Pumping groundwater has shifted the tilt of Earth's axis by 79 centimetres
About 99 percent of the world's liquid fresh water sits underground, in the pores and cracks of rock and soil. Some of it is over a million years old. People now pump so much that cities are sinking, coastal wells are turning salty, and Earth's axial tilt has moved about 79 centimetres.
Groundwater fills the spaces in soil and the fractures in rock below the water table, the depth at which everything is saturated. A rock layer or loose deposit that can yield useful quantities is called an aquifer. Rain, streams and rivers recharge it from above; springs, seeps and flows into the ocean drain it naturally. Sedimentary formations and the thick gravels that fill big river valleys make the most productive aquifers, while fractured crystalline rock usually gives far less.
It moves astonishingly slowly. In Australia's Great Artesian Basin, one of the largest confined aquifer systems on Earth at almost 2 million square kilometres, trace elements show that some water is more than a million years old. Ages rise steadily from the eastern recharge zone toward the west, implying the water creeps nearly 1,000 kilometres at roughly a metre per year. Deep water that soaked in thousands of years ago is sometimes called fossil water.
Human dependence is enormous. Groundwater supplies about half the world's drinking water, 40 percent of irrigation and a third of industrial water, and roughly 2.5 billion people rely on it alone. In the United States underground reservoirs hold far more than every surface reservoir and lake combined, the Great Lakes included. Seven of the ten biggest users, among them India, China and Pakistan, are in the Asia-Pacific region and together account for about 60 percent of global extraction. The steady temperature of shallow groundwater, around 10 degrees Celsius in some places, even lets it cool homes in summer and feed efficient heat pumps in winter.
Because storage is so large relative to recharge, overuse can continue for years before damage shows. Global depletion is estimated at 100 to 300 cubic kilometres a year, driven mostly by irrigated farming in dry regions. Land sinks as aquifers empty, as in Bangkok and California's Central Valley, which has lost several metres of elevation. Pollution is hard to see and harder to clean, and natural contaminants such as arsenic and fluoride, along with salt pushed inland by heavy pumping, can make wells unsafe.
Source: Groundwater
