.
Results of research
All this was suggested in the research paper ‘Declining groundwater level and aquifer dewatering in Dhaka metropolitan area, Bangladesh: causes and quantification’ by M A Hoque of the Department of Petroleum and Georesources Engineering, Shah Jalal University of Science and Technology; M Mozammel Hoque of Institute of Water and Flood Management, Bangladesh University of Engineering and Technology (BUET) and Kazi Matin Ahmed of the Department of Geology, University of Dhaka.
According to the research paper, groundwater is generally preferred as a source of potable (filtered and drinkable) water in the developing world because of its ready availability and natural protection from contamination. It is commonly used for irrigation and to supply industrial and domestic needs.
The paper also mentions a lack of proper understanding of the groundwater system, in terms of resource utilisation, as one of the major limitations to the effective management of groundwater resources.
It is mentioned in the research paper that the upper part of the aquifer in most of the city has been dewatered, with the exception of the northeast and southeast parts of the city, where the clay layer is the thickest and extends the deepest.
It is also mentioned in the paper that the recent development of a depression in the Dhanmondi area is most likely due to the impact of high pumping associated with the high-rise apartment boom, which started in the early 1990s. This converted the once low-density residential area to a high-density area along with installation of many private abstraction wells for uninterrupted water supply.
The research paper further discloses that the volume of water withdrawal from the aquifer was 160 to 186 million cubic metres (MCM) per year before 1989 and after 1990 it increased by more than a factor of 1,000 between 1970 and 2003. Dhaka WASA has been increasing its production to keep pace with the demand by installing more wells.
The change in number of tube wells is more than 700 per cent from the 49 wells in 1970 to the 389 wells in 2003. The frequency of installation of new tube wells has increased since 1990. Most of the Dhaka WASA production wells were installed after 1995.
It is also evident that the rate of water-level drop in the city is about 2.5 meters per year in recent years, which was about one meter per year during the late 1990s. The huge depression in the water level is within the city area, centering in and around the abstraction areas. This sort of drop in level is not occurring just outside the city, which is evident from observation and wells calculation.
The tiers of water problems
M Mozammel Hoque, professor of Institute of Water and Flood Management, BUET says ‘three major crises are occurring due to drop in groundwater level. Firstly, the common problem of water crisis: the situation now is such that a lot of deep tube wells have become dysfunctional and the deeper the tube wells need to be installed, the cost is increasing exponentially. Secondly, land subsidence is occurring due to the fall of groundwater levels. Subsidence (sinking of the ground surface; the process may occur over an aquifer that is slowly draining and decreasing in volume because of pore collapse) occurs when the upward pressure of groundwater decreases over the soil pressure, which keeps the balance. This causes landslide.’
In the research paper, it is mentioned that four rivers — Buriganga, Turag, Balu and the Tongi Khal (canal) form the Dhaka metropolitan area borders. These local rivers feed the flood plains and marshy lands in and around the city. Like other parts of the country the climate of the city is tropical monsoon. In the long-term this means an annual rainfall of over 2,000 mm, and about 80 to 90 per cent of this occurs during monsoon (May to October). Almost all the researchers, Dhaka WASA, senior officials and ex-Rajuk chief town planners say that all the rivers around Dhaka are heavily polluted and even though water from these rivers are big sources of surface water, it cannot be used due to illegal discharge of wastes into these rivers.
‘Therefore, due to groundwater level decrease, the river water is filling the space with huge amount of pollution. All these rivers are heavily polluted from mainly dyeing and other industrial waste such as from tanneries, plus domestic waste, hospital waste and many other forms. These wastes come to the river due to improper disposal method. Waste should always be treated using treatment plants, which is never done and the hospital wastes are heavily carcinogenic which causes cancer,’ Mozammel added.
Mozammel also says that due to the city’s expansion, the construction of unplanned pavements, rainwater cannot infiltrate or seep into the ground, which is why the groundwater recharge is decreasing. Another source of groundwater recharge is the low-lying areas and canals that are filled up constantly.
