Groundwater is water that exists beneath the Earth’s surface within the pores, voids, and fractures of soil, sediment, and rock formations, where it occupies the saturated zone below the water table. In stormwater management and hydrology, groundwater represents a critical component of the hydrologic cycle and serves as a major source of baseflow to streams, as well as a vital supply for drinking water and irrigation.
Groundwater originates primarily from precipitation that infiltrates into the ground and percolates downward through the unsaturated zone until it reaches the saturated zone, where all available pore spaces are filled with water. The upper surface of this saturated zone is known as the water table, which can fluctuate based on seasonal conditions, precipitation patterns, and withdrawals.
Groundwater is stored and transmitted through geologic formations known as aquifers. These formations vary in their ability to store and transmit water depending on their porosity and permeability. Coarse-grained materials like sand and gravel typically form productive aquifers, while fine-grained materials like clay restrict groundwater movement.
In the context of stormwater management, groundwater plays an important role in maintaining streamflow during dry periods, supporting wetlands and ecosystems, and influencing the effectiveness of infiltration-based practices such as infiltration basins and trenches. Recharge of groundwater occurs when stormwater infiltrates into the subsurface, making practices that promote infiltration valuable for sustaining groundwater levels.
However, groundwater is also vulnerable to contamination. Pollutants carried in stormwater, such as nutrients, metals, hydrocarbons, and pathogens, can infiltrate into the subsurface and degrade groundwater quality if not properly managed. Because groundwater moves slowly and is difficult to remediate once contaminated, protection through proper stormwater design and land use practices is essential.
Groundwater is a foundational element of watershed systems, linking surface water processes with subsurface storage and flow, and playing a key role in both water supply and ecological health.