Groundwater: What It Is

Groundwater

Groundwater is freshwater stored underground in the tiny pores, cracks, and open spaces within soil, sand, and rock. It represents the vast majority (95%)  of Earth’s available liquid freshwater—far more than the water visible in lakes and rivers.

A common myth is that groundwater usually exists as underground rivers or cavernous lakes. In most places, it behaves more like water held in a giant underground sponge, saturating the porous materials beneath our feet.

Underground Anatomy

A cross-section of the ground reveals several layers, each defined by how it stores and transmits water:

• The Unsaturated Zone: This is the top layer of earth closest to the surface. The spaces between dirt and rock here contain mostly air, with some moisture that feeds plant roots.
• The Water Table: The boundary where the unsaturated zone ends and the fully saturated ground begins. Depending on local geology and rainfall, the water table may be near the surface or hundreds of feet below it. In parts of Prince William County, it can be relatively shallow.
• The Saturated Zone: Located beneath the water table, this is the region where every single crevice, crack, and pore is completely filled with water. This trapped water is what we call groundwater.

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What Is an Aquifer?

When an underground geological formation holds enough water and is permeable enough to allow that water to move, it is called an aquifer. Aquifers are typically composed of gravel, sand, sandstone, or fractured limestone.

• Porosity vs. Permeability: To be a good aquifer, rock needs high porosity (plenty of empty space to hold water) and high permeability (those spaces must be interconnected so water can flow through them). Clay, for example, is porous but has very low permeability, meaning water gets trapped and cannot flow.

·         Slow-Moving Flow: Groundwater does not rush like a surface stream. It creeps through pore spaces and fractures, often moving only inches or feet per day. Because of this slow movement, water in deep aquifers may remain underground for thousands of years. In the Piedmont section of Prince William County, groundwater is often much younger—ranging from months to roughly a decade.

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The Recharge and Discharge Cycle

Groundwater is an active, shifting stage of the global hydrologic (water) cycle:  

1.      Recharge: Rain and melting snow soak into the ground, and some of that water travels downward to the water table. The portion that survives plant uptake, evaporation, and shallow soil storage is called recharge. Forest loss and impervious surfaces—such as roads, buildings, patios, and even compacted suburban lawns—reduce the amount of water that can recharge groundwater.

2.      Discharge: Groundwater naturally moves from higher-pressure areas toward lower-pressure areas. Where the land surface intersects the water table, groundwater can emerge as springs or seep into wetlands, lakes, and streams. This groundwater contribution, known as baseflow, helps sustain streams and rivers during dry periods.