What is groundwater?

Groundwater is water that collects or flows beneath the Earth’s surface, percolating through and filling the spaces in soil, sediment, and porous rocks, as well as fractures in hard rock. Groundwater originates from rain, melting snow and ice, irrigation, surface water, and infiltrated stormwater. It fills aquifers so that wells can withdraw water, and emerges at the land surface as springs. Over 60 percent of Washington residents get their drinking water from groundwater. Groundwater is also used for irrigation, agriculture and more.

Why does it matter?

Groundwater provides drinking water through wells and base flows for streams and rivers. Base flows are important for fish during dry summer months.

Various chemicals used or dumped on the ground’s surface can infiltrate into the ground, or leak from septic systems, and end up in groundwater. Once these pollutants enter underground waters, they can be difficult to detect and remove, polluting drinking and irrigation water. Because groundwater supplies many streams, especially in the late summer, pollutants in groundwater can end up in those rivers and streams, harming both humans and wildlife alike.

Groundwater and water quality

Water quality is of particular importance when managing groundwater. A large portion of Pierce County has been designated a Sole Source Aquifer by the Environmental Protection Agency and most residents rely on groundwater as their only source of drinking water. Preventing contamination of groundwater is a significant health issue. Ironically, discharging waste and stormwater to the ground is a common practice for removing pollutants. Soil can provide excellent filtration. 

Another threat to groundwater is pollution by fertilizers, pesticides, and waste from septic tanks, all of which can seep down into aquifers from the soil surface.

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Tacoma, WA 98409-7322

This landform cross-section shows the role of groundwater in the water cycle.