Groundwater

• 1. Introduction

  ›

  Groundwater is the water hidden beneath Earth's surface, tucked into rock fractures, soil pores, and the spaces between grains of sand and gravel. It is the world's largest source of readily accessible fresh water, accounting for about 30 percent of the global supply. Over 2 billion people rely on it as their primary water source. Underground reservoirs in the United States alone hold far more water than every surface reservoir and lake in the country combined, including the Great Lakes. Yet most of us have never seen it, never thought about it, and almost certainly take it for granted every time we turn on a tap. How did this invisible resource become so indispensable? What happens when it runs out? And why is the ground beneath Bangkok sinking into the sea?

• 2. Beneath Our Feet

  ›

  About 99 percent of the world's liquid fresh water is groundwater. That figure reframes everything we think we know about where fresh water lives. The lakes, rivers, and reservoirs we can see and photograph hold only a small fraction of what lies below. Global groundwater storage is roughly equal to all the fresh water locked in snow and ice packs, including both poles combined.

  Not all groundwater behaves the same way. Water that infiltrated the ground millennia ago, sitting in deep aquifers far from any recharge zone, is sometimes called fossil water. In the Great Artesian Basin of central and eastern Australia, one of the largest confined aquifer systems on Earth, hydrogeologists have traced water that is more than 1 million years old. The basin extends across almost 2 million square kilometres. Water enters it along the Eastern Divide, then flows westward across the continent, growing older as it travels. To have moved nearly 1,000 kilometres from its recharge point in 1 million years, that water travels at an average rate of roughly 1 metre per year.

  Groundwater also acts as a thermal buffer. The high specific heat capacity of water, combined with the insulating effect of soil and rock, keeps groundwater at relatively steady temperatures. In places where that temperature hovers around 10 degrees Celsius, it can be pumped through radiators during hot weather to cool buildings, or used as a heat source for heat pumps during cold seasons.

• 3. The World's Thirst

  ›

  Groundwater accounts for roughly half of the world's drinking water, 40 percent of its irrigation water, and about a third of the water used for industrial purposes. About 2.5 billion people depend on it solely to meet their basic daily needs. Those numbers make it the most accessed freshwater source on the planet.

  Global freshwater withdrawal was around 600 cubic kilometres per year in 1900. By 2017 that figure had risen to 3,880 cubic kilometres per year. The rate of increase was especially steep between 1950 and 1980, running at roughly 3 percent per year, driven by rapid population growth and an explosion of groundwater development for irrigation. As of 2022, the growth rate has slowed to about 1 percent annually, roughly in step with current population increases.

  Seventy percent of all extracted groundwater goes to agriculture. In India, 65 percent of all irrigation draws on groundwater, and about 90 percent of all extracted groundwater there is used for farming. The Asia-Pacific region is the world's largest groundwater abstractor, home to seven of the ten countries that pump the most. Bangladesh, China, India, Indonesia, Iran, Pakistan, and Turkey together account for roughly 60 percent of total global groundwater withdrawal.

  In Libya, Muammar Gaddafi's Great Manmade River project pumped large volumes of groundwater from aquifers beneath the Sahara to populated coastal areas. The project proved less expensive than seawater desalination, but those aquifers are likely to run dry within 60 to 100 years.

• 4. When the Ground Sinks

  ›

  Mexico City, built on a former lake bed, has experienced subsidence at rates of up to 40 centimetres per year. The San Joaquin Valley in California sank by as much as 8.5 metres in the first half of the 20th century due to groundwater removal. New Orleans, Louisiana is now below sea level, partly because of the extraction of groundwater from aquifer systems beneath it. Bangkok, Thailand, faces a projected 5.138 million people exposed to coastal flooding by 2070, a figure shaped by the combined effects of subsidence and sea level rise.

