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Nuclear Waste Program

Groundwater Primer

Groundwater is just one part of the continuous water cycle as water moves on, in, and around planet Earth. Groundwater is defined as water occupying the pore space between sand and gravel below the ground surface.

The area between the ground surface and the area where all pores are saturated is called the vadose zone. In many places at Hanford, the vadose zone is contaminated. In some locations, the contaminated soil may be dug up and moved to a safe storage area: the Environmental Restoration Disposal Facility. In others, the contamination is too deep to dig up. We face difficult decisions about how to deal with those areas and prevent further contaminants from reaching groundwater. Currently, 60 square miles of groundwater on the Hanford Site are contaminated above drinking water standards.

Hanford Groundwater Contamination Sources

Underground Storage Tanks

There are 177 tanks at Hanford storing more than 55 million gallons of high-level waste. Sixty-seven single shell tanks are known, or suspected, to have leaked. Past releases have amounted to about 1 million gallons.

Pits & Burial Grounds

Solid and liquid wastes in barrels were buried in trenches, pits, or unlined landfills. As the containers break down, contaminants enter the soil.

Injection Wells

Also known as reverse wells, injection wells served as disposal areas for liquid contaminants by pumping them directly back into the soil.

Cribs, Ponds & Trenches

Cooling and waste water was directed to storage ponds, trenches, cribs or French drains (perforated pipes allowed liquid to release into rock-lined, soil-covered trenches).

Waste Discharge from Reactors & Processing Plants

Some facilities at Hanford disposed of waste directly to the soil outside the facility, and accidental spills also occurred.

Graphic showing sources of Hanford groundwater contamination 

The water table is the top of the saturated area, which also is known as an aquifer. Aquifers may be perched, or separated from one another, by clay or rock layers through which water will not flow unless a fracture is available.

Some groundwater was trapped in the ground over the course of geologic time, which resulted in deep aquifers. Some water is near the surface and must be recharged regularly to maintain its flow. At Hanford, the stratigraphy or layers of rock, such as sand and clay determine where water is trapped.


The hyporheic zone is an area beneath and parallel to a surface water source/body, such as a river, stream, or lake, comprised of the sediments in which shallow groundwater and surface water mix and flow. The flow in this zone is an important factor in surface and groundwater interchanges, affecting things like fish spawning, pollutant concentrations, and water purification processes.

Hanford groundwater used to be closer to the surface because manufacturing weapons materials required millions of gallons of water.  Much of that water was discharged to the ground, recharging the groundwater at a faster rate than would occur naturally through precipitation.