It is known that the rapid rise in the urban population is a major constraint to development of infrastructure and services, including water supply, sanitation, sewerage and drainage services.
Kazi Matin Ahmed, professor of department of geology, University of Dhaka said one of the biggest reasons for groundwater decrease is that most of the water bodies, low lying areas, ponds and lakes are losing water because they are being filled-up for usage as land. And water demand is increasing day-by-day due to migration and construction of multistoried buildings causing excessive withdrawal of groundwater.
He thinks that after heavy rain, there should be ways to channel rain water from the roof which can reach the ground and this should be in every house.
‘Since our drainage system is not good, and due to artificial low-lying areas and canals being filled-up, rainwater cannot reach the ground. And in the last 25 to 30 years, groundwater decreased almost 60 to 70 meters’, Kazi Matin added.
Speaking to Kazi Golam Hafiz, an urban planning team leader and consultant for the urban planning organization Development Design Ltd, and retired chief town planner of Rajuk, said that the three major reasons why groundwater is diminishing are firstly, siltation – for which dredging needs to be done. Secondly, illegal filling of water bodies and thirdly, migration into Dhaka.
Future counteractive steps
Hafiz thinks from a planning perspective, the recent decision to protect Hatirjheel and maintain it as a natural water reservoir is commendable. ‘Government must look into the matter. No further filling should be done. The construction of Hotel Hilton should be stopped. More natural reservoirs should be increased and government should acquire more low-lying areas, and increase the depths for storage of rainwater during monsoon’.
He continued ‘Government must dredge rivers and canals to increase depth and water flow and should check the rate of migration. WASA has to outsource and by the use of deep tube wells, raise water from places out of Dhaka and bring them to the city via pipeline. Plants like Sayedabad Surface Water Treatment Plant, should use the water of upper valley Northern Region Area Rivers Jamuna, Meghna, Sitalakhya and then Dhaleswari. Finally, alternative has to be thought of using deep tube wells in the city’.
The WASA official says, ‘In the crisis we are looking at right now, we have to start using surface water more to reduce pressure over groundwater and more water treatment plants must be installed to clean river water. But it is a matter of policy of the government since the installation cost is huge. For example, the first phase of Sayedabad Surface Water Treatment Plant cost Tk 630 crore and the second phase, which will start shortly, is set to cost Tk 830 crore and will take almost three years to construct. But installation of deep tube wells costs Tk 50 to 60 lakhs.’
‘We have plans to construct a plant in Khilkhet area,’ he reveals. ‘WASA now supplies in ten zones of Dhaka city which has a total of 482 wells and recently, a ground and surface water survey and investigation on total water resources of Dhaka city has been conducted by the Institute of Water Modeling (IWM) which will reveal the overall situation of ground and surface water resources of Dhaka. And through this, our next step will be decided.’
According to Mozammel, groun-dwater withdrawal is now more than recharge and in areas like Gulshan, Uttara and Motijheel it is the highest since more expansion is going on, the lesser recharge is taking place.
‘Land subsidence increases if the major soil characteristic of a city is clay, but that is not the case in major parts of Dhaka. However, Nikunja as an area might be more subsidence-prone. According to a research conducted in 2002 and based upon conditions and factors of that time like population density, living area, water usage and demand, we calculate that parts of Dhaka like Mohakhali, Gulshan, Badda and nearby areas will go through 196 metres land subsidence till 2010 and that will be the maximum. Then again, this is based on figures from 2002 and it might differ,’ Mozammel says.
Mozammel also says that if we are looking for solutions towards the water supply, water contamination and land subsidence problems, to increase supply, the recharge rate of groundwater has to be increased and so will the dependence on surface water. ‘Pollutants and wastes have to be treated from rivers and investigation must be done before giving permission to any new industry. The government must also look into its treatment plant design. The permit of any industry should be cancelled if they do not construct treatment plants. And finally, artificial recharge has to be made by making wells and by promoting low-lying areas around the city to store rainwater and even bring less contaminated water from far away places outside Dhaka and fill those wells.