Changes in the Groundwater Regime of Dhaka City: A ...

Changes in the Groundwater Regime of Dhaka City: A Historical Perspective Kazi Matin Ahmed, Mohammad Saiful Islam and Sarmin Sultana

Department of Geology, University of Dhaka, Dhaka 1000

INTRODUCTION History of urbanisation in Dhaka city dates back to the 15th century and flourished as a Maslin Palli during the Mughal time. Dhaka has continued to increase in size with rapid growth in population and history of the city is divided into Pre-Mughal, Mughal, British, Pakistan and Bangladesh stages (Islam Khan, 2000). However, Dhaka expanded very rapidly since it became the capital of independent Bangladesh in 1971. Population increased from 2.2 million in 1975 to 10.2 million in 2000, 12.4 million in 2005 and projected population for 2015 is 16.8 million. Dhaka became the 15th largest megacity in terms of population size in 2000 and quickly rose to 11th place in 2005 with a population of 12.4. The projected population in 2015 is 16.8 and would become the 9th largest megacity by that time (UN, 2006). Urbanization has profound impacts on the hydrological cycle which include effects on surface water, such as increasing the magnitude of surface runoff and causing deterioration in river water quality (El Arabi, 1999; Foster, 2001; Jiao et al, 2008). Radical changes in groundwater recharge regime also occur due to increase in paved area (Lerner, 1990). Additional threats have been posed to urban groundwater systems by the possible impacts of climate change (Cromwell et al, 2007). Groundwater utilisation in the Indian subcontinent dates back to prehistoric time as evidenced by the presence of water wells in Harappa and Mohenjo-Daro. D’Souza (2006) notes the existence of dug well and step wells in the region long before the British role started. Availability of potable water is a fundamental requirement for any community to survive and flourish. Dhaka started its journey on the north bank of the Buriganga and today it has an extension of over 300 km2 (Figure 1). Initial water supply for the Dhaka dwellers came from surface water sources such as rivers, canals and ponds. Groundwater utilisation started at much later stage and eventually has become the prime source of potable water on the big population. There is very limited historical record of the groundwater conditions of Dhaka city for the first few hundred years. Organised data collection on aspects of groundwater started only in the later part of the last century. This paper attempts to present the recorded historical changes in groundwater regime of Dhaka city.

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India India

Bangladesh DHAKA

Bay of Bengal

Figure 1: Location map of Dhaka City. WATER SUPPLY FOR AN EXPANDING CITY The current population of Dhaka is about 15 million, and by 2015 this may have risen to more than 20 million people. The existence of a highly productive aquifer beneath Dhaka - the Dupi Tila sands aquifer - has to date enabled the staged development of a reliable and good quality groundwater-based water supply. Currently, 84% of the total supply is provided by groundwater where the rest (16%) comes from surface water sources. Demand for water is rising rapidly and groundwater abstraction is increasing annually (Figure 2). The long-term sustainability of groundwater abstraction in Dhaka has previously been called into question (Hasan et al 1998b). Intensive exploitation of the Dupi Tila aquifer has led to decline of water levels and modification of the recharge regime, with adverse impacts on groundwater quality (Ahmed et al 1999; Hoque et al, 2007) and quantity (Morris et al 2003). Dhaka has a long history as a regional capital due to its strategic position at the head of the Bay of Bengal. It is situated at a point on the River Buriganga where the slight elevation of the Madhupur Tract gives some protection against flooding. Four hundred years ago, in the pre-Mughal period, Dhaka was restricted to an area of 1.5 km2 at the junction of the Buriganga and Dholai Khal rivers. At the beginning of the 17th century it became the capital of the Mughal viceroys, and a centre for their expansion of trade. After the decline of Mughal power in the 18th century, came the period of the British Raj (1764-1947). In 1874 the Chadnighat Water Works were constructed on the River Buriganga to provide water for Dhaka, which became the capital of the East Bengal and Assam Province in 1905. At the time of partition of India, when Dhaka became the capital of East Pakistan, the city had expanded to cover an area of 40 km2. In 1941 2

the Department of Public Health Engineering (DPHE) drilled the first borehole in Dhaka, initiating groundwater development for public supply. By the 1960s, groundwater was the principal source for public supply. In 1963 the Dhaka Water Supply and Sewerage Authority (DWASA) was established (Haq 2006) and by 1966 groundwater abstraction had reached 30 Mm3 per year, with surface water providing less than 10 Mm3 per year. Water Demand

