Aquifer a geological group-formation or group of formations, or a part of a formation capable of yielding a significant amount of water to a well or spring. Three aquifers have been identified in Bangladesh: an upper aquifer, herein called the composite aquifer; the main aquifer; and a deep aquifer which lies below the main aquifer and is separated from it by clay layers of varied thickness. In most areas the upper two aquifers are probably hydraulically interconnected.

An upper clay and silt unit underlies the composite aquifer, composed of very fine to fine sand, in places interbedded or mixed, with medium sand and very thin layers of clay. The thickness of this part ranges from a few meters in the northwest to as much as 60m in the south of the country. At places, these sands and thin clay layers form a composite aquifer, with poor to moderate water-bearing characteristics, capable of yielding water to shallow wells. Over most of the country it represents the uppermost water-bearing zone.

Beneath the composite aquifer, medium and coarse-grained sandy sediments, in places interbedded with gravel, are commonly encountered. These sediments form the main aquifer and occur to depths of about 140m below the ground surface. At places, the zone has thin interbeds of clay. This aquifer, in most of the country, is either semi-confined and leaky or consists of stratified, interconnected, unconfined water-bearing zones which are subjected to delayed drainage. It occurs at depths ranging from less than 5m in the northwest to more than 75m in the south of the country.

The transmission properties of the main aquifer are good to excellent over most of the country but deteriorate to the south and the east. Average transmissivities exceed 2,000 sq m/day and range from 100 to over 6,000 sq m/day and horizontal hydraulic conductivities range from 5 to 200 m/day. The storage co-efficient of the aquifer computed from pumping test data, ranges from 3,10-4 to 72,10-3 with the average value being 2,10-3.

Underlying the main aquifer is a deeper water-bearing unit, herein referred to as the deep aquifer, which has been exploited by tubewells in Dhaka and in the coastal areas. This water-bearing unit is separated from the overlying main aquifer by one or more clay layers of varied thickness. [Md Sazzad Hossain]