Declining groundwater level and aquifer dewatering in Dhaka metropolitan area, Bangladesh: causes and quantification
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- Hoque, M.A., Hoque, M.M. & Ahmed, K.M. Hydrogeol J (2007) 15: 1523. doi:10.1007/s10040-007-0226-5
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Large abstraction by water-wells has been causing a linear to exponential drop in groundwater level and substantial aquifer dewatering in Dhaka, Bangladesh. The city is almost entirely dependent on groundwater, which occurs beneath the area in an unconsolidated Plio-Pleistocene sandy aquifer. Analysis shows that the pattern of water-level change largely replicates the patterns of change in the rate of groundwater abstraction. Contribution of the aquifer storage to the abstraction is estimated to be more than 15% in the year 2002. This abstraction has caused a sharp drop in water level throughout the city and turned into two cones of depression in the water level. Upper parts of the aquifer are already dewatered throughout the area, with the exception of part of the northeast and southeast corner of the city. It is calculated that about 41 million cubic metres (MCM) of the aquifer dewatered by the year 1988, which increased to 2,272 MCM in the year 2002. Water-level decline may increase non-linearly due to limiting vertical recharge in areas where the aquifer is dewatered and may severely threaten the sustainability of the aquifer.
KeywordsOver-abstraction Groundwater level Aquifer dewatering Dhaka metropolitan Bangladesh
Des prélèvements massifs dans les puits ont causé des chutes de niveau piézométriques linéaires à exponentielles et un dénoyage substantiel de l’aquifère à Dhaka, au Bangladesh. La ville est presque intégralement dépendante des eaux souterraines, contenues sous ce secteur dans un aquifère de sables non consolidés plio-pléistocènes. Les analyses démontrent que l’allure des variations de niveau reproduit celle générée par des prélèvements en eau souterraine. La contribution des réserves propres de l’aquifère aux prélèvements est estimée à plus de 15% sur l’année 2002. Le prélèvement considéré a occasionné une chute de niveau à l’échelle de la ville, et est marqué par deux cônes de dépression majeurs. Les franges supérieures de l’aquifère sont déjà dénoyées sur le secteur, à l’exception des extrémités nord-est et sud-est de la ville. Selon les calculs, environ 41 millions de mètres cubes (MCM) de l’aquifère ont été dénoyés en 1988, cette quantité augmentant ensuite jusqu’à 2,272 MCM en 2002. La baisse du niveau piézométrique peut s’amplifier de manière non-linéaire sur les secteurs dénoyés, du fait d’une réalimentation limitée, menaçant sérieusement la pérennité de l’aquifère.
La extracción fuerte mediante pozos de agua ha causado un descenso lineal a exponencial en el nivel de agua subterránea y un drenaje significativo del acuífero en Dhaka, Bangladesh. La ciudad depende casi completamente de agua subterránea la cual ocurre debajo del área en un acuífero arenoso no consolidado del Plio-Pleistoceno. Los análisis muestran que el patrón de cambio del nivel del agua replica en gran parte los patrones de cambio en la tasa de abstracción de agua subterránea. La contribución del almacenamiento del acuífero a la abstracción se estima en más del 15% en el año 2002. Esta abstracción ha causado un descenso brusco en el nivel del agua en toda la ciudad habiéndose transformado en dos conos de depresión en el nivel del agua. Las partes superiores del acuífero se encuentran drenadas en toda el área, con la excepción de la porción nororiental y la esquina suroriental de la ciudad. Se calcula que aproximadamente 41 millones de metros cúbicos (MMC) del acuífero se habían drenado para el año 1988 lo cual aumentó a 2,272 MMC en el año 2002. El descenso del nivel del agua puede incrementar de modo no lineal debido a recarga vertical limitada en áreas donde el acuífero es drenado y puede amenazar severamente la sostenibilidad del acuífero.
Like many natural resources, groundwater is being exploited at an increasing rate all over the world. Groundwater is generally preferred as a source of potable 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. A lack of proper understanding of the groundwater system, in terms of resource utilization, is one of the major limitations to the effective management of groundwater resources.