  Subsidence happens because groundwater does physical work. In its natural state, the hydraulic pressure of water in pore spaces supports some of the weight of the sediments above. When that water is pumped out, the pressure drops and the aquifer compresses. In unconsolidated aquifers, this means the silt and clay layers sandwiched between water-bearing sands begin to compact under the weight of everything above them. Much of that compression is permanent. The aquifer not only collapses but loses some of its capacity to hold water in the future.

  The GRACE satellites have gathered data showing that 21 of Earth's 37 major aquifers are actively being depleted. In the Punjab region of India, groundwater levels have dropped 10 metres since 1979, and the rate of depletion is accelerating. In California, Texas, and India, the water table in some areas has fallen by hundreds of feet because of intensive well pumping. A 2021 study found that between 6 and 20 percent of roughly 39 million investigated groundwater wells worldwide are at high risk of running dry if local groundwater levels decline by even a few metres.

• 5. Salt, Poison, and Pollution

  ›

  Groundwater pollution most often results from improper disposal of wastes on land. The list of major sources is long: industrial and household chemicals, garbage landfills, excessive fertilizers and pesticides, industrial waste lagoons, mine tailings and process wastewater, industrial fracking, oil field brine pits, leaking underground storage tanks and pipelines, sewage sludge, and septic systems. Polluted groundwater is far less visible than polluted rivers, and far harder to clean up.

  Arsenic and fluoride have been identified as priority contaminants at the global level, though which chemicals rank as priority threats varies by country. Salinity presents a different kind of problem. Groundwater salinity can vary dramatically over short distances, which makes risk assessment within a single region difficult. Salinity is often worst in coastal zones, particularly in Bangladesh and parts of India, where excessive pumping reverses the natural seaward flow of groundwater and draws in ocean water instead.

  The Ghyben-Herzberg equation describes the physics of seawater intrusion in porous coastal aquifers: for every 1 foot of freshwater head above sea level, there are about 40 feet of fresh water sitting above the saltwater below. When pumping reduces that head, the saltwater wedge rises. The Biscayne Aquifer near Miami and the New Jersey Coastal Plain aquifer are among the coastal aquifers already experiencing saltwater intrusion from a combination of overpumping and sea level rise. Along roughly 15 percent of the US coastline, the majority of local groundwater levels already sit below sea level.

  Climate change adds another layer. The temperature of Viennese groundwater rose by 0.9 degrees Celsius between 2001 and 2010, and by 1.4 degrees between 2011 and 2020. Research from the Karlsruhe Institute of Technology and the University of Vienna projects that under a medium emissions pathway, between 77 million and 188 million people will live in areas where groundwater exceeds the highest drinking-water temperature threshold set by any country by the year 2100.

• 6. Governance and What Comes Next

  ›

  Groundwater governance is complicated by a basic cultural assumption: in many places, water beneath private land is treated as private property. That perception makes top-down regulation difficult. Legal frameworks need to cover more than just who may drill a well. They need to protect recharge zones, regulate abstraction volumes, and govern the relationship between groundwater and connected surface water systems.

  Human groundwater pumping has already shifted Earth's axial tilt by 31 inches. Global groundwater depletion runs between 100 and 300 cubic kilometres per year, driven mainly by the expansion of irrigated agriculture in drylands. Direct redistribution of water by human activities amounts to around 24,000 cubic kilometres per year, which is roughly double the global groundwater recharge rate.

  In the Netherlands and Sweden, groundwater and the ground itself are increasingly treated as components in district heating and cooling networks, acting as seasonal thermal buffers. Deep aquifers are also being investigated for carbon capture and sequestration. In tropical Africa, pumping from groundwater storage has been explored as a way to strengthen climate resilience in water and food systems. The Arab region, one of the most water-scarce areas on Earth, depends on groundwater as the primary source in at least 11 of the 22 Arab states, and over-extraction has caused water table declines across heavily populated and agricultural zones. The way groundwater is governed in the decades ahead will shape whether that resource survives long enough to be governed at all.