Dhaka city on the verge of subsidence
Dhaka city, the capital of the country has always been the core of political, cultural, and educational movement and evolution. Home of about 15 million people, the city is now worn out providing supplies to growing demand for housing, infrastructure, water supply and sanitation. Among many civic problems faced by city dwellers inadequate drinking water-supply, most of which comes from groundwater, is one of the critical issues that draws periodic attention, particularly during the dry season (March to May) when so-called "load shedding" of electricity across the city especially disrupts groundwater abstraction (withdrawal) from its deepest levels in the underlying aquifer.
For the rest of the year, when people move on forgetting their short-term pain and suffering to some degree, unfortunately, very little is done to combat this acute crisis of drinking water-supply and dwindling groundwater levels. Over the last few years, many research papers and newspaper articles reported the extent of this problem and proposed several mitigation options. The objective of this article is to remind us, for one more time, of the potential catastrophic consequences of rapidly declining groundwater levels in Dhaka city.
In 1963, Dhaka Water Supply and Sewerage Authority (DWASA) was established in order to provide better services to the city dwellers. Currently, there are some 546 water-supply pumps. DWASA supplies about 1.9 million cubic metres (MCM) of water a day against city's daily demand of 2.2 MCM. About 85% of the current demand is met of which only 15% comes from surface water treatment plants. This figure clearly shows the overwhelming dependence on groundwater supplies that come from underlying aquifers.
The total groundwater abstraction from licenced production wells operated by the DWASA and private (mainly industrial) operators is around 700 MCM per year. In addition to DWASA tubewells, there are more than 1000 privately managed deep tubewells that are primarily unlicenced and no abstraction data are available. Although the quantity of water abstracted by many unlicenced tubewells is not known but estimated to be significant since it meets the demands for areas that are not connected to the DWASA water-supply network.
What happens when the rate of groundwater abstraction is too high? If we look at the long-term groundwater-level hydrograph at a monitoring well in Mirpur area managed by Bangladesh Water Development Board (BWDB) we see a steady decline in water-level since the mid-1980s. Recently, groundwater level is declining at even greater (>2 m/year) rate. Groundwater level in 2007 was about 70 m below ground level (bgl), which was only a few meters below ground level during the 1970s.
Why is pumping too much groundwater in Dhaka city a problem? To answer these questions one has to understand the groundwater system sitting underneath the city and its adjacent areas. Most part of Dhaka city is located on the southern fringe of the Madhupur Tract composed of red-brown clay. Underlying the clay, the sand body belongs to a geological unit called the "Dupi Tila Formation" which forms regional alluvial aquifers not only for Dhaka city but for most parts of north-central Bangladesh. Groundwater has been abstracted for urban and industrial water supplies (mostly in Dhaka, Narayanganj and Gazipur areas), and irrigation purposes (rest of the north-central region) from the Dupi Tila aquifers for last few decades.
As a result, groundwater levels started to fall at variable rates in north-central parts of Bangladesh. Most rapid and higher rates of decline occur over Dhaka city and adjacent areas where groundwater abstraction is the highest in the country. These aquifers are replenished (known as recharge) each year during the monsoon season when rain and flood water finds its way into the aquifer slowly percolating down through overlying soils and sediments. The rate of recharge varies depending on the property of soil and geology of the area. Unfortunately, the recharge rate over Dhaka city is much slower than that of the adjacent floodplain areas.
Let us now look at the water balance in and around Dhaka city and see if we are using more water than the rate of annual groundwater recharge. Estimation of actual groundwater recharge to the aquifer is a difficult task, which requires a lot of information regarding aquifer and soil property, groundwater pumping, rainfall, and evaporation. A recent study at the University College London, UK has estimated that the mean annual recharge for Dhaka city and its adjacent area is 300-350 MCM, which is much less than the annual abstraction of about 700 MCM.
Where does the remaining water supply come from? Abstraction of groundwater, therefore, includes actual recharge water plus some fraction of aquifer storage. In the recent years, the aquifer depletes its storage in order to meet increased demand for water supplies in the city and, consequently, groundwater levels are declining.
Where are the areas in Dhaka city with deep groundwater table? One can see the dry-season (April-May) groundwater table (2007), contoured from weekly monitored water-level in BWDB wells, on a satellite image (taken from Google Earth) which shows three distinct areas of deep water-table; such areas are located in Dhanmondi, Mirpur, Motijheel Thanas where dry-season water-table is located 50 m below ground level. Groundwater table in the central part of the city (Moghbazar, Tejgaon, Rampura, Mohakhali, and Gulshan) is between 40 and 50 mbgl. Groundwater table in Mohammadpur and around the northern boundary of the city (Uttara) is shallower than 30 mbgl.
Why is falling groundwater table a concern? There are many consequences associated with groundwater storage depletions. These include drying up of production wells, reduction of water in streams and lakes, deterioration of water quality, increased pumping and water supply costs, and land subsidence.
Rapid decline in groundwater levels means sands of the aquifer are dewatered leaving pore spaces to be filled mainly with air. Moreover, continuing decline in groundwater levels and increased urbanization can reduce the water content in overlying soils and sediments. Currently, water-dependent ecology within Dhaka city is primarily sustained by the water held in pore-spaces in soils and Madhupur clay. In a particularly prolonged dry year, these water-sustained ecosystems (lakes, trees, and parks) can be threatened to substantial damage and even partial extinction.
One of the major threats to the city due to declining groundwater levels is land subsidence, which can be triggered by earthquakes of greater magnitudes. Recently, a series of earthquakes of magnitudes ranging from 4 to 5.2 on the Richter scale jolted Dhaka and other parts of the country. The epicentre of the earthquake was estimated to be approximately 45 km southeast of Dhaka city. Although no damage to the infrastructure of the city was reported but there is a great potential of collapse of infrastructures and land subsidence associated with earthquakes, particularly in areas of greater groundwater-storage depletion. Such phenomena were observed in many parts around the world such as Bangkok City (Thailand), Mexico City (Mexico), several places in California State, USA and so forth.
What can we do to improve subsurface groundwater conditions? Many mitigation options have been proposed by university researchers and engineers over the last decade and some are underway which include (i) proper maintenance of existing surface water treatment plants and construction of more such water-supply system, (ii) construction of groundwater well field (analogous to gas field) outside of Dhaka city where groundwater storage is not falling (one such well field has been recognised in Singair Upazila of Manikganj district), (iii) rejuvenation of canals and wetlands in and around Dhaka city to enhance groundwater recharge to aquifers, and (iv) artificial recharge to aquifers through a number of measures of which relatively cheap and most popular methods are rainwater harvest, dug well and borehole recharge via natural or injection, basin or pond recharge, and storage reservoirs.
Recently, Aquifer Storage and Recovery (ASR) method is gaining acceptance worldwide as a water-storage technology. ASR essentially involves injecting water (such as rainwater) into an aquifer through wells during the rainy season and then pumping it out during the dry season. This method has widely been used in many parts of the USA, Australia, and Europe. However, a clear knowledge of water quality with a better understanding of sustainable treatment processes in aquifers is necessary to enable water utilities to take full advantage of this technology.
Currently, research on artificial groundwater recharge is being conducted at the Department of Geology, University of Dhaka and the team has already identified potential sites in Dhaka city for artificial recharge.
Above all, public awareness campaigns on this matter should be undertaken to improve the current condition. Involvement of private sectors alongside government initiatives is critical to alleviate the current situation and build a sustainable water-supply system for Dhaka city.