Potential and challenges of managed aquifer recharge in an over exploited aquifer of Dhaka city

Abstract

The present study analyzed the role of Managed aquifer recharge (MAR) on the improvement of water supply condition in Dhaka, the capital of Bangladesh. Dhaka is one of the most challenging megacities with regards to its sustainable water resources management, in particular drinking water supply. Population growth would create additional drinking water demand in the near future (year 2025). The population of Dhaka City is presently about 14 million and according to present trends in population growth, that number will most likely increase to 22 million by the year 2025. According to previous studies, due to over-exploitation of the regional aquifer system the current groundwater resources trend is non-sustainable. It results in very fast decrease in groundwater levels of about 2 to 3 m/y. New water resources management strategies are needed to confirm drinking water supply and sustainable groundwater development (i.e., halt of groundwater decline). MAR would help to restore groundwater resources in Dhaka city by using, for example, collected rainwater. This thesis briefly explores the potential, viability, and challenges with respect to the implementation of Managed Aquifer Recharge (MAR) as a contribution to sustainable water resources development in Dhaka City. Rainwater harvesting together with water capturing from the open spaces can meet up to 20%-30% of the present water supply demand in Dhaka City. Though the pheripheral rivers are polluted, nearby big rivers (such as Meghna) can be a source of water during the monsoon. The estimated volume of storage for the upper Dupitila aquifer is about 1120 Mm3. Hydraulic conductivities of the Dhaka City aquifer would allow for the dispersion of recharged water with low costs of recovery, making MAR viable. Lithologs and 3D block diagrams reveal that the top most clay layer ranges between 8 and 52 m in most places. Considering the top impermeable layer thickness (TIL) and land cover classification, four primary MAR techniques have been suggested: (1) infiltration basin (TIL thickness: 0-10 m), (2) cascade type recharge trench/pit (TIL thickness: 10-32 m), (3) Aquifer storage, transfer and recovery, ASTR (32-52 m), and (4) use of natural wetlands to recharge the water collected from open spaces. The regional groundwater flow direction, from North-West and North-East towards Dhaka City, may allow the use of the aquifer as a treatment facility and transport medium for groundwater development, if spreading basins are installed in the greater Dhaka City area. Preliminary hydrogeochemical investigations reveal that in some places groundwater is already polluted by industrial waste. Therefore, a comprehensive geochemical model is required to identify potential geochemical processes related to the infiltration or injection of storm water. Nevertheless, the preliminary evaluation of the potential of MAR implementation in this region, which is based on available conventional and nonconventional water resources, aquifer characteristics, and applicability of MAR technologies as well as water treatment requirements, shows that MAR is viable and can play a key role in sustaining water resource development amidst increasing pressures on the current water resources of Dhaka City, Bangladesh.