First, flood mitigation schemes, intended to protect infrastructure built on floodplains, have had the unintended consequence of reducing
aquifer recharge associated with natural flooding. Second, prolonged depletion of groundwater in extensive aquifers can result in land
subsidence, with associated infrastructure damageas well as, third,
saline intrusion. Fourth, draining acid sulphate soils, often found in low-lying coastal plains, can result in acidification and pollution of formerly freshwater and
estuarine streams.
Overdraft , short periods of recovery were mostly driven by extreme weather events that typically caused flooding and had negative social, environmental and economic consequences. of the aquifer Groundwater is a highly useful and often abundant resource. Most land areas on
Earth have some form of aquifer underlying them, sometimes at significant depths. In some cases, these aquifers are rapidly being depleted by the human population. Such over-use, over-abstraction or overdraft can cause major problems to human users and to the environment. The most evident problem (as far as human groundwater use is concerned) is a lowering of the water table beyond the reach of existing wells. As a consequence, wells must be drilled deeper to reach the groundwater; in some places (e.g.,
California,
Texas, and
India) the water table has dropped hundreds of feet because of extensive well pumping. The
GRACE satellites have collected data that demonstrates 21 of Earth's 37 major aquifers are undergoing depletion. A lowered water table may, in turn, cause other problems such as
groundwater-related subsidence and
saltwater intrusion. Another cause for concern is that groundwater drawdown from over-allocated aquifers has the potential to cause severe damage to both terrestrial and aquatic ecosystemsin some cases very conspicuously but in others quite imperceptibly because of the extended period over which the damage occurs.continue to decline. Fresh-water aquifers, especially those with limited recharge by snow or rain, also known as
meteoric water, can be over-exploited and depending on the local
hydrogeology, may draw in non-potable water or saltwater intrusion from hydraulically connected aquifers or
surface water bodies. This can be a serious problem, especially in coastal areas and other areas where aquifer pumping is excessive.
Subsidence Subsidence occurs when too much water is pumped out from underground, deflating the space below the above-surface, and thus causing the ground to collapse. The result can look like craters on plots of land. This occurs because, in its natural equilibrium state, the
hydraulic pressure of groundwater in the pore spaces of the aquifer and the aquitard supports some of the weight of the overlying sediments. When groundwater is removed from aquifers by excessive pumping, pore pressures in the aquifer drop and compression of the aquifer may occur. This compression may be partially recoverable if pressures rebound, but much of it is not. When the aquifer gets compressed, it may cause land subsidence, a drop in the ground surface. In unconsolidated aquifers, groundwater is produced from pore spaces between particles of gravel, sand, and silt. If the aquifer is confined by low-permeability layers, the reduced water pressure in the sand and gravel causes slow drainage of water from the adjoining confining layers. If these confining layers are composed of compressible silt or clay, the loss of water to the aquifer reduces the water pressure in the confining layer, causing it to compress from the weight of overlying geologic materials. In severe cases, this compression can be observed on the ground surface as
subsidence. Unfortunately, much of the subsidence from groundwater extraction is permanent (elastic rebound is small). Thus, the subsidence is not only permanent, but the compressed aquifer has a permanently reduced capacity to hold water. The city of
New Orleans, Louisiana is actually below sea level today, and its subsidence is partly caused by removal of groundwater from the various aquifer/aquitard systems beneath it. In the first half of the 20th century, the
San Joaquin Valley experienced significant subsidence, in some places up to due to groundwater removal. Cities on river deltas, including Venice in Italy, and
Bangkok in Thailand, have experienced surface subsidence; Mexico City, built on a former lake bed, has experienced rates of subsidence of up to per year. For coastal cities, subsidence can increase the risk of other environmental issues, such as
sea level rise. For example, Bangkok is expected to have 5.138 million people exposed to
coastal flooding by 2070 because of these combining factors. combined with increasing levels of salt in surface waters. As a consequence, major damage has occurred to local economies and environments. Aquifers in surface
irrigated areas in semi-arid zones with reuse of the unavoidable irrigation water losses
percolating down into the underground by supplemental irrigation from wells run the risk of
salination. Surface irrigation water normally contains salts in the order of or more and the annual irrigation requirement is in the order of or more so the annual import of salt is in the order of or more. Under the influence of continuous evaporation, the salt concentration of the aquifer water may increase continually and eventually cause an
environmental problem. For
salinity control in such a case, annually an amount of drainage water is to be discharged from the aquifer by means of a subsurface
drainage system and disposed of through a safe outlet. The drainage system may be
horizontal (i.e. using pipes,
tile drains or ditches) or
vertical (
drainage by wells). To estimate the drainage requirement, the use of a
groundwater model with an agro-hydro-salinity component may be instrumental, e.g.
SahysMod.
Seawater intrusion Aquifers near the coast have a lens of freshwater near the surface and denser seawater under freshwater. Seawater penetrates the aquifer diffusing in from the ocean and is denser than freshwater. For porous (i.e., sandy) aquifers near the coast, the thickness of freshwater atop saltwater is about for every of freshwater head above
sea level. This relationship is called the
Ghyben-Herzberg equation. If too much groundwater is pumped near the coast, salt-water may intrude into freshwater aquifers causing contamination of potable freshwater supplies. Many coastal aquifers, such as the
Biscayne Aquifer near Miami and the New Jersey Coastal Plain aquifer, have problems with saltwater intrusion as a result of overpumping and sea level rise. Seawater intrusion is the flow or presence of seawater into coastal aquifers; it is a case of
saltwater intrusion. It is a natural phenomenon but can also be caused or worsened by anthropogenic factors, such as
sea level rise due to
climate change. In the case of homogeneous aquifers, seawater intrusion forms a saline wedge below a transition zone to fresh groundwater, flowing seaward on the top. These changes can have other effects on the land above the groundwater. For example, coastal groundwater in California would rise in many aquifers, increasing risks of flooding and
runoff challenges.
Pollution can be spread via a groundwater well which is contaminated with fecal pathogens from
pit latrines
Climate change , Pakistan The impacts of climate change on groundwater may be greatest through its indirect effects on irrigation water demand via increased
evapotranspiration. In the tropics intense precipitation and flooding events appear to lead to more groundwater recharge. For the higher altitudes regions, the reduced duration and amount of snow may lead to reduced recharge of groundwater in spring. Groundwater-based
adaptations to climate change exploit distributed groundwater storage and the capacity of aquifer systems to store seasonal or episodic water surpluses. They incur substantially lower evaporative losses than conventional infrastructure, such as surface dams. For example, in
tropical Africa, pumping water from groundwater storage can help to improve the
climate resilience of water and food supplies.
Climate change mitigation The development of
geothermal energy, a
sustainable energy source, plays an important role in reducing CO2 emissions and thus
mitigating climate change. Groundwater is an agent in the storage, movement, and extraction of geothermal energy. In pioneering nations, such as the Netherlands and Sweden, the ground/groundwater is increasingly seen as just one component (a seasonal source, sink or thermal 'buffer') in
district heating and cooling networks. Deep aquifers can also be used for
carbon capture and sequestration, the process of storing carbon to curb accumulation of carbon dioxide in the atmosphere. == Groundwater governance ==