An aquifer is a natural underground area where large quantities of ground water fill the spaces between rocks and sediment. According to Idaho's Ground Water Quality Rule (IDAPA 58.01.11.007.02: pdf on Department of Administration Web site), to be considered an aquifer in Idaho, the area must produce "economically significant quantities of water to wells and springs."

In an aquifer, ground water can move sideways, up, or down in response to gravity, differences in elevation, differences in pressure, and differences in the physical properties of the aquifer. Depending on the aquifer, the water can move from very fast (as much as hundreds of feet per day in fractured rock aquifers) to very slow (as little as a few feet per year in very fine grained sedimentary aquifers). Read more on the water cycle and ground water movement.

An aquifer is defined according to the types of rocks and sediment in which it resides and the geologic conditions that formed or surround it. Just a few of the ways an aquifer can be described include:

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

• Unconfined—An aquifer that is not overlain by a layer of impermeable rock or soil. Water in a well will naturally stay at the level of the water table. As water is removed from the well, the water table at that place is lowered, causing the surrounding ground water to flow toward the well.

• Fractured—An aquifer where the water fills spaces produced by broken or shattered rock that would otherwise be impervious, such as basalt or granite.

• Sedimentary—An aquifer located in sedimentary materials, such as loose gravels and sands.

• 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 (an area where air fills most of the spaces in the soil and rock).

These categories are not mutually exclusive. For example, an aquifer may be described as a confined, fractured basalt aquifer.

Approximately 70 major aquifers have been identified in Idaho. They are found throughout the state. Below are descriptions of some of the major types of aquifers found in Idaho. View a map of Idaho's major aquifers (pdf 213 kb, 1 page).

Valley fill aquifers are generally found in the state's intermountain valleys. The sediments and rocks comprising these aquifers were loosely deposited some time ago 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 many spaces between each other to hold water. One example of a valley fill aquifer is the Spokane Valley-Rathdrum Prairie Aquifer located in northern Idaho.

Fractured basalt aquifers are found in southern Idaho, the Lewiston-Moscow basin, and the Weiser area. Basalt is a fine-grained rock formed by the cooling and hardening of volcanic material. It tends to contain many fractures through which water easily moves. In addition, thin areas of sediment have been deposited between the basalt layers. These areas provide additional space for water storage or movement. Idaho's major basalt aquifer is the Eastern Snake River Plain Aquifer.

Sedimentary and volcanic aquifers are found primarily in southern Idaho. They typically contain a mixture of the loose gravels, sands, silts, and clays that comprise valley fill aquifers, intermixed with areas containing basalt, shale, and sandstone rocks that have a more consistent structure. One example of this type of aquifer is the Boise Valley Aquifer.

Aquifers in Idaho are categorized based on vulnerability of the ground water, existing and future beneficial uses of the ground water (such as domestic, industrial, agricultural, or aquaculture water supply), existing water quality, and social and economic considerations. Idaho's Ground Water Quality Rule (IDAPA 58.01.11.150.02: pdf on Department of Administration Web site) defines three aquifer categories in Idaho; each receives a different level of protection.

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 (pdf on Department of Administration Web site) can be applied to sensitive resource aquifers.

General Resource aquifers are protected by the standards in DEQ's Ground Water Quality Rule (pdf on Department of Administration Web site). All aquifers in Idaho other than the Spokane Valley-Rathdrum Prairie Aquifer are general resource aquifers.  

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 (pdf on Department of Administration Web site). There are currently no aquifers in Idaho designated as other resource aquifers.

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. A sole source aquifer is an aquifer that has been designated by the U.S. 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. Read more about Idaho's sole source aquifers.

The program for protecting sole source aquifers was established by the Safe Drinking Water Act of 1974. The EPA designates an aquifer as a sole source based upon a petition from an individual, company, association, or government entity. Once an aquifer has been designated as sole source, federal financial assistance cannot be used for any project that might contaminate the aquifer. Federal money can be used to help plan or design a project in such a way that it will not contaminate the aquifer. Learn more about EPA's Sole Source Aquifer Protection Program.