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.
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).