Dhaka is located in the central part of Bangladesh on the River Buriganga. It covers an area greater than 250 km2 and is home to more than 10 million people. It is one of the fastest growing megacities in the world and the population of the city is rapidly increasing. 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.
Dhaka is dependent primarily on groundwater for the urban water supply; about 84% of the present municipal water supply comes from groundwater and 16% is from surface water (WASA 2003). The dependence on groundwater for domestic, industrial, and commercial water supply in the city area was more than 95% prior to the commissioning of a large surface water treatment plant (Sayedabad Surface Water Treatment Plant) in 2002. Systematic groundwater development began in the city of Dhaka in 1949 and available records show that groundwater abstraction in the city has increased drastically in recent decades (Ahmed et al. 1999). Water tapped from the Dupi Tila aquifer in the city of Dhaka is free from arsenic, the main constraint on groundwater use for drinking purposes in the southern part of the country (e.g. BGS/DPHE 2001; Ahmed et al. 2004). To meet the ever-increasing demand of the city dwellers, DWASA (Dhaka Water Supply and Sewerage Authority) has undertaken a program for abstraction of more groundwater from the aquifer beneath the city. As of 2003, DWASA produced 1,160 million litres of groundwater per day through 389 DTW (Deep Tube Wells; WASA 2003). The number of private boreholes has also increased substantially to 970 wells (WASA 2000), and abstraction through these wells remains unquantified but is likely to be significant. Though the city is largely dependent on groundwater, currently there is no management plan in place, and large-scale uncontrolled groundwater development is occurring. Lack of proper understanding of the groundwater system is one of the major constraints on sustainable development of groundwater in the megacity. The objectives of this work are to correlate the trend of groundwater level decline and aquifer dewatering with over-abstraction, and to quantify the abstraction in relation to recharge and aquifer storage.
Stratigraphy of the city of Dhaka region (Modified after Morris et al. 2003)
Function in aquifer system
The flood plain Area
Alluvial silt, sand and clay
Late Pleistocene to Holocene
The Madhupur Tract Area
Swamp, levee, and riverbed sediments
Madhupur Clay Formation
Silty clay member, Fluvio-deltaic sand
Dupi Tila Formation
Dupi Tila clay stones Fluvio–deltaic sands
Known lower aquitard
Methods and materials
The present work assembles data on groundwater abstraction and groundwater level/piezometric head from DWASA and BWDB (Bangladesh Water Development Board), respectively. Some borehole lithologs and geophysical resistivity logs were also collected from the same sources.
DWASA serves the city dwellers through seven distribution zones, six of which are within the city limits (Fig. 3). Zone-wise yearly abstraction data for DWASA and private production wells for the year 1983–2003 have been incorporated in this study to unveil the reason behind the nature and variability of the piezometric surface with time. A small part of the abstraction data for DWASA production wells and some of the data for the private wells are estimated. DWASA abstraction data on individual zones for the years 1990–1992 are missing, but total abstraction data for those years are available. Abstraction for the individual zone was estimated as a percentage of the total abstraction and trend in the data was also considered. Private abstraction data are missing for the years 1983–1990 and 2001–2003, but the total abstraction has been estimated by assuming that private abstraction during the missing years is 0.36 of DWASA abstraction. This ratio is obtained from the average of the available private abstraction data in comparison to the DWASA abstraction.
