Water found below Earth’s surface is called groundwater. It fills the tiny spaces between grains of sand, gravel, and rock, and it sits in cracks within solid stone. Groundwater accounts for 99% of all liquid freshwater on Earth, making it far more abundant than every river, lake, and reservoir combined. About one quarter of all water used by humans comes from this hidden supply.
Where Groundwater Sits Underground
The ground beneath your feet isn’t solid all the way down. Soil and rock contain small openings called pores, and the way water fills those pores creates two distinct layers. The upper layer, called the unsaturated zone (or vadose zone), has a mix of air and water. Pore spaces here are mostly air-filled, with thin films of water coating the solid particles. Below that sits the saturated zone, where every available pore space is completely filled with water. The boundary between these two layers is the water table.
The depth of the water table varies widely. In wet, low-lying areas it can be just a few feet below the surface. In arid regions or hilltops, it may be hundreds of feet down. It also shifts with the seasons, rising after periods of heavy rain and dropping during dry spells.
How Water Gets Underground
Rain and snowmelt are the primary sources. When precipitation hits the ground, some of it soaks downward through soil and rock in a process called infiltration. It eventually reaches the saturated zone and becomes groundwater. This replenishment process is called recharge.
The details depend heavily on rainfall patterns. Long, steady rain tends to soak in gradually and recharge groundwater effectively. In drier climates, extreme storms and the flooding they cause may be the only events powerful enough to push water deep into the ground. Research in northern China found that rainfall greater than 40 millimeters per day (about 1.6 inches) was needed to meaningfully recharge shallow groundwater. After a single extreme downpour, water bypassed the soil layers and flowed quickly into aquifers. After sustained heavy rain over several days, a slower, mixing process dominated, blending older soil moisture with new rainfall on its way down.
What an Aquifer Is
An aquifer is a body of rock or sediment that holds groundwater and allows it to flow. Not all underground rock qualifies. The rock needs two properties: porosity (enough open space to store water) and permeability (those spaces need to be connected so water can move between them). Coarse materials like sand and gravel tend to be highly permeable. Clay, by contrast, can hold a lot of water in its pores but grips it so tightly that the water barely moves. A thin layer of water clings to each mineral grain due to electrical attraction, and if the connections between pores are smaller than that clinging layer, water is effectively trapped.
There are three main types of aquifers:
- Unconfined aquifers extend up to or near the land surface. The water table marks their upper boundary, and they’re in direct pressure contact with the atmosphere. These are the most common and the easiest to access with shallow wells.
- Confined aquifers are sandwiched between layers of impermeable rock or clay. Because they’re sealed off, the water inside is often under considerable pressure, sometimes from being recharged at a higher elevation. When that pressure is high enough, drilling into the aquifer sends water flowing to the surface without any pumping. These are called artesian wells.
- Perched aquifers are small, isolated pockets that sit above the main water table. They form when a layer of impermeable material (like a lens of clay) traps water above it, creating a “mound” of groundwater separated from the deeper saturated zone by dry rock below.
How Fast Groundwater Moves
Groundwater moves, but slowly. Unlike a river flowing at miles per hour, groundwater creeps through pore spaces at rates that can range from a few feet per year to a few feet per day, depending on conditions. The flow is driven by gravity and pressure differences, moving from areas of higher pressure to lower pressure. The speed depends on the permeability of the rock and the steepness of the pressure difference (called the hydraulic gradient). In practical terms, water moves faster through loose gravel than through dense sandstone, and faster down a steep underground slope than across a flat one.
This slow movement has consequences. It means groundwater can take years, decades, or even thousands of years to travel from its recharge area to where it’s eventually pumped out or flows into a spring. It also means that once groundwater is contaminated, cleaning it up is extremely difficult and slow.
What’s Dissolved in Groundwater
As water moves through soil and rock, it dissolves minerals along the way. The most common dissolved substances are calcium, magnesium, sodium, potassium, bicarbonate, sulfate, chloride, nitrate, and silica. This is why well water often tastes different from treated city water and why it can leave mineral deposits on fixtures.
The concentration of dissolved minerals varies enormously. Water that has traveled through limestone picks up calcium and magnesium, making it “hard.” Water near agricultural areas or urban development often carries higher levels of dissolved solids because fertilizers, road salt, and other chemicals leach into the ground. According to U.S. Geological Survey data, concentrations above 500 milligrams per liter give water a noticeably salty taste. The highest groundwater mineral concentrations in the U.S. have been measured in aquifers beneath the Denver Basin, the High Plains, and the desert basins of the Southwest.
How People Access Groundwater
Wells are the primary way humans tap into groundwater. A standard well is a hole drilled or dug down past the water table into an unconfined aquifer. A pump then lifts the water to the surface. These wells depend entirely on mechanical pumping because there’s no natural pressure pushing the water up.
Artesian wells work differently. Because confined aquifers are under pressure from the weight of overlying rock and from water entering the aquifer at higher elevations, the water naturally rises up the well shaft. In some cases it flows freely at the surface with no pump needed at all. Many artesian wells still use pumps to increase flow, but their energy costs are lower because nature does part of the work.
What Happens When Too Much Is Pumped
Groundwater isn’t an unlimited resource. When extraction outpaces natural recharge, water levels drop and the ground itself can physically sink. This process, called land subsidence, happens because removing water from the pore spaces reduces the pressure holding up the layers of sediment above. Fine-grained materials like clay compact under the weight, and that compaction is largely permanent.
The Mekong Delta in Vietnam illustrates the scale of this problem. Over 25 years of increasing groundwater pumping, water levels in some aquifers dropped more than 20 meters (65 feet), with hotspots near Ho Chi Minh City seeing drops of 40 meters. The result: an estimated average of 18 centimeters (7 inches) of land sinking across the delta, with some areas sinking more than 30 centimeters. Present-day sinking rates in Ho Chi Minh City reach about 7.3 centimeters per year, which is roughly 25 times faster than local sea level rise. For a low-lying delta region, this combination of sinking land and rising seas creates a serious flooding threat.
Similar problems have appeared worldwide. Parts of California’s Central Valley, Mexico City, and Jakarta have all experienced significant subsidence from groundwater depletion. Once the ground compacts, the aquifer’s storage capacity is permanently reduced, meaning it can never hold as much water as it once did, even if pumping stops entirely.