Department of
Environmental Quality

A sole source aquifer is an aquifer that has been designated by EPA as the sole or principal source of drinking water for an area. As such, a designated sole source aquifer receives special protection. EPA designates an aquifer as a sole source based upon a petition from an individual, company, association, or government entity. 

Three of Idaho's aquifers—the Eastern Snake River Plain Aquifer, the Spokane Valley-Rathdrum Prairie Aquifer, and the Lewiston Basin Aquifer—are classified as sole source aquifers. View a map of Sole Source Aquifers in EPA Region 10 (including Idaho).

Eastern Snake River Plain Aquifer

The Eastern Snake River Plain Aquifer was designated a sole source aquifer in 1991. It provides the sole source of drinking water for nearly 200,000 people in southeast and south central Idaho.

The aquifer stretches across much of south central Idaho and is Idaho's largest basalt aquifer, covering an area of approximately 10,800 square miles. In 1980 alone, around 630 billion gallons of water were withdrawn from the aquifer to irrigate approximately 900,000 acres of farmland. The aquifer also discharges nearly 2.6 trillion gallons of water each year to the Snake River. The ability to supply these large quantities of water makes it one of the most productive aquifers in the nation.

The most productive part of the aquifer is the upper 300 - 500 feet, where ground water flows the most rapidly (the total thickness of the aquifer is estimated to be more than 5,000 feet). In this upper portion, the water flows generally from northeast to southwest. The total ground water storage in the upper 500 feet of the aquifer is estimated at 200 to 300 million acre-feet, which is approximately the equivalent of Lake Erie.

Spokane Valley-Rathdrum Prairie Aquifer

Designated as a sole source aquifer in 1978, the Spokane Valley-Rathdrum Prairie Aquifer was the first aquifer in Idaho and the second in the nation to receive sole source designation. The aquifer originates at the southern end of Lake Pend Oreille in northern Idaho and extends west under the Rathdrum Prairie in Idaho and the Spokane Valley in Washington, underlying approximately 321 square miles of land.

The aquifer serves as the principal source of drinking water for more than 400,000 people. Because of this, the Spokane Valley-Rathdrum Prairie Aquifer is specially categorized by Idaho as a sensitive resource aquifer as well as being designated by the EPA as a sole source aquifer. Both of these designations afford the aquifer special protection. Through Idaho's sensitive resource designation, the aquifer cannot be degraded unless it is demonstrated that the change is a justifiable result of necessary economic or social development.

The aquifer is an unconfined, valley fill aquifer. This means no barrier limits or blocks the flow of water down into the aquifer from the surface. Because the rocks and sediments in the aquifer fit very loosely together, water moves relatively quickly through the aquifer. In some places, water has been estimated to move at a rate of 50 feet per day.

Lewiston Basin Aquifer

The Lewiston Basin Aquifer (previously called the Russell Aquifer) was designated a sole source aquifer in 1988. The aquifer provides all domestic water to Clarkston, Washington, and the Lewiston Orchards Irrigation District in Idaho, in addition to providing some domestic water for the city of Lewiston, Idaho.