Groundwater is surface water which has filtered through permeable soils and rocks until stopped by impermeable layers below, being cleaned in the process. It accumulates as aquifers, which may be thousands and millions or years old and slowly seep to the surface as springs or flow underground and feed rivers and lakes. The rates of water movement are extremely slow compared with surface flows, and depletion of groundwater in excess of its recharge rate is effectively use of a non-renewable resource.
Huge underground reservoirs are being drained at alarming rates in many parts of the world. Arid climates aggravated by lower than average rain fall, complex groundwater systems that receive limited recharges, population growth, uncontrolled drilling, increasing per head use of water, mismanagement of water supplies and little knowledge about underground water resources exacerbate the problem. Properly irrigating the main arid and semi-arid regions of the world would require the use of the total continental run-off (a physical impossibility) and water projects so large as to risk substantial changes in regional global climate. Present rapid depletion of ground-water resources throughout the world will soon lead to widespread local shortages. It is believed that irrigation will in most areas contribute too little and too late.
Trends documented by the the World Bank and Worldwatch Institute, and reported to the Rio+5 conference in 1997, at that underground water tables have begun falling in much of the United States, India, China, northern Africa, southern Europe and the Middle East.
Overuse of groundwater is now ubiquitous in parts of China, India, Mexico, Thailand, the western USA, north Africa and the Middle East. Water tables beneath Beijing have been dropping 1-2 metres a year and a third of its wells have reportedly gone dry.
Only about 2.5 million square kilometres (2% of all ice-free land) is now irrigated by groundwater, at great cost and with many unwelcome side-effects. As an example, Libya is constructing a 500 km pipeline to take water, pumped out of hundreds of wells dug beneath the Sahara. There is concern that this $25 billion project will drain wells supplying Egypt and the Sudan. The prairie region of the USA, responsible for 23% of agricultural production, is irrigated by an aquifer which is drying up. By 2010 it may be too low to pump. In 1993, one-fifth of pumped groundwater in the USA was not renewable.
In the interim period between handover of responsibilities for water management in the Gaza Strip from the Israelis to the Palestinians, the amount of unlicensed pumps and uncontrolled water volume increased enormously. In 1993 around 300 new irrigation pumps were operating without any restriction. The salt level in wells of one village was almost twice those recorded 18 months earlier (now above 5,000 parts per million, which is no longer fit for human use or up to 90% of agricultural uses).
Over-abstraction has led to seawater intrusion along shorelines, causing salinization of coastal agricultural lands. As a result, some arable land, such as that on the Batinah coastal plain of Oman, has been completely lost (UNEP/ESCWA 1992). It is estimated that the saline interface between the sea and groundwater advances at an annual rate of 75-130 metres in Bahrain (UNEP/ESCWA 1991). In Madras, India, salt water intrusion has moved 10 km inland, rendering many irrigation wells useless (UNEP 1996b). Salt water intrusion is of particular concern in small island states, where the limited groundwater supply is surrounded by salt water.
Groundwater supplies about one-third of the world's population, and is the only source of water for rural dwellers in many parts of the world. Excessive withdrawal of groundwater, in quantities greater than the ability of nature to renew the aquifers, is now widespread in parts of the Arabian Peninsula, China, India, Mexico, the former Soviet Union and the United States. The water table has dropped by tens of metres in many places where there is intensive groundwater use. An estimated 65 per cent of public water supplies in Europe come from groundwater sources, and groundwater withdrawal in the European Union rose by 35 per cent between 1970 and 1985 (EEA 1995).
In El Salvador, water was a serious problem in 1997. Just in the capital, the subterranean water supplies dropped a meter a year, and sooner or later those aquifers would run dry.
Humanity is overpumping (at rates higher than recharge) groundwater stored during the last glacial period by some trillions of gallons a year. Another ice age will be required to refill some depleted aquifers.