What Is Groundwater?

Groundwater Definition

Groundwater is stored in—and moves slowly through—underground geological formations called aquifers. Aquifers are typically made up of gravel, sand, sandstone, limestone, or fractured rock. Water can move through these materials because they have well-connected spaces that make them permeable.

Despite what many people picture, groundwater does not flow through underground rivers or lakes. It fills the tiny pore spaces and fractures within rock and soil, more like water saturating a sponge than water flowing through a pipe. The top of the zone where those spaces are filled with water is called the water table. Above the water table, the pore spaces contain a mix of air and water; below it, they are fully saturated.

Groundwater moves slowly—often just inches to a few feet per day, depending on the size and connectivity of the spaces in the surrounding material. That slow movement through soil and rock is one reason groundwater is naturally filtered and generally more stable in quality than surface water. It is also why contamination, once it reaches an aquifer, can be difficult to reverse.

Groundwater supplies are replenished—or recharged—by rain and snowmelt that seeps down through the soil and into the aquifer below. In some regions, groundwater is used faster than it is naturally replenished, leading to declining water levels over time.

Where is Groundwater Found?

Groundwater is stored in aquifers—underground formations of soil, sand, gravel, or porous rock that hold and transmit water. Aquifers exist throughout the United States, beneath virtually every landscape. The depth, extent, and productivity of aquifers vary significantly by region and local geology.

Types of aquifers

There are two main types of aquifers relevant to private well owners:

How deep is groundwater?

There is no single answer—groundwater depth varies widely by region, local geology, and seasonal conditions. In some areas, the water table sits just a few feet below the surface. In others, it may be hundreds of feet down. A private well must be drilled deep enough to reliably reach the saturated zone and maintain access even during dry periods when the water table drops.

What are aquifers made of?

The most productive aquifers are made of materials with large, well-connected pore spaces that allow water to move through—gravel, sand, sandstone, and fractures or solution openings in limestone or granite. Dense, unfractured rock and clay hold very little groundwater and cannot easily transmit water. The composition of the aquifer material also affects water quality—groundwater dissolves minerals from the rock it passes through, which gives it its unique chemical character.

How Does Groundwater Form?

Groundwater is part of the hydrologic cycle—the continuous movement of water through the atmosphere, land surface, and underground. It begins through a process called recharge.

When precipitation falls to Earth’s surface, it follows one of three paths:

1. 1Some evaporates from the soil or through vegetation and returns directly to the atmosphere.
2. 2Some runs off along the land surface into streams, rivers, lakes, and oceans.
3. 3Some infiltrates—it soaks into the ground, moving downward through the soil until it reaches the water table and enters the aquifer below, becoming groundwater.

Once underground, groundwater continues moving slowly, following gravity and pressure gradients. Eventually, it may discharge naturally at springs, seeps, or stream beds—or it may remain underground for decades, centuries, or longer before reaching a natural outlet or being pumped out by a well.

About a quarter of all U.S. rainfall eventually becomes groundwater. The time it takes for precipitation to travel from the surface to the water table—and then through the aquifer—depends on local geology and depth, and can range from days to thousands of years.

How is Groundwater Used?

Groundwater is one of the most important natural resources in the United States. It supports drinking water, agriculture, and industry.

Drinking water

Groundwater provides about 44 percent of U.S. drinking water—either through a public water system or a private well. The USGS reports that groundwater provides drinking water for more than 90 percent of the rural population. More than 13 million U.S. households rely on private water wells.

Agriculture

Irrigation is the largest single use of groundwater in the United States. According to the USGS, approximately 57 billion gallons of groundwater are pumped daily for agricultural irrigation. The High Plains Aquifer—the nation’s largest, underlying roughly 174,000 square miles from Texas to South Dakota—supplies more than 90 percent of its pumped water for crop production. Representing about one-third of all U.S. irrigated agriculture, it generates approximately $20 billion annually in food and fiber.

Industry

Groundwater supplies approximately 18 percent of all water used by U.S. industries. It is used extensively in industrial cooling, manufacturing processes, and commercial operations. The United States uses approximately 82 billion gallons of fresh groundwater daily across all uses.

Is Well Water Groundwater?

Yes. Water from a private water well is groundwater. When a contractor drills a well, the casing penetrates below the water table into the saturated zone. A pump draws groundwater up through the casing and into your home’s plumbing system.

This has two important implications for private well owners:

Natural filtration: As precipitation moves downward through layers of soil and rock on its way to the aquifer, physical particles are filtered out. This is why groundwater is generally clearer and more stable in quality than surface water. The rock and soil act like a slow, natural filter.

But not automatically pure: Filtration removes physical particles, not dissolved chemicals. Groundwater can contain naturally occurring substances, such as arsenic, radon, manganese, and iron, that are present in the surrounding geology. Human activities such as agricultural runoff, failing septic systems, underground storage tank leaks, and industrial releases can also introduce nitrates and other contaminants that reach the aquifer.

Groundwater Quality: What Well Owners Need to Know

Groundwater quality varies by region, local geology, and nearby land use. Most aquifers protected by deep rock and soil layers yield clean, safe water—but no groundwater source is guaranteed to be contaminant-free without testing.

Naturally occurring contaminants

Groundwater dissolves minerals from the rock it passes through. Depending on local geology, water in your aquifer may contain iron, manganese, arsenic, fluoride, hardness-causing minerals (calcium and magnesium), or naturally occurring radioactive elements such as uranium. These are not the result of pollution—they are part of the aquifer’s natural chemistry. Some exist at harmless levels; others require treatment to meet safe drinking water standards.

Human-caused contamination

Agricultural chemicals, septic waste, petroleum products, road salt, and industrial discharges can infiltrate groundwater. Shallow, unconfined aquifers are more vulnerable to surface contamination than deep, confined aquifers.

What well owners should do

NGWA recommends that private well owners test their water at minimum once a year—and conduct additional targeted testing if the property is near agricultural operations, gas stations, industrial facilities, known spill sites, or if the well has ever flooded. A licensed water well contractor can also inspect your well’s physical condition to ensure the casing and wellhead are properly sealed and protected.

Threats to Groundwater: Depletion and Contamination

Groundwater is a renewable resource, but it is not inexhaustible. Two main threats affect groundwater supplies across the United States.

Groundwater Fast Facts

Frequently Asked Questions About Groundwater