Water level and piezometric analysis
Information on the Bangladesh Water Development Board (BWDB) water level monitoring bores (extracted from the BWDB electronic database)
Well ID (old)
Well ID (new)
Geographic area (thana)
Screen depth (m)
Height of measuring level from PWD (m)
There are some ambiguities in the BWDB data with regard to the reference measurement point and water-table information for observation wells DH124 and DH112. In both of these stations, the water level is ‘measured’ below the well depth, which is impossible. In the case of DH112, the height of measuring level for the later/replacement setup was used and depth of the screen assumed to be below the water table. It was difficult to sensibly estimate an elevation for the measuring point for DH124. Due to this difficulty, data for DH124 station for 2001 onward were not used in the long-term hydrograph. It is assumed that the information in the database is not updated for the replacement setup. Analysis of the data from those stations with a height of measurement level from original well setups shows a large shift which may be due to a change in elevation in the measuring point. The water table at DH124 shows more than 10 m difference (e.g. 41.38 m on 10 December 2001 and 51.80 m on 01 July 2002) within 6 months, and it fell below the well depth mentioned in the database. There is also a data gap for the period of December 2001 to June 2002, indicating some sort of adjustment-time for the new set up as because the water table dropped below the depth of the well; however the information regarding the new set up is not in the database. The authors tried without success via personal communication to collect the missing database information. This big difference in the measurements might have appeared due to a different (localized) hydraulic regime at the new depth or interference by production well(s) nearby. The maximum drop in the water table (53.75 m below the ground surface) was recorded at DH124 station in 2003.
Long-term hydrographs of water-level elevation with reference to the PWD datum (1985 to 2003) were prepared for individual observation stations. Data for the last week of December for the respective years of all 14 stations were used to construct water-level elevation-contour surfaces for the years 1988, 1993, 1998 and 2002. Seasonal fluctuations were absent in the data and did not influence overall water level for any given period within the year. Therefore, December data were arbitrarily chosen as the data for yearly contouring. Missing data were estimated with respect to earlier and later records. It is assumed that the observation wells are recording the water level of the same aquifer throughout the area. Contouring was based on data from observation wells from the Dhaka metropolitan and neighbouring region using Surfer 8.4 (surface mapping and computing software) (Golden Software, Golden, Colorado, USA). Kriging geo-statistical method (e.g. Cressie 1990) with a linear variogram model was used in each case of grid data generation.
Dewatered volume calculation
The base of the top clay is taken as the upper piezometric surface and the piezometric surface of 1988, 1993, 1998 and 2002 is taken as the lower surface for dewatered volume calculation for the respective years. The elevation of the piezometric surface was within the clay before groundwater development (e.g. Morris et al. 2003), indicating the initial confined nature of the aquifer. It is anticipated that the bottom of the clay, as the top piezometric surface, is a conservative estimate for the dewatered volume derivation because the water table was actually within the clay. In each surface grid construction, a planar earth projection UTM (universal transverse mercator) in metres is used to obtain the volume in cubic metres.
Quantification of groundwater abstraction in terms of contribution from recharge and storage
Total abstraction (MCM)
Yearly average abstraction (MCM)
Dewatered volume of aquifer (MCM)
Yearly average dewatered volume of aquifer (MCM)
Dewatered pore-volume (MCM) (for porosity 15%)
Yearly average dewatered pore-volume (MCM)
Yearly average abstraction from recharge (MCM)
Yearly average abstraction from recharge (%)
Yearly average abstraction from storage (%)
Piezometric surface construction
Discussion and conclusions
Groundwater level drop is a city-wide phenomenon in the hydrogeological regime. The rate of decline has reached 2.5 m/year in recent years in the main part of the city. This scenario follows the increasing rate of groundwater withdrawal over the city. The area has been experiencing massive abstraction of groundwater for the last three decades and the density of the wells is increasing with time. The water level in the aquifer is declining because withdrawals are exceeding recharge. Large and prolonged abstraction has modified groundwater flow directions by reversing hydraulic gradients towards the city centre producing cones of depression in the piezometric surface around large pumping centres (Fig. 7). The cones of depression are lowering vertically and widening horizontally owing to the increased installation of production wells and to the spread of the production wells into new areas. The positions of the piezometric depressions are largely ruled by the concentrated pumping locations of DWASA. 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 piezometric surfaces and stationary water-level results indicate that the metropolitan area is hydraulically separated from the surrounding region. This standalone hydraulic nature may be controlled by the hydrostructural features and stratigraphic settings. Stratigraphically, the low transmissivity Dupi Tila aquifer may always remain in semi-isolation from the surrounding high transmissivity flood plain aquifer. On the other hand, fault controlled rivers around the metropolis would have a link with the aquifer in the region to establish a more localized hydraulic system in Dhaka by surface water to groundwater interaction. A separate model study for the area shows that connection of the surrounding rivers with the aquifer can generate such a restricted groundwater flow system with existing pumping set up (MA Hoque, Bangladesh University of Engineering and Technology, unpublished data, 2006). If the city aquifer is isolated from the outside aquifer, water cannot be drawn laterally and recharge to the Dupi Tila aquifer can only occur from the surface or from surrounding rivers. This might have been significantly accelerating the drawdown in the city and causing more induced recharge. Due to the lack of site characterization, it remains uncertain whether the water-level data used for outside the city represent the same aquifer as the one inside the city (Dupi Tila aquifer), or whether the outside aquifer, in connection with the Dupi Tila aquifer, is underlain by the aquifer represented by the data.
