Assessment and modeling of groundwater hydrology in selected zones of Dhaka city

Abstract

Water supply and service systems at any region throughout the world closely linked with ground water and aquifer systems since about 97 % of the earth’s fresh water supply is stored in the underground formations. So understanding of ground water hydrology is significant and important while dealing with the critical issues of water production from ground water aquifer systems of any area. Mega city Dhaka is the residence of over 15 million people. Dhaka Water and Sewerage Authority (DWASA) is entrusted with supply of piped water to Dhaka Metropolitan City and its adjacent area. To meet the growing demand for water, DWASA mainly depends on deep tubewell extracting ground water. The rate of water extraction from existing aquifers and the declining level of groundwater are alarming and if unabated, will cause serious shortages, land subsidence, and further environmental degradation. According to DWASA, 87 percent of its pipelined water supply in the capital is from groundwater sources and only 13 percent from surface water treatment plants. DWASA supplies 2077.20 million litres of water daily against the current demand for 2,250 million litres. The present study tried to indicate the critical issues relating to ground water production and ground water aquifer systems. Extensive studies had done to assess the locations and conditions of deep tubewells under DWASA zone 5, 8 and 9 as well as other aspects including water depletion scenario, formation of soil layers, characteristics of aquifer systems, hydrologic properties and surface water infiltration rate. For soil formation and hydrologic properties analysis, 3D stratigraphy model is developed and MODFLOW LPF package is simulated. GIS modeling is used for spatial assessment. Total 96 tubewells were used for spatial analysis. DWASA proposed tubewell to tubewell distance should be 365 meter for normal adequacy of withdrawal of water from aquifer and for giving facility of aquifer to get recharged naturally. But almost all the tubewells are within 365m area of interference of any deep tubewell. Average ground water depletion found in this zone is 25.8m. Even, this type of alarming ground water depletion, DWASA installing or planning to install deep tubewell to deeper position than before. Daily water production in this zone from deep tubewell is 393.68 ML. Around 15.57 MLD water adds in the supply lines of this zone from Surface water treatment plant at Saidabad. Considering this additional water actual deficit is 45.17MLD. One of the important aspects of this study was to estimate hydrologic parameters with a view to use those in hydrologic modeling to get an overview of ground water aquifer system beneath the selected zone. Hydraulic head, drawdown, specific capacity and transmissivity data are computed from the existing well information. 66 borehole data are analyzed to identify types and properties of soil layer to 200m depth below ground. 3D stratigraphy model has been identified 60 separate layers of soil solid with respect to types and depth. From 0m to 200m below ground, representative values of coefficient of grain uniformity (U) of different types of soil varies from 1.77 to 2.11 (excluding clay). Five separate empirical formulae have been used to determine soil hydraulic conductivity (K) values and among those K values from Kozeny-Carman formula are found maximum. It has given coarse soil K value is 78.83 m/day and for fine sand it is 26.38 m/day. K values of clay soil could not be determined for lack of soil grain properties data. To estimate that, field level infiltration rate had been measured at six spots on top soil layer. Assignment of required information in the developed model, K values and aquifer thickness from MODFLOW simulation separated Slitcher equation from others and justified as valid formula to determine K value with soil propertied in the selected areas of Dhaka city. In the final stage, the subsurface flow budget has been estimated from stratigraphy model and on the basis of some assumptions.