Evaluation of ground water abstraction for water supply in Dhaka city

Abstract

OWASA is responsible for supplying potable water for the city dwellers. At present total water requirement in the Dhaka city is about 1620 MLD (million liter per day) but OWASA is only supplying 828.09 MLD, 94% of this water comes form underground source and the rest 6% from two small surface water treatment plants. Ground water which is the principal source of drinking water supply, has depleted considerably. The rate of lowering of piezometric water table was 0.5m per year in the eighties which increased to 0.75m in early nineties. In some locations such as Gopibag, Gulbag the present rate of water level lowering has been found to be more than 2 m per year. Comparing to the present situation, it is clear that Dhaka city ground water level has been depleting very rapidly. The situation should not go any long. Moreover the Government is actively considering the fourth interim project of OWASA and it will provide an additional 121 new and replacement deep tubewells to its system. Obviously then the situation would be further deteriorated. 'Dhaka Regional Ground water and subsidence study' conducted by OWASA in 1991 suggested a safe limit of maximum allowable depth of water level which was 20-25m for the central part of city. The study also predicted a maximum water level of 24 m below ground surface in Motijheel area in 2010. But some of the DTWs in DWASA indicate a static water level greater than 40m bellow ground level. As OWASA has no regular groundwater level monitoring system, the overall condition of the city groundwater level cannot be assessed exactly at this time. However BWDB.piezometers provide adequate data on piezometric levels in the upper aquifer, but additional piezometers are required in the main aquifer. No doubt the situation of groundwater abstraction is very much alarming for thc city of Dhaka. It may result in the failure of some of the existing pumping wells to collect water in near future and increase in the effective pressure on underlying soil layer by about 37.7%. The incremental weight may cause a settlement of 4.9cm. which would be uniform in nature because of the presence of a thick consolidated clay layer at the surface beneath Dhaka City. The high cohesion of this clay layer is likely to prevent any differential settlement which could be detrimental to the growth of the city. Vegetation is not likely to be affected by the lowering of water table as the water table in the upper aquifer is not very much affected by abstraction of water from the deep aquifer and the intermediate vadose zone contributes water to the root zone.