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Figure 2: Population, demand and supply of water in Dhaka city. The independence of Bangladesh in 1971 stimulated the development of groundwater (Ahmed et al 1999). At that time, the city had a population of less than two million and groundwater abstraction was less than 50 Mm3 per year. By 1992, 150 public supply (PS) boreholes, up to 180 m deep, managed by DWASA and distributed across the city, were yielding a total of 170 Mm3 per year. By 1998 this had risen to 190 PS boreholes and 200 private boreholes abstracting in total 310 Mm3 (Ahmed et al 1999), and by late 2000 DWASA was pumping approximately 400 Mm3 from a total of more than 330 boreholes (Morris et al 2003). Haq (2006) reported that abstraction was close to 455 Mm3 per year from 430 public supply boreholes across a service area of 350 km2 in 2006. Current abstraction is 1782 MLD from 533 boreholes which is 88% of the total supply as per MIS of October 2009. Apart from DWASA wells, there are 691 private wells of varying water capacity which also abstract from the same aquifers underneath the city (DWASA data accessed at www.dwasa.org.bd). GEOLOGY OF DHAKA CITY AREA Islam Khan (2000) reported that first urbanization in Dhaka started on the Old Natural Levee of the Buriganga during the Sultani or Pre-Mughal period (1299-1608). The same study also reported that suitable land elevation, favorable geological settings, hydrological characteristics played significant role for the expansion of the city towards north. Dhaka lies at the southern end of the Plio-Pleistocene Madhupur Tract and current Dhaka is bordered by the rivers Turag, Buriganga, Balu and Tongi Khal. Figure 3 shows the surface geology along with the historical development of the city. It is evident from the figure that surface geology played a role in early expansion of the city which was restricted mainly to the Madhupur Tract. Current developments mostly take place in the low-lying areas along the

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margins of the Tracts. The Madhupur Tract region is floored by the reddish brown Madhupur Clay of varying thickness. The low lying areas along the edges are covered by Holocene alluvial silt and clay and marshy clay. Pliocene Dupi Tila sands underlies the Madhupur Clay and is not exposed anywhere in the city.

Figure 3: Map showing geology and areal expansion of Dhaka city. HYDROGEOLOGY AND AQUIFER SYSTEM The Madhupur Clay overlies a sequence of fine to coarse-grained sands of the Dupi Tila Formation and termed as the Dupi Tila aquifers as shown in Figure 7 (DWASA and IWM, 2008). Fine-grained, silty sands at the top of the Dupi Tila Formation grade downwards into fine/medium-grained sands, and medium/coarse-grained sands with gravels towards the base. A discontinuous clay layer divides the Dupi Tila system into two aquifers: an upper fine-grained aquifer (the upper Dupi Tila aquifer 1), approximately 40–50 m thick, and a lower coarsegrained aquifer (the upper Dupi Tila aquifer 2), approximately 80 m thick. Recently, the basal clay layer has been penetrated by boreholes drilled for exploration beyond 200 m depth (Haq 2006). The basal clay is 10-15 m thick, and overlies a deeper sequence of sands which is more than 100 m thick, lithologically resembling the Dupi Tila Formation. This has been termed the ‘lower Dupi Tila aquifer’.

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HISTORICAL CHANGES OF GROUNDWATER REGIMES Historical Records of Water Level The earliest record of depth to water level in and around Dhaka was given by Taylor (1840). According to him water level was below 18 to 22 feet (6 to 7m) below the ground. He also reported the occurrences of springs (natural discharge of groundwater) in Madhupur and Palash, close to Dhaka city. Systematic recording of groundwater level started in Dhaka towards the end of 1960. Available data show that water level was very close to the ground surface even in 1980s. However, the water level of the Upper Dupi Tila aquifer started to fall due to high rate of abstraction and reduction of natural recharge due to urbanisation. Figures 4 and 5 present two long-term hydrograph for Dhaka city: one for Mirpur and the other one for Sutrapur. Both the well show very similar trend, i.e. continuous decline in water level since eighties. However, there is significant difference in the magnitude of decline. At Sutrapur water level lies about 15m below the datum plane whereas at Mirpur, the water level lies 60m below the datum. Both the hydrographs present water level of the Upper Dupi Tila aquifer. The water level of the Lower Dupi Tila Aquifer is being monitored for the last few years but the data was not available.

Figure 4: Historical trend of water level in Dhaka city as recorded at Mirpur Observation Well of BWDB.

Figure 5: Historical trend of water level in Dhaka city as recorded at Sutrapur Observation Well of BWDB.

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Figure 6: Dugwells of Dhaka: a) well-maintained and used one in Bashabo; b, c, d) abandoned or rarely used ones in old Dhaka Historical Records of Rainfall As rainfall is the main source of recharge to groundwater, historical rainfall data has been collected from the Bangladesh Meteorology Department (BMD). Data from 1953 to 2007 do not show any declining on increasing trend in annual total rainfall. Rainfall for the time varied from 1100mm to more than 3000mm and the long-term average is slightly over 2000 mm (Figure 7).