Rivers around Dhaka appear to be recharging the aquifer to a new hydrodynamic equilibrium in response to pumping indicated by the water-level contours and associated flow pattern. The flow pattern in the shallower horizon would primarily be from the surrounding rivers towards the cones of depression, and water in the deeper horizon may be slow-moving and very old. Oxygen (O) and Hydrogen (H) stable isotope studies (Darling et al. 2002) confirm the large-scale leakage of river water in the upper part of the aquifer beneath the city of Dhaka and 3H/3He isotope data demonstrate that the water at 75 m in the area between Motijheel and south of Dhanmondi is younger than 20 years (Darling et al. 2002), indicating that water from this depth has recently been recharged. Another earlier study (Hasan 1999) used temporal patterns in EC (electrical conductivity) distribution to prove that pumping has induced recharge in the southern part of the aquifer from the polluted Buriganga River. These earlier studies also support the water-level contours that show flow directions towards the city centre from the surrounding rivers (Fig. 7).
The rate of drawdown is increasing with abstraction even though the aquifer is recharged from the surrounding river. Abstracted water is usually supplied by more water entering the groundwater system (increased recharge), less water leaving the system (decreased discharge), and removal of the water from the storage, or some combination of these (Alley et al. 1999). In the case of the Dhaka groundwater system, abstraction water is coming from all three sources as indicated by the hydraulic gradients towards the city centre, facilitating enhanced recharge and decreased discharge and dewatering of the aquifer due to contribution of the aquifer storage. BWDB (1991) quantify the groundwater withdrawal in terms of recharge: 94% of the withdrawal is dependent on recharge (vertical and horizontal recharge and decreased discharge), and the remaining 6% of the withdrawal comes from storage, causing permanent decline in the water level. The present study found 0.61 and 1.86% of the abstraction to be derived from the aquifer storage in the years 1988 and 1993, respectively. The proportion of average yearly abstraction from recharge decreases in favour of the proportion from the storage. The contribution of the storage is increasing with time and reached 15.68% of abstraction in the year 2002, which is most alarming and causing substantial dewatering of the aquifer. Dewatering is attaining a maximum vertical recharge rate that is proportional to the water-level difference between the water-table elevation in the top clay layer and the elevation of the bottom of the top clay layer in the city of Dhaka. The abstraction from storage will increase non-linearly where dewatering of the aquifer occurs, as the vertical recharge rate reaches its maximum.
This study concludes that the pattern of water-level change in the city largely follows the rate and spread of groundwater abstraction. This increasing nature of the groundwater abstraction could restrict the sustainability of the hydrogeologic regime because the contribution of aquifer-storage is non-linearly increasing, vertical recharge has reached its maximum in most parts of the city, and pumping induced recharge from rivers may, in turn, pollute the aquifer.
The research reported here represents part of a postgraduate diploma (2004) research work by the first author at the Institute of Water and Flood Management, BUET. The first author is very grateful to Dr. H. A. Michael for her encouraging review, critical comments and suggestions on the manuscript. We also thank Mr. S. Cook for his comments on the manuscript. The authors are very grateful to the anonymous reviewers for their suggestions in improving the quality of the manuscript.