Groundwater resource evaluation of Dhaka city using modflow

Abstract

Dhaka Water Supply and Sewerage Authority (DWASA) is traditionally dependent on groundwater for drinking water supply. In addition, most of the industries and big user groups are utilizing groundwater by sinking private tubewells. The Dupi Tila aquifer, lying underneath the Madhupur clay has been providing major portion of the good quality drinking water in Dhaka City. A surface water treatment plant at Saidabad is now operating in order to balance the required demand but still the supply is far lagging behind the demand. The rechargeable surface area is decreasing day by day due to construction of buildings, roads and concrete pavements in unplanned urbanization. Besides, a thick compact upper clay layer in the subsurface is also responsible for reduced vertical recharge. Various models have been used for the evaluation of groundwater resource in Dhaka City but the current interest is to use one of the most popular groundwater flow simulation model, MODFLOW, since it is versatile and easy to handle. The Dhaka City and its immediate covering Buriganga-Turag-Tongi Khal-Balu circular waterways are divided into 91 rows and 59 columns, which has created 5369 square cells for each layer. The coarse cells are being 500m by 500m squares at the periphery of the study area, while in the metropolitan area the cell sizes have been reduced to 250m by 250m squares. Cells outside the model domain act as inactive cells and do not take part into the modeling system. The necessary data are incorporated into model to run the program. The result in terms of groundwater potentials are compared with observed groundwater potentials and calibration was done by adjusting the parameters until the model result reproduce the observed responses within acceptable limit. Sensitivity tests of the parameters are performed in order to identify the most important parameters, which have relatively greater effect on the behavior of the simulated hydro-geological system of the regIOn. One of the purposes of the present study is to plan the future development of groundwater resource depending on the availability of groundwater and keeping the water level above specified threshold in order to limit adverse impact to a specified minimum. So, the scenarios that have been tested in the current study with the given boundary conditions are, i) Scenario 01: Continuation of groundwater development at the current volume of withdrawal (i.e., at the volume withdrawn in 2003) until 2010, ii) Scenario 02: Increasing the private withdrawal at a rate of 10% each year from 2003 to 2010, iii) Scenario 03: Increasing the DWASA withdrawal at a rate of 10% each year from 2003 to 2010 and iv) Scenario 04: Increasing the DWASA withdrawal at a rate of 10% each year from 2003 to 2010 and then keeping the withdrawal rate constant until 2020. The prediction runs under different scenarios give some idea about groundwater levels of the aquifers below Dhaka City for a given set of policy and management practices of stressing the reserve and yield. Scenario 01 shows if the current (2003) volume of withdrawal remains constant up to 2010, the declining trend of groundwater level diminishes significantly with a steady-state condition reached by 2009. The contribution from peripheral rivers and existing ground surface recharge reduce that declining trend. Scenario 02 shows if private withdrawal of water increases at 10% rate each year from 2003 to 2010, the groundwater table will decline to a level of -47 mPWD to -64 mPWD. On the other hand, Scenario 03 shows if DWASA withdrawal of water increases at 10% rate each year from 2003 to 2010, the groundwater table will decline to a level of -49 mPWD to -66 mPWD. However, the both cases predicted groundwater table much higher than the existing condition (2003). This situation might prove disastrous, as existing tubewells yield will reduced or even go out of production due to excessive lowering of pumping water levels. Abstracting water under this worse condition would necessitate extension of boring length and further lowering of pumps below, which would eventually increase installation and pumping costs to bring water to the surface. Scenario 04 increasing the DWASA withdrawal at a rate of 10% for each year from 2003 to 2010 and then keeping the withdrawal rate constant until 2020 shows declination of groundwater table to a level of -49 mPWD to -66 mPWD in the year of 2010 and with a few meter more declination, almost steady state condition reach after 2017. The predicted water balance for different scenarios shows that the depletion of aquifer storage is increasing every year. It also indicates an alarming situation that may evolve in near future since the declining trend of groundwater table is increasing enormously.