Mahabub-ul-Alam, S.M.; (1990) Hydrogeology of part of south-eastern Bangladesh. Doctoral thesis (Ph.D), UCL (University College London).

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Abstract

The present study covers 10,223 km2 of the Ganges, Jamuna and Meghna river's delta, at the head of the Bay of Bengal, which was deposited in a geosynclinal environment. The morphometry of the delta was controlled by three geomorphological units, recognised as geological formations. The main aquifer comprises primarily the sandy Dupitila Formation, of Pliocene age, which is composed of unconsolidated sands showing a gradation of grain size. Finer sands are at the top with the sequence coarsening downwards; the coarse sand at the bottom is usually associated with gravels and pebbles. The thickness of the aquifer ranges from 30 m to 120 m. The aquifer is almost entirely covered by a clay/silt aquiclude of thickness ranging from <3 m to >300 m. The loss of ground water storage during the dry season is restored rapidly and regularly by the rising water table reflecting monsoon conditions. The complicated flow behaviour suggests that all the perennial rivers maintain direct hydraulic continuity with the ground water. The flow-net analysis also reveals uniform distribution of water resource potential. Borehole pumping tests indicate a true confined condition for the smaller eastern part while leaky confined conditions prevail elsewhere. The maximum and minimum transmissivity range from >1400 m2/d to <400 m2/d while the storativity ranges from 4.1 x 10-3 to 3.7 x 10-4 and the maximum leakage (BL= 368 m) is recorded around Burichong. The values of well loss factor (Ct) and aquifer loss factor (Bt suggest highly efficient well conditions as deduced from the calculation of well efficiency and the analysis of yield-drawdown curves. The ground water towards the outcrop appears to have been an essentially NaHCO3 type in nature but, as it moves, hydrochemical evolution modifies the ground water into a mixed NaHCO3/CI type. Three major and two minor ion exchange fields were identified. The expanded Durov plot was most successful in identifying the existence of two different types of water, the dominant type being the ion-exchanged water and the less dominant but equally well defined type being old brackish water accompanied by some reverse ion exchange water. The existence of reverse ion exchange water is further suggested by the changing chemistry along flow (flow line 2 and 3). The brackish waters are segregated in few locations and are not linked to one another. Other than the brackish water, the ground water is found to be suitable both as potable as well as irrigation sources. The saturation indices calculated by means of the PHREEQE hydro-chemical model for Calcite and Dolomite establishes a unique linear relationship emphasising that the older and deeper waters are mainly super-saturated and the young waters are under-saturated with respect to these minerals, with few exceptions. The water balance computation and the present rate of withdrawal indicates a huge recharge surplus so that the aquifer may be regarded as in a youthful state of development. The perennial yield is estimated to be about 5.7 x 109 m3/year and the present rate of ground water withdrawal is about 37 percent of the perennial yield, suggesting a huge ground water potential yet to be exploited.

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