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Figure 7: Historical trend of rainfall in Dhaka city. Historical Changes in Sources of Water Supply and Types of Water Wells A comprehensive description of water supply scenario of Dhaka was given by Taylor (1840). According to Taylor most people in Dhaka used to drink water from jheel or morass. There were very few wells in the city. However, the European and wealthier natives used to bring water from Sitalakhya. Because of poor quality of water, water borne diseases were very common as described by Taylor (1840). Cholera epidemic was common in Dhaka and death in large numbers was common. Dugwells became relatively safer source of water. It is not evident when first dug well was sunk in Dhaka. However, there is reference to a dug well sunk by Guru Nanak who visited Dhaka in the 16th Century on his way from Dhibru (Assam) to Jaganath Puri (Orissa). There appeared to be scarcity of drinking water and during his stay, Guru Nanak had a well constructed. The water of the well was believed to possess miraculous power. Hindus, Muslims, Sikhs came to drink the wish fulfilling water of this well (Ralhan, 2002). Guru Nanak came to Dhaka by boat, which anchored at Shivpur village in the north of Dhaka. Later it took the name of Rayer Bazar and Dhanmandi. According to Dr. Trilochan Singh, an author, the erstwhile Pakistan Government requisitioned the entire area including the well. The government hacked the area into several small plots and leased them out to people for housing. It is learnt that Guru Nanak's well now lies at House No. 278, Road No. 26, Dhanmandi residential area. The allotee of this plot of land constructed a building there in 1968. The dugwells might have provided water for Dhaka dwellers for long time. Taylor (1840) mentioned about need for artesian wells in Dhaka to prevent cholera epidemic. However, the quality of water was not very good as evident from the report of the Civil Surgeon in 1869. The report stated that water of all dug wells of the city was contaminated; water from the rivers and canals was full of pathogens. There are mentions of dugwells even in the 1968 as a source of drinking water for Dhaka dwellers (Khan and Stockard, 1968). Dugwells stopped functioning in Dhaka due to decline in water level, availability of better water lifting technology and poor quality of water. Still today there are a few dugwells found in the city, some of them are being used. Some of the existing dugwells in Dhaka city have been shown in Figure 6. Most of the existing dugwells are found in the areas where there is a alluvial

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cover on top of the Madhupur Clay. One dugwell in Mirpur was used by the Pakistani occupation forces/collaborators in 1971 to dump dead bodies of killed Bengalis which is now known as the Jallad Khana well. Hand tubewell technology was developed in Europe in 19th Century and might have been introduced to this part of the world in later dates. It was not possible to find any record when first hand tube well was installed in Dhaka. Hand tube wells have been a major course of water due to availability of good quality water at shallow depth just underneath the Madhupur Clay. Hand tube well only works when the water level is within 6m from the ground surface. Initial conditions in Dhaka were ideal for hand tube wells as water existed under piezometric pressure. When large scale water abstraction started from the Upper Dupi Tila 1 aquifer, piezometric surface started to decline and gradually went below the suction limit of the hand pumps. There is rarely any hand tube wells today operational on the Madhupr Clay covered areas. However, there are a few hand wells still operating in the areas close the peripheral rivers. Treated water supply began in Dhaka, partly, for the first time in 1878 based on treated water from the river Buriganga. The Chadnighat water treatment plant, providing treated water till to date, was the first installation for municipal water supply in Dhaka. It is likely that Dhaka Municipality also installed hand tube wells to provide water for the city dwellers. First municipal water supply well was installed in Dhaka in 1940 at Narinda with the capacity to supply 11,000 gallons water per hour (Khanum, 1991). Since then, use of groundwater continued to increase as a source of water supply. Wells drilled at the Upper Dupi Tila Aquifer 1 has been the source of water supply for some time. Due to large scale abstraction for decades, a considerable part of the aquifer has become dewatered (Hoque et al, 2007) as many of the WASA’s production wells went dry. Later new generation of wells were drilled in the Upper Dupi Tila Aquifer 2 which has been providing water since 1980s. Due to rapid decline in the water level, wells installed in this aquifer are being abandoned as well. As there is very little natural recharge to the Upper Dupi Tila aquifer, the pumping has exceeded the safe yield and water is being mined. The deepest water supply well installation started at the end of last century which abstract water from the Lower Dupi Tila aquifer. These wells, drilled at a depth of more than 300m, are likely to continue supply water for next decades. Water in this aquifer still exists under piezometric pressure and level rises well above the aquifer. However, there are very limited knowledge about the recharge mechanism to this aquifer and amount of water that can be withdrawn sustainably. This aquifer is considered to be very important of Dhaka City water supply in the coming years. If this one is not properly managed, even deeper wells may have to be drilled which will become very expensive. Quality may also become a constraint for water from the aquifer deeper than 300m. The Figure 8 illustrates the evolution of wells in Dhaka city with approximate time lines based on available literature and information.

