Everyone is familiar with water in the form of rain, rivers, lakes and seas.  But  groundwater is less well-known.  Here are the essentials, in Question and Answer form.

What is groundwater?
Underground water in saturated layers of porous rock or sand known as aquifers (1).

How does it get there?
Mainly by infiltration of rainwater through soil and porous rock. Eventually the water reaches a layer of non-porous rock, above which it accumulates.

Does groundwater remain there forever in the absence of human intervention?
Many aquifers are recharged by rainwater, and discharge water through springs or seeps when the water table (ie level) is high, resulting in a gradual turnover of the water they contain (1).  However, some aquifers in arid countries such as Libya contain water that has been undisturbed for thousands of years (2).

How much groundwater is there worldwide?
Roughly a hundred times as much as all the water in surface rivers and lakes (3).

How is it extracted?
Usually by drilling a borehole and installing a tubewell, a long narrow tube often made of PVC.  Water is then lifted by a manual, electric, diesel or solar-powered pump (5, 6).

How is it used?
Agriculture is the largest use globally. Groundwater is also widely used for domestic water supply (7).

Which countries extract the most groundwater?
India (25%), followed by China and the US (10% each) (7).

How much groundwater is extracted worldwide?
About 1,000 cubic kilometres annually, or 25% of all water withdrawn for human use (the rest being mainly from surface rivers and lakes). The amount is growing fairly rapidly (7).

Is this sustainable?
This is complicated.  Groundwater that is regularly recharged by rainfall is a renewable resource, and the relevant comparison is between annual extraction and annual recharge.  Where it is not recharged, no rate of extraction can be sustained indefinitely, and the question then is how long the groundwater reserves will last.  Worldwide, annual extraction is about 10% of annual recharge and about 0.0001% of total reserves (7).  However, some groundwater is too deep to be accessible, some is polluted, and our knowledge is incomplete.  The global totals appear sustainable in the short term, but the longer-term position is unclear.

Aren’t water tables already falling rapidly in some parts of the world?
Yes.  There is a geographical mismatch between demand for and availability of groundwater,  resulting in depletion of aquifers in some regions such as parts of India and Pakistan (8).  Where aquifers extend under land occupied by many farmers, with no property rights over the groundwater or regulation of extraction, there can be a form of ‘tragedy of the commons’.  As water tables fall, deeper tubewells are needed to extract water, pumping costs increase, and  pollutants can become more concentrated.  However, much groundwater is under-exploited, especially in South America, Sub-Saharan Africa and Indonesia (8).

Why might a farmer in an area with high rainfall and/or a surface irrigation network choose to invest in a tubewell?
Key factors are flexibility and reliability (9). Rainfall may be variable or seasonal. Water delivery from large surface irrigation systems is often unreliable due to seasonal factors, poor maintenance, or control of allocation by local elites. A tubewell can provide a year-round flexible supply, independent of any other party.  Where rainfall is seasonal, groundwater may support a second crop.  A more reliable water supply can also enable a farmer to switch to higher-value crops that because of their water requirements would otherwise be too risky (9).

How serious a problem is groundwater pollution?
Pollutants can accumulate in aquifers where there are regular inflows from agricultural drainage, urban wastewater or landfill sites.  Groundwater may also contain natural contaminants such as arsenic or radon gas (10).  However, water volumes in deep aquifers are often so large that concentrations remain low. Much groundwater is considered safe for human consumption (11). Shallow groundwater is more likely to be unsafe.  There have been widespread health problems from drinking unsafe groundwater in Bangladesh (12) and China (13).

What about pollution and agricultural use?
Salination of groundwater is a widespread problem (14).  Crops irrigated with saline water may suffer from reduced yields, and very saline water may be unuseable.  There is also a risk that pollutants in water may be absorbed by crops and enter the human food chain.  This is hard to evaluate since effects may be long-term and dependent on quantities consumed.  Concerns to date in this respect have centred more on urban wastewater than on groundwater (14).

Notes & References

1.  McGinley M (2011) Aquifer, in Encyclopaedia of Earth http://www.eoearth.org/article/Aquifer

1. FAO Aquastat: Libya (Version May 2006)   http://www.fao.org/nr/water/aquastat/countries_regions/libya/index.stm

3.  Water in rivers and lakes is about 0.13 million cubic kilometres (4). Global groundwater reserves are estimated at between 7 and 23 million cubic kilometres (7).

1. Townsend C, Begon M & Harper J (3rd edn 2008) Essentials of Ecology  Blackwell Publishing  p 380
2. Water Aid,  Tubewells and Boreholes   http://www.wateraid.org/uk/what_we_do/sustainable_technologies/technology_notes/243.asp
3. WHO, Factsheet 2.3 Boreholes and Tubewells   http://www.who.int/water_sanitation_health/hygiene/emergencies/fs2_3.pdf

7.  Comprehensive Assessment of Water Management in Agriculture 2007  Water for Food, Water for Life: A Comprehensive Assessment of Water Management in Agriculture  London: Earthscan, and Colombo: International Water Management Institute  p 398-9

8.  Comprehensive Assessment as above, p 402-3

9.  Comprehensive Assessment as above, p 409-11

1. Water Encyclopaedia: Pollution of Groundwater   http://www.waterencyclopedia.com/Oc-Po/Pollution-of-Groundwater.html
2. US Environmental Protection Agency  Drinking Water from Household Wells  pp 2-3   http://www.epa.gov/privatewells/pdfs/household_wells.pdf
3. WHO  Arsenic in Drinking Water   http://www.who.int/water_sanitation_health/dwq/arsenic/en/
4. People’s Daily Online, Nov 5 2010: Most Northern Plain Groundwater Unsafe to Drink   http://english.peopledaily.com.cn/90001/90776/90882/7188979.html
5. Comprehensive Assessment as above, p 432-4