In many cities of the world including cities of Pakistan, huge amount of ground water is drawn through pumps every day for domestic, agricultural and industrial uses. Due to this under ground water table is sinking down and impurities are concentrating. But is this activity can cause sinking of earth's surface and damage the infrastructure like buildings etc. present on the surface specifically in cities? Practical experience and research based answer will be helpful to understand the issue.
In general, lowering the water table is a relatively slow process. Unlike surface water, the groundwater table does not decrease simultaneously across the entire aquifer, but in the immediate vicinity of the borehole.
The flow time of groundwater is much shorter than that of surface water, due to its movement through pores and cracks. When the boreholes pump water from these aquifers, the level of the water table near the boreholes / wells drops in the form of a depression cone. In this cone of depression, the groundwater flows to the borehole.
Subsidence is yet another particularly widespread effect of over-pumping, with some notable examples in a number of large cities in China, Japan, Mexico and the United States. Land subsidence occurs when excessive amounts of groundwater have been removed from a porous aquifer. As a result, the porous aquifer materials compact and collapse, resulting in a lowering of the soil surface in the area (www.sjra.net). Subsidence can cause many problems, such as: changes in the elevation of the soil surface; damage to structures such as rainwater and wastewater pipes, roads, railways, canals, dikes and bridges; structural damage to public and private buildings; and damage to boreholes. Most often, however, subsidence increases the risk of flooding.
Regards
Thanks Bachir Achour for detailed answer which is helpful to understand the issue.
Mr Bachir Achour is very correct in his response. However, in perched and fractured basement aquifer environments where groundwater recharge is mainly by rainwater, the lowering of groundwater table can be fast depending on their discharge rate and topography of the area. Similar situations occur in coastal aquifers that discharge gradually into drying water bodies through seepages in dry seasons. You can look at my work https://doi.org/10.1016/j.jafrearsci.2018.03.012.
In all cases, the associated subsidence usually results in damage to infrastructure and farmlands.
Thanks Anthony E. Akpan for your valuable views and suggestions to understand the issue.
Sink holes have appeared in cities including Miami, FL (USA). How much of this is related to over-pumping ground water is unclear. Incidences of increased earthquake activity have been reported in the state of Oklahoma, USA, as a result of horizontal drilling (fracking) and these incidents have decreased once fracking activities were cut down. This latter observation is not related to pumping ground water but one can see the loose connection
Appreciate the answers of Dr. Bachir & Dr. Anthony. Land subsidence with respect to over extraction/development of groundwater, is a common phenomenon in regions covered by sedimentary formations having confined aquifers with artisan condition. When the groundwater is extracted heavily from the confined aquifers, exceeding the recharge rate, the pressure within the confined aquifer reduces, thereby creating compaction of the aquifer leading to land subsidence. If the land subsides, then many infrastructures like buildings, roads, pipe lines, etc., will be damaged.
Hence, while exploiting groundwater from an artisan aquifer to avoid compacting of the aquifer and resultant land subsidence,.some of the routine aspects of groundwater management practices are:
1. Drawdown in a confined aquifer should not exceed below the base of the
confining stratum.
2. Artisan pressure should not be lowered below the atmospheric pressure in the region.
Confined aquifer repository that is overexploited has its pressure built up by interstitial water reduced. This action causes subsidence that can pull down houses and other high rising infrastrures in cities.
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