Groundwater is water located beneath the Earth’s surface, filling spaces within porous rock formations and soil. It accounts for approximately 99% of the planet’s liquid freshwater. Though unseen, groundwater plays a part in the continuous movement of water, known as the water cycle. This reservoir interacts with surface water bodies and the atmosphere, making it an interconnected component of Earth’s natural systems.
How Water Becomes Groundwater
Water enters the ground through infiltration, where precipitation soaks into the soil surface. This water moves downward through the unsaturated zone, also known as the vadose zone, where pores in the soil and rock are partially filled with water and air. Gravity pulls the water deeper until it reaches the water table, the boundary between the unsaturated and saturated zones.
Below the water table, all spaces in the soil and rock are completely filled with water. This process is called groundwater recharge. Recharge occurs when precipitation exceeds evaporation, often in cooler, wet seasons or during prolonged rainfall. The rate of recharge depends on factors like soil properties, water table depth, and precipitation intensity and duration.
Where Groundwater Resides and How It Moves
Once below the water table, groundwater is stored in geological formations called aquifers. These formations, made of materials like sand, gravel, or fractured rock, possess porosity and permeability to hold and transmit water. Porosity refers to the open space within the rock or soil that stores water, while permeability describes how easily water flows through those interconnected spaces.
Groundwater moves slowly through aquifers, driven by gravity and differences in water pressure, known as the hydraulic gradient. This movement is not like an underground river, but rather akin to water seeping through a sponge’s tiny pores and cracks. Flow rates vary significantly, from a few feet per day to a few feet per year, depending on aquifer permeability and hydraulic gradient steepness. Groundwater generally flows from areas of higher hydraulic head to lower hydraulic head, meaning it can move downward, laterally, or even upward.
How Groundwater Rejoins Surface Water
Groundwater eventually re-enters the visible water cycle through discharge. This occurs naturally through springs, where the water table intersects the land surface. Groundwater also provides baseflow to rivers and streams, maintaining their flow during dry periods, and seeps into lakes and wetlands, sustaining these ecosystems. Discharged groundwater can also make its way to oceans, contributing to coastal waters.
Human activities also contribute to groundwater discharge, primarily through water extraction via wells. Pumping from wells near streams, for instance, can reduce stream flow as groundwater is drawn to the surface for various uses. This extraction represents another pathway for groundwater to re-enter circulation.
The Importance of Groundwater
Groundwater is important for both human societies and natural ecosystems. It serves as a major source of drinking water, supplying half of the U.S. population and contributing to the daily water needs of 2.5 billion people globally. Beyond drinking, groundwater is used extensively for irrigation in agriculture, accounting for 43% of all water used for this purpose worldwide, and also supports industrial processes.
Ecologically, groundwater sustains wetlands and provides baseflow to rivers and streams, particularly during dry seasons when surface water sources diminish. It supports diverse aquatic and terrestrial ecosystems, influencing plant distribution and forming unique habitats like caves and oases in arid regions. The availability and quality of groundwater directly impact the health and functioning of many natural environments.