Groundwater

Additionally, groundwater is susceptible to saltwater intrusion in coastal areas and can cause land subsidence when extracted unsustainably, leading to sinking cities (like Bangkok) and loss in elevation (such as the multiple meters lost in the Central Valley of California).

These issues are made more complicated by sea level rise and other effects of climate change, particularly those on the water cycle.

Due to its slow rate of turnover, groundwater storage is generally much larger (in volume) compared to inputs than it is for surface water.

Deep groundwater (which is quite distant from the surface recharge) can take a very long time to complete its natural cycle.

The Great Artesian Basin in central and eastern Australia is one of the largest confined aquifer systems in the world, extending for almost 2 million km2.

This process usually occurs in the vadose zone below plant roots and is often expressed as a flux to the water table surface.

[16] Global groundwater storage is roughly equal to the total amount of freshwater stored in the snow and ice pack, including the north and south poles.

This makes it an important resource that can act as a natural storage that can buffer against shortages of surface water, as in during times of drought.

[17] The volume of groundwater in an aquifer can be estimated by measuring water levels in local wells and by examining geologic records from well-drilling to determine the extent, depth and thickness of water-bearing sediments and rocks.

Unconsolidated to poorly cemented alluvial materials that have accumulated as valley-filling sediments in major river valleys and geologically subsiding structural basins are included among the most productive sources of groundwater.

[22] About 2.5 billion people depend solely on groundwater resources to satisfy their basic daily water needs.

[22] Salinity in groundwater makes the water unpalatable and unusable and is often the worst in coastal areas, especially due to Saltwater intrusion from excessive use, which are notable in Bangladesh, and East and West India, and many islan nations.

[25] Stressing the fact that regional shallow groundwater warming patterns vary substantially due to spatial variability in climate change and water table depth these researchers write that we currently lack knowledge about how groundwater responds to surface warming across spatial and temporal scales.

In Libya, for example, Muammar Gaddafi's Great Manmade River project has pumped large amounts of groundwater from aquifers beneath the Sahara to populous areas near the coast.

[37] Though this has saved Libya money over the alternative, seawater desalination, the aquifers are likely to run dry in 60 to 100 years.

[37] Groundwater provides critical freshwater supply, particularly in dry regions where surface water availability is limited.

The demand for groundwater is rapidly increasing with population growth, while climate change is imposing additional stress on water resources and raising the probability of severe drought occurrence.

Second, prolonged depletion of groundwater in extensive aquifers can result in land subsidence, with associated infrastructure damage – as well as, third, saline intrusion.

[42] Fourth, draining acid sulphate soils, often found in low-lying coastal plains, can result in acidification and pollution of formerly freshwater and estuarine streams.

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.

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.

[52] In the first half of the 20th century, the San Joaquin Valley experienced significant subsidence, in some places up to 8.5 metres (28 feet)[53] due to groundwater removal.

Major land degradation problems of soil salinity and waterlogging result,[58] combined with increasing levels of salt in surface waters.

It is a natural phenomenon but can also be caused or worsened by anthropogenic factors, such as sea level rise due to climate change.

[62] In the case of homogeneous aquifers, seawater intrusion forms a saline wedge below a transition zone to fresh groundwater, flowing seaward on the top.

Groundwater pollution can occur from on-site sanitation systems, landfill leachate, effluent from wastewater treatment plants, leaking sewers, petrol filling stations, hydraulic fracturing (fracking) or from over application of fertilizers in agriculture.

The impacts of climate change on groundwater may be greatest through its indirect effects on irrigation water demand via increased evapotranspiration.

[19]: 106 Global sea level rise due to climate change has induced seawater intrusion into coastal aquifers around the world, particularly in low-lying areas and small islands.

[69] Groundwater-based adaptations to climate change exploit distributed groundwater storage and the capacity of aquifer systems to store seasonal or episodic water surpluses.

[19]: 110 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.