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An aquifer is a natural underground geological formation where large quantities of ground water fill spaces between rocks and sediment. To be considered an aquifer in Idaho, the geological formation must produce economically significant quantities of water to wells and springs. There are about 70 major aquifers in Idaho.

  • Types
  • Categories
  • Sole Source

An aquifer is defined according to the types of rocks and sediment in which it resides and the geologic conditions that surround it. These descriptions are not mutually exclusive. For example, an aquifer may be described as a confined, fractured basalt aquifer.

Confined—Overlain by one or more layers of impermeable rock or soil that restrict water to within the aquifer. The water is confined under pressure, and drilling into a confined aquifer releases that pressure and causes the water to rise. These wells are sometimes called artesian wells.

Unconfined—Not overlain by a layer of impermeable material. Water in a well will naturally stay at the level of the water table. As water is removed from the well, the water table is lowered, causing the surrounding ground water to flow toward the well.

Fractured—Water fills spaces produced by broken or shattered rock that would otherwise be impervious such as basalt or granite. Basalt is a fine-grained rock formed by the cooling and hardening of volcanic material. It may contain fractures through which water easily moves. Sedimentary deposits between the basalt layers, also provide material for water storage or movement.

Basalt—Idaho's basalt aquifers underlie the Eastern Snake River Plain Aquifer the Lewiston-Moscow basin, and the Weiser area.

Sedimentary—Located in sedimentary materials, such as loose gravels and sands. The sediments and rocks comprising these aquifers were loosely deposited over time by air, water, or glacial activity on the earth's surface. As more material was deposited, these sediments and rocks generally remained in a loose configuration with space between each other to hold water. Northern Idaho’s Spokane Valley-Rathdrum Prairie Aquiferis a sedimentary aquifer.

Perched—A small aquifer that is separated from a main aquifer below it by an impermeable layer of rock or soil and an unsaturated zone (i.e., an area where air fills most of the spaces in the soil and rock).

Idaho’s aquifers are categorized based on the vulnerability of the ground water, existing and future beneficial uses such as domestic, industrial, agricultural, or aquaculture water supply, existing water quality, and social and economic considerations. Idaho's "Ground Water Quality Rule" (IDAPA defines three aquifer categories in Idaho and each receives a different level of protection.

Sensitive Resource - Sensitive resource aquifers require the strongest level of protection. The Spokane Valley-Rathdrum Prairie Aquifer is the only aquifer designated as a sensitive resource aquifer in Idaho. Standards stricter than those outlined in DEQ’s Ground Water Quality Rule can be applied to sensitive resource aquifers.

General Resource - General resource aquifers are protected by the standards in DEQ’s Ground Water Quality Rule. All aquifers in Idaho other than the Spokane Valley-Rathdrum Prairie Aquifer are general resource aquifers.

Other Resource - Other Resource aquifers require the lowest level of protection and may have standards that are less strict than those in the Ground Water Quality Rule.

A sole source aquifer is an aquifer that has been designated by the United States Environmental Protection Agency (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, Spokane Valley-Rathdrum Prairie Aquifer, and Lewiston Basin Aquifer—are classified as sole source aquifers. View a map of Sole Source Aquifers in EPA Region 10 (including Idaho).

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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 southeastern 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.

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 Rathdrum Prairie 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.

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.

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