Groundwater hydraulic resource beneath the surface of the ground, consisting largely of surface water seeped down. Such water accumulates in porous strata and soils. Groundwater represents around 30% of freshwater resources of the earth, while lakes and rivers correspond to less than 1%. The largest volume of freshwater is stored in glaciers (69%). Groundwater is one of the components of the hydrological cycle, stored underground in rock layers called aquifers - it is a result of infiltration of rainfall and surface water with which it maintains a close relationship. When this water reaches the groundwater table (ie the groundwater's upper level), it begins a long, slow journey underground, moving at rates ranging from a few millimetres to a few meters per day.
The zone above the water table is called the 'unsaturated zone' and the zone beneath 'saturated zone'. The soils and rocks in the unsaturated zone remove the main impurities, while the rocks in the saturated zone filter and purify the water even further. It then usually reappears at the earth's surface free of pathogens, and pollutants. Because of this process, groundwater is usually of excellent microbiological quality and of adequate chemical quality for both irrigation and potable purposes. Aquifers present the advantage of having large to huge storage capacity with a very low flow and are easily accessed through dug wells or tubewells, either pumped or flowing (artesian wells) or through springs. The importance of aquifers depends on the geological nature of the layers, for example, sedimentary formations represent good aquifers while hardrocks have limited capacity for storage.
Groundwater in Bangladesh occurs at a very shallow depth where the recent river-borne sediments form prolific aquifers in the floodplains. In the higher terraces, the Barind and Madhupur tracts, the Pleistocene Dupi Tila sands act as aquifers. In the hilly areas, the Pliocene Tipam sands serve as aquifers. The groundwater table over most of Bangladesh lies very close to the surface and fluctuates with the annual recharge discharge conditions. Recharge to aquifers in Bangladesh is mainly from vertical percolation of rainwater and floodwater. Rivers and other standing water bodies provide local recharge to the nearby aquifers. The main component of discharge is the withdrawal of groundwater by different types of tubewells. A minor component of discharge is natural flow towards lower gradient. The groundwater level is at or very close to the surface during the monsoon whereas it is at maximum depth during the months of April and May. This trend is common over most of Bangladesh except Dhaka City and the barind tract. Because of uneven distribution of water resources, dry season demand for water in Bangladesh is mainly met by groundwater.
Presently about 80% of the people in rural Bangladesh depend on groundwater for drinking. Urban water supply is also largely dependent on groundwater. In Dhaka City more than 95% of the supply comes from groundwater and the remainder is provided by treated surface water. Groundwater also contributes largely to irrigation and under 1995 conditions 72% of the total coverage was provided by groundwater. The Master Plan Organisation (MPO) in 1989 estimated a reserve of 25,750 million-meter cube (MMC) of groundwater in Bangladesh. Of this 1,686 MMC is unavailable and 900 MMC remains in reserve for domestic cum industrial use, and a maximum of 12,809 MMC is available for agriculture.
The most serious constraint is the largescale arsenic contamination of groundwater. Before the discovery of arsenic contamination 97% of the rural people used to rely on groundwater. Now the coverage has come down to 80%. Apart from arsenic the other quality issues include high concentration of iron, high salinity in coastal areas, high concentration of manganese etc. Faecal colliform is also a problem, particularly for shallow groundwater. Decline in groundwater level in the dry season due to larger withdrawal for irrigation is another constraint for water abstraction by certain technologies. Most commonly used hand tubewells cannot pump water if the level goes below 6m. For large production wells like the wells in Dhaka city, continuous lowering of water level means higher and higher production cost. Ground subsidence is another adverse impact of lowering groundwater level. Although predicted, there is no sign of land subsidence due to groundwater withdrawal in Bangladesh. Indiscriminate disposal of industrial and municipal wastes in large cities cause groundwater quality degradation as seen in Dhaka city. [Kazi Matinuddin Ahmed and Md Alamgir Hossain]