Groundwater Modelling of Dhaka City and Surrounding Areas and Evaluation of the Effect of Artificial Recharge to Aquifers

Shireen Akhter, Md. Sarwar Hossain

Dhaka city lies on the Madhupur Clay overlying the Plio-Pliestocene Dupi Tila Formation which forms the principal aquifer. Massive withdrawal of water from this aquifer caused fall of water table at an alarming rate which might provoke subsidence, ecological and environmental hazards [1]. It has been found that the aquifer system of the study area can be divided in to four aquifers and four aquitards up to a depth of 400m. The long term hydrographs for the observation wells within the Dhaka City shows a sharp decline of water level with little or even no fluctuation which indicates over exploitation of aquifers [2]. The water table contour maps of wet and dry season show a pointed cone of depression in the central part of the study area. The research work has been carried out to predict the future response of groundwater level after 20 years for increasing abstraction and assessment of the usability of the artificial recharge of aquifers of the Dhaka City using groundwater modelling technique. An eight layer transient groundwater flow model was set up with time steps of a month using MODFLOW. Two model scenarios were considered for model prediction. For the first scenario it has been found that after 20 years the elevation of groundwater level will decline to about -120m in the central part of the Dhaka City, and the mega cone of depression will spread over an area of about 1962 km2. For the second scenario substantial water is injected into the aquifer for recharging the aquifer artificially. The quantity of artificially recharged water was equal to the abstraction of water in Dhaka City. The model predicted that the elevation of the groundwater level after 20 years of pumping will not decline anymore and will be fixed at 70m which is the present minimum groundwater level. The area of the mega cone of depression around Dhaka city will reduce by 945km2 for the next 20 years. As a consequence of artificial recharge predicted environmental degradation in and around Dhaka City can be prevented.