Groundwater is the fresh water found beneath the Earth’s surface, stored within the geological layers of soil and rock. This subsurface water constitutes almost all of the planet’s liquid freshwater that is not frozen in ice caps and glaciers. It is an important resource, serving as the source for approximately 50% of municipal, domestic, and agricultural water supplies in the United States. Globally, over 2.5 billion people depend exclusively on this hidden reservoir for their water needs.
The Journey Down: How Water Enters the Ground
The process of water reaching its underground storage begins with precipitation, such as rain or snowmelt, that lands on the ground surface. Water that does not immediately run off or evaporate begins to move downward in a two-stage process. The initial movement, known as infiltration, involves water passing through the soil surface and into the top layers of the earth.
As the water continues to be pulled down by gravity, it enters the process of percolation, which describes its movement through the deeper soil and rock layers. The maximum rate at which a soil can absorb water is called its infiltrability, which decreases as the soil becomes more saturated. This downward path takes the water through the Unsaturated Zone, also referred to as the Zone of Aeration.
In the Unsaturated Zone, the spaces between the soil and rock particles contain both air and water. The water here is often held tightly by capillary forces and is generally not accessible by wells. This zone is important for supporting plant life and agriculture.
The Mechanics of Underground Storage
The ability of the ground to store and transmit water depends on the physical properties of the geological material it passes through. Two specific properties, porosity and permeability, dictate where and how much water can be held.
Porosity is the measure of the empty space, or voids, within a rock or sediment. The amount of porosity determines the maximum volume of water a material can hold. Materials like unconsolidated sand and gravel often have high porosity. Conversely, dense rocks like granite have low porosity unless they are fractured.
Permeability is a separate property that describes how easily water can flow through the material. This is determined by how well the individual pore spaces are connected. A material can be highly porous, like clay, but have very low permeability because its pores are too small and poorly connected.
For water to be readily extracted, the geological layer must possess both sufficient porosity to hold the water and high permeability to allow it to move toward a well. If the pore spaces are isolated or blocked, the water is effectively trapped, even if the storage volume is high.
Aquifers: The Primary Underground Reservoirs
Water that successfully percolates downward eventually reaches the Saturated Zone, where all the spaces and fractures in the rock are completely filled with water. The water within this zone is formally known as groundwater. The upper surface of the Saturated Zone is called the Water Table, which fluctuates seasonally depending on the rates of recharge from precipitation.
A geological formation that contains and transmits significant quantities of this groundwater is called an Aquifer. Aquifers are the primary structures that function as underground reservoirs. They are typically composed of permeable materials like sand, gravel, or fractured rock. These formations allow water to be pumped to the surface for human use.
Aquifers are categorized based on their relationship to the layers directly above and below them. An Unconfined Aquifer, or water table aquifer, is the most common type, where the upper boundary is the water table and is in direct contact with the atmosphere through permeable material. It is recharged relatively quickly by water percolating from the surface directly above it.
In contrast, a Confined Aquifer is situated between two layers of impermeable or poorly permeable rock, often called aquicludes or aquitards. These confining layers prevent the direct vertical flow of water. The water in a confined aquifer is typically under pressure, which causes the water level to rise above the top of the aquifer when a well is drilled into it.