Groundwater is the source of roughly 40% of water used for public supplies and about 39% of water used for agriculture in the U.S. According to USGS, it moves slowly, typically at rates of 3-25 inches (7-60 cm) per day in an aquifer. This means water could remain in an aquifer for hundreds, even thousands of years.
Why it’s important
Groundwater is critically important for both people and the environment because it is one of the most widely used and relied upon natural resources. Groundwater provides nearly half of all drinking water worldwide and supplies around 90% of rural domestic water.
Groundwater feeds rivers, wetlands and lakes, with many aquatic and terrestrial ecosystems depending on it to maintain flow and habitat. Industries can rely on groundwater for cooling, processing and manufacturing due to its consistent quality and temperature.
Unlike surface water, groundwater can act as a natural buffer during droughts. Communities often turn to groundwater when rivers and reservoirs run low.
What is groundwater?
Groundwater permeates the ground to fill cracks and other openings in beds of rock and sand that makes up a saturated zone beneath the land surface. The upper surface of this zone is called the water table. Rain that is absorbed into land moves through soil and sand until it reaches the water table. Groundwater does not form underground lakes and streams, instead soil, sand and rock hold onto the water like a sponge. If the groundwater flows naturally out of the rocky material, or if it can be removed by pumping, it is called an aquifer.
What is a water table?
The water table is the upper surface of the saturated zone in the ground, according to USGS. Above the water table sits the unsaturated zone where the soil and rock contain both air and water. The level of water in the table is dynamic – it fluctuates with recharge from precipitation or infiltration and natural drainage into streams and rivers.
What is an aquifer?
An aquifer is a water-bearing geologic formation that is saturated with groundwater and can yield usable amounts of water to wells or springs. It must have sufficient permeability and saturation to transmit water at rates useful for extraction or natural discharge to springs or wells.
What is the simple definition of groundwater?
Simply put, groundwater is the water that fills the cracks and spaces in soil, sand and rock beneath the Earth’s surface. It comes mainly from rain and snow that soaks into the ground and is stored in pores and fractures underground.
Where is groundwater found?
- Unsaturated zone (vadose zone): Water is present, but the spaces in soil and rock also contain air.
- Saturated zone: All the pore and fractures are completely filled with water. The top of this zone is called the water table.
- Aquifers: Groundwater is often stored in aquifers, which are underground layers of permeable rock, sand or gravel that can hold and transmit water.
How does groundwater form?
- Precipitation: Rain, snow, sleet or hail that falls to the ground.
- Infiltration: Some of this water soaks into the soil rather than running off into rivers or evaporating.
- Percolation: The water moves downward through the soil, sand and porous rock, filtering along the way.
- Recharge: Eventually, the water reaches deeper layers where all the spaces are filled with water.
What are examples of groundwater?
Examples of groundwater in different contexts include:
- Wells: Water pumped from residential, agricultural or municipal wells comes from aquifers.
- Springs: Natural springs occur where groundwater flows out of the ground.
- Aquifers: Large underground layers of sand, gravel or rock that store and transmit water.
- Subsurface storage: Groundwater that slowly discharges into rivers, lakes, wetlands or coastal areas feeds surface water during dry periods.
- Confined and unconfined systems: In confined aquifers, water is trapped between impermeable layers, sometimes under pressure, as in artesian wells. In unconfined aquifers, the water table is open to recharge from the surface.
How is groundwater formed?
The water begins its journey high above the surface, in the form of rain, snow, or other precipitation. Some of that water flows over land into rivers and lakes, but a portion seeps into the soil, slowly working its way downward through layers of sand, gravel and rock.
As it travels, the water filters naturally, collecting in the pores and cracks of underground formations known as aquifers. Over time, these saturated zones fill with water that can be tapped by wells, emerge in springs, or feed streams and wetlands during dry periods. This process, called recharge, is how nature continually replenishes the groundwater that communities, farmers and ecosystems rely on every day.
Understanding this journey is crucial for stormwater professionals. By managing runoff, promoting infiltration and protecting recharge areas, we can help maintain the delicate balance of groundwater resources, ensuring clean water is available for both people and the environment.