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Figure 8: Historical changes in type and depth of water wells in Dhaka city Historical Changes in Surface Water Bodies Though Dhaka was once famous for its river courses and other surface water bodies, most of those surface water sources have been lost due to impacts of urbanization. Water bodies have been filled and converted into housing blocks, roads, etc. These conversions bring in significant changes in the hydrologic cycle. As paved area increases, natural recharge to the underlying aquifers decreases and surface runoff increases. Islam Khan (2000) listed the following impacts of loss of surface water bodies: - Reduced infiltration/sub-surface flow; - Reduced ground water recharge; - Changes in the flow regime of the khals; - Increased surface runoff. Figure 9 illustrates the impacts of urbanization on natural recharge.

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Figure 9: Impact of urbanisation on natural groundwater recharge and surface runoff. Current groundwater flow regime The natural pattern of groundwater flow at the southern end of the Madhupur Tract has been much disturbed by intensive groundwater development for the water supply of Dhaka since the early 1970s. Large-scale groundwater abstraction has resulted in an extensive cone of depression centred on the city, over large parts of which the aquifer has become unconfined. Drawdown has increased by approximately 0.75 m per year since the late 1970s at the Motijheel observation well, and over the last 30 years the cone of depression in the aquifer has developed to such stage that it now extends to the major river courses that delineate the southern portion of the Tract (Ahmed et al 1999). Rate of decline has increased substantially over the recent years and a decline rate of more than 2m/year has been recorded in Mirpur observation well. Figure 10 presents the depth distribution of groundwater in and around the Dhaka city based on monitoring data from BWDB. It is evident from the map that depth to water level reached to more than 60m in some locations inside the city (Figure 10).

Figure 10: Depth to groundwater level in and around Dhaka city. 10

GROUNDWATER QUALITY There is no record of groundwater quality from the historical past and it is not possible to decipher what changes have taken place in the quality of Dhaka city groundwater through time. Groundwater quality data is available from 1970 onwards. Limited monitoring data of BWDB show that concentration of Chloride and Nitrate has increased over the time of record (Ahmed et al, 1998). Various surveys demonstrated increase in electrical conductivity (EC) of groundwater of the upper Dupi Tila aquifer (Ahmed et al, 2010). There is marked difference between water quality of the upper Dupi Tila and lower Dupi Tila aquifer. Sultana (2009a) identified that the upper Dupi Tila groundwater chemistry has been altered by anthropogenic processes whereas the lower Dupi Tila aquifer is unaffected by the surface water processes. In areas like Hazaribag, water quality is relatively poor compared to other areas (Zahid et al, 2006). Groundwater in the vicinity of the Buriganga is influenced by the leakage of contaminated surface water (Darling et al., 2000). However, by and large, quality of Dhaka groundwater is within acceptable limit for all parameters including arsenic. CONCLUSIONS As Dhaka continues to expand, the Dupi Tila aquifer beneath the city will eventually be unable to meet the rising demand for water (Haq 2006). Protection of the aquifer for use as a strategic supply of high quality water is an important option for water managers, an alternative to continuing water level decline (Morris et al., 2003) and unabated deterioration of groundwater quality. The borehole network for monitoring groundwater levels should be expanded to cover the entire area within the Rivers Buriganga, Balu, Turag and Tongi Khal. Shallow water level decline in multiple aquifer systems can be used to indicate over-abstraction at an early stage (Rushton 1998; Hasan et al 1998a). Therefore the water level monitoring boreholes should be completed to monitor water levels in both the upper and the lower aquifers. As demand for water would continue to grow, alternative sources have to be explored. It is possible to bring water to the country by developing well fields outside the zone influenced by the huge cone of depression. It is also possible to augment the declining water resources of Dhaka city by artificial recharge as there are good potentials for this option (Sultana, 2009b). If the current practices of producing more water from the Dupi Tial aquifers continue, adverse environmental impacts such as lowering of water table, degradation of water quality and risk for land subsidence would continue to increase. It is extremely important to undertake proper groundwater management strategy for Dhaka city to ensure sustainable water supply for the future generations. REFERENCES Ahmed, K M, Sultana, S, Hasan, M A, Bhattacharya, P, Hasan, M K, Burgess W G & Hoque, M A, 2010. Groundwater Quality Contrasts between Upper and Lower Dupi Tila Aquifers in Megacity Dhaka, Bangladesh. GQ10: Groundwater Quality Management in a Rapidly Changing World (Proc. 7th International Groundwater Quality Conference held in Zurich, Switzerland, 13–18 June 2010). Ahmed, K.M., Hasan, M.K., Burgess, W.G., Dottridge, J., Ravenscroft, P. and van Wonderen, J. 1999. The Dupi Tila aquifer of Dhaka, Bangladesh: hydraulic and hydrochemical response to

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