Study on groundwater depletion and land subsidence in Dhaka city

Abstract

Groundwater is the primary source of safe water to meet the essential demands of people in Bangladesh. Dhaka the capital city, with its enOlmous population takes a huge toll on its groundwater resources. In order to meet this growing demand various organizations mainly DWASA and private users have already installed nearly 2000 high capacity DTWs. Continuous pumping with so many wells from an area less than 450 km2 has rendered the groundwater resources in stressed condition. This is likely to cause some adverse environmental impact like land subsidence. Many countries of the world have already experienced land subsidence of severe magnitude due to excessive groundwater pumping. Formation beneath the city of Dhaka is also susceptible to such phenomenon. This gives an early indication for an urgent need to alleviate pressure on the upper aquifers being exploited and explore for more suitable and sustainable sources to supplement the present water supply. DWASA is entrusted with supply of piped water to Dhaka Metropolitan City and its adjacent area. At present 75% city area is under DWASA water supply coverage, out of which 82% is from groundwater sources. In an attempt to find alternate sources DWASA has started exploiting the deep aquifer at a depth of around 300 m. But the source and recharge mechanism of this aquifer is yet unknown and can not support the development of .J a long tenn strategy for water supply of Dhaka City. From the previous studies, the development of Ground Water and Land Subsidence Model by EPC and MMP (1991) and its updating by BUET (2000) have been the most important and relevant to the present study. In the wake of recent development the previous model has become an inadequate tool to simulate the deep aquifer. So the current A, study was undertaken to setup a groundwater and land subsidence model that is flexible enough to simulate deep aquifer system and any other new changes. A versatile groundwater modeling tool called 'MODFLOW-2000 with Argus ONE' has been used for the current study.

MODFLOW-2000 uses the coupling of law of conservation of mass and Darcy's equation to simulate groundwater flow. The subsidence module is integrated into MODFLOW -2000 to compute the land subsidence using the groundwater model. The subsidence is computed basing on the Terzaghi's theory of one dimensional consolidation. The groundwater model under the current study covers an area of 8,778 km2 and includes the districts of Dhaka, Manikganj, Gazipur, Narshindi, Narayanganj and Munshiganj. The model area is discretized to 500 m grid inside Dhaka City. In the vertical direction the model is defined by 6 geologic layers. The model was calibrated by matching the hydro graphs within and outside the Dhaka Region. The calibrated model was then used to estimate the land subsidence in and around Dhaka City. The simulated hydrographs at important places of Dhaka City confirms the declining trend of groundwater level. The piezometric levels at Motijheel and Gulshan are found to be -52 and -43 m PWD respectively in December 2004. The corresponding subsidence as estimated by the model is maximum 4.9 cm. Results of model scenario runs indicate that the groundwater in the upper Dupi Tila aquifer would stabilize by April 2006 if no increase of abstraction is allowed after December 2004. Subsidence results for the same scenario shows that subsidence almost ceases with the stabilization of groundwater. Results of another scenario shows abstraction from deep aquifer with 94 wells in operation would deplete the groundwater by 20 m in 2020. But the situation in deep aquifer can only be confirmed after collecting observed data for a complete hydrological cycle. If the current situation prevails the condition in the upper aquifers will worsen and many existing wells will go inoperative. Recharge from rainfall has become negligible due to lowering of water level and increased urbanization. The reduced storage capacity of the aquifers may cause land subsidence of such magnitude that may affect life and property. All these facts suggest that a comprehensive policy must be undertaken to counter/prevent the possible adverse effects on groundwater resource of Dhaka City.