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.
