Author: Shovon Barua
Dhaka, the capital and central city of Bangladesh, with a population of over 15 million, suffers from both water scarcity and water pollution. Dhaka city is dependent mainly on groundwater resources from fluvio-deltaic Dupi Tila aquifer of Pliocene age. The water productivity is about 2180 MLD (million liter per day) against the current demand for 2250 MLD that is withdrawn by about 622 DWASA tube wells with 3040 km long pipeline network. City of Dhaka immediately requires alternative water resources. This study was conducted to investigate the water quality of existing dug wells to determine the potential of dug wells to use as alternative source of safe water.
Geology of Dhaka City
Physiographically Dhaka city is situated in the southern half of the Madhupur Tract and Floodplain area with southern river system. Regional elevation of the area gradually declines towards Buriganga River on the south and the elevation ranges from 10m to 17m but is generally around 14m above mean sea level. Geologically Dhaka city belongs to Bengal Foredeep and is situated in the Pleistocene uplifted block (Madhupur Tract) within the passive margin surrounded by subsiding floodplains bounded on the west by a series of NW-SE trending en-echelon faults including the Dhamrai, Maijail and Kaliakoir ones. Land surface is covered by gray floodplain and non-Cretaceous floodplain soils. Stratigraphically, Old Dhaka is characterized by hundreds of meters thick unconsolidated sequence of fluvio-deltaic deposits many composed of gravels, sands, silts and clays of Plio-Pleistocene age.
There were as many as 120-150 Dug wells in Old Dhaka during the British tenure. Now-a-days the figure is reduced to 50-60. Among them thirty six dug wells water samples were collected randomly and four river water samples were collected from four different boat terminals in Buriganga River during both dry and wet periods. Analytical results of samples have been evaluated and compared with various drinking water quality parameters following guidelines of World Health Organization (WHO, 2011), Department of Environment, Bangladesh (DOE, 1997) and United States Environmental Protection Agency (USEPA, 2011).
The hydrochemical classification shows that both dug wells and river water samples are Ca2+-HCO3– type indicating that the dug wells receive water from the adjacent Buriganga River. Above 50% dug well water samples exceed the DW standard of Ca2+, K+ and Fe2+ ions and below 25% Buriganga river water samples are within the acceptable limits during both dry and wet periods. Calcium carbonate and bleaching powder are used to treat water from dug wells for domestic purposes. Plastering is employed to avoid the caving in and also to keep the dug well wall dry and algae-free. It has not been deciphered whether the dug wells are being recharged from surface water flowing directly into the well or groundwater that has recharged via riverbank filtration. It is urgent to ensure the proper maintenance of these historical dug wells by the concerned authorities as well as to mitigate water scarcity problem using dug well as alternative water resource.