Groundwater is caused by precipitation soaking into the ground and collecting in the spaces between rocks and soil below the surface. It accounts for 99% of all liquid freshwater on Earth, making it by far the planet’s largest accessible freshwater reserve. The process that creates it is straightforward: rain or snow falls, water seeps downward through soil and rock, and gravity pulls it into underground storage zones called aquifers.
How Water Gets Underground
The journey from raindrop to groundwater happens in two stages. First, water infiltrates the soil by passing through the surface layer. Then it percolates, moving deeper through the soil and rock under the pull of gravity. Plants and shallow root systems absorb much of what enters the top layer, and some water evaporates back into the atmosphere. Only the water that makes it past these obstacles continues downward until it reaches a zone where every gap between rock particles is completely saturated.
The top of that saturated zone is the water table. Everything below the water table is groundwater. Everything above it is unsaturated, with air still occupying some of the spaces between soil grains. The water table isn’t fixed at one depth. It rises after heavy rain and falls during dry periods, and it responds to how much water humans pump out of the ground.
What Feeds Groundwater Supply
The primary source of groundwater is precipitation: rain and snowmelt that percolates downward over weeks, months, or years. But precipitation isn’t the only contributor. Rivers, lakes, and streams can also lose water to the ground, especially in areas where the water table sits lower than the bottom of the surface water body. Irrigation water that isn’t taken up by crops seeps downward too, sometimes becoming a significant source of recharge in agricultural regions.
Snowmelt plays an especially important role in colder climates. As snowpack melts gradually in spring, it delivers a slow, steady supply of water to the soil surface, giving it more time to soak in rather than running off. This slow release is one reason mountainous regions often serve as critical recharge zones for aquifers hundreds of miles downstream.
Why Soil Type Matters
Not all ground lets water through at the same rate. Sandy soil can absorb 30 millimeters of water per hour or more, while clay allows only 1 to 5 millimeters per hour. That difference is enormous. Sandy loam falls in the 20 to 30 mm/hour range, standard loam handles 10 to 20, and clay loam sits at 5 to 10. These rates determine how much rainfall actually reaches groundwater versus running off the surface into streams and rivers.
When rain falls faster than the soil can absorb it, the excess flows across the surface as runoff. This is why a gentle, sustained rain recharges groundwater far more effectively than a sudden downpour of the same total volume. The downpour overwhelms the soil’s capacity, and most of the water never makes it underground.
How Aquifers Store Water
Once water percolates deep enough, it collects in aquifers: layers of permeable rock, sand, or gravel that hold and transmit water. There are two main types, and they behave quite differently.
An unconfined aquifer sits relatively close to the surface with no impermeable barrier above it. Its upper boundary is the water table itself, which rises and falls freely with rainfall and usage. These aquifers respond quickly to drought because they depend directly on recent precipitation. They’re also more vulnerable to contamination from the surface.
A confined aquifer is sandwiched between layers of impermeable material like clay or shale, both above and below. Because the water is trapped under pressure, it sometimes rises on its own when a well punctures the confining layer. These deeper aquifers are more protected from surface pollution, but they recharge far more slowly. In arid regions, the water in confined aquifers can be thousands or even hundreds of thousands of years old.
How Old Groundwater Can Be
Groundwater age varies wildly depending on depth and geology. In shallow, unconfined aquifers with plenty of rainfall, groundwater is often just a few decades old. Scientists define “young” groundwater as water that entered the aquifer after 1950, identifiable by trace chemicals and isotopes released into the atmosphere by human activity during that era.
“Old” groundwater, by contrast, entered the aquifer before 1950 and is often more than 1,000 years old. In thick aquifers or arid regions where very little new water trickles in, groundwater can date back 30,000 years or more. The oldest groundwater on Earth, measured using isotopes that decay over extremely long timescales, is hundreds of thousands of years old. This ancient water is essentially fossil water, a nonrenewable resource on any human timescale.
What Groundwater Picks Up Along the Way
Water is an effective solvent, and as it moves through rock and soil, it dissolves minerals it encounters. The specific chemistry of groundwater depends on what types of rock it passes through and how long it stays in contact with them. Water flowing through limestone picks up calcium. Water in contact with feldspar minerals absorbs sodium and potassium. Iron, manganese, and sulfur compounds are common additions depending on local geology.
This is why well water tastes different in different places, and why some groundwater is naturally “hard” (high in dissolved calcium and magnesium) while other sources are soft. In most cases, these dissolved minerals are harmless or even beneficial. But in some geological settings, groundwater naturally contains arsenic, fluoride, or other elements at concentrations that create health concerns without treatment.
How Urbanization Reduces Groundwater
Pavement, buildings, and other impervious surfaces fundamentally disrupt the process that creates groundwater. A study modeling Los Angeles found that urbanization redirected up to half of infiltrating water in heavily developed watersheds, doubling the share of precipitation that became surface runoff from roughly 15% to 30%. In downtown LA, the amount of water stored in the soil column during a rain event dropped from over 200 millimeters under natural conditions to approximately 35 millimeters after development. Deep drainage, the water that would eventually become groundwater, declined by 47% across the study area and by 68% in the most urbanized watershed.
This pattern repeats in cities worldwide. Every parking lot, road, and rooftop intercepts rain that would otherwise soak into the ground. The water instead flows into storm drains and out to rivers or the ocean, bypassing the underground entirely. Some cities now use permeable pavement, rain gardens, and infiltration basins to counteract this effect, deliberately routing stormwater back into the soil to maintain groundwater levels.
Why Groundwater Levels Change
The water table is constantly shifting in response to the balance between what goes in and what comes out. On the input side, precipitation, snowmelt, and seepage from surface water all add to the supply. On the output side, natural discharge into springs and rivers, evaporation where the water table is close to the surface, plant roots pulling water up, and human pumping all remove it. In coastal areas, tides and wave action also influence groundwater levels.
When pumping exceeds recharge over long periods, the water table drops. Wells need to be drilled deeper, pumping costs rise, and in extreme cases aquifers compact permanently, causing the land surface above to sink. This land subsidence is irreversible because the compressed pore spaces that once held water can never reopen to their original size. Parts of California’s Central Valley have sunk nearly 30 feet over the past century from excessive groundwater pumping, a visible consequence of drawing out water faster than nature puts it back.