Groundwater Resources
Idaho relies heavily on underground water supplies for agricultural, domestic and industrial uses. Groundwater is the largest reservoir of fresh water on the earth and is the predominant source of drinking water in many western states.

In 1995, groundwater provided more than 95% of Idaho’s drinking water from private and municipal wells. The amount of groundwater used in southeast Idaho, alone, was more than 800 million gallons per day.

Part of Idaho’s groundwater resource includes geothermal groundwater, or water naturally heated within the earth’s crust, that is available at or near the surface, either through wells or in natural springs.

Idaho has the third highest number of geothermal springs in the continental U.S.: some 258 active springs, with average water temperature of about 120 F, and individually as high as 203 F.

Idaho also has more than 600 geothermal water wells which provide heated water for recreation, aquaculture, and heating. Idaho's state capital building is heated with geothermal water, and Boise’s Warm Springs Avenue was the first geothermal heating district in The United States.

Areas of porous rock and sediment which hold water and give it up easily enough to provide water for a beneficial use are called aquifers (for drinking water and other non-thermal supplies). Where the water is heated geothermally, the groundwater resource is usually termed a geothermal reservoir.

Ground water issues forth in springs where water flows naturally from rock onto the surface of the land. Springs may seep from places where the water table intersects the land surface. Water may also flow out of the ground along fractures.

In most cases these springs contain water that has fallen upslope, been absorbed into the ground, and spent a few weeks to thousands of years traveling to the point of issue (the average amount of time spent in an aquifer by a water molecule called residence time).

Although most irrigation water today is provided from surface sources and wells, many an early homestead was located next to a spring.

Idaho has numerous aquifers which comprise an essential part of the state's overall water supply. Figure A shows the locations and extents of Idaho’s types of aquifers and Figure B shows some of the most important in the state.

Unconsolidated aquifers hold water in pore spaces between the grains of sand and gravel in loose sediment, and are commonly known as valley-fill aquifers (e.g. The Rathdrum Prairie, Payette River valley, and lower Portneuf River valley).

Those which hold water in the cracks and pore spaces of solid rock are classified as consolidated aquifers. Idaho has two major types of consolidated aquifers: those in basalt (e.g. the Eastern Snake River Plain and Clearwater Plateau), and those in other volcanic and sedimentary materials that have been compacted and solidified. The amount of water available in these consolidated aquifers usually depends upon the size, number, and interconnection of cracks in the rock.

Sedimentary/volcanic aquifers in Idaho contain a mixture of unconsolidated sedimentary material, sedimentary rock (sandstone and shale), and basalt (e.g. The Treasure Valley, Salmon Falls/Rock Creek).

The most famous aquifer in Idaho is that of the Snake River Plain, which controls the economy of much of southern Idaho north and west of Pocatello (Stearns and others, 1938). Three million acres of farmland on the Snake River Plain are irrigated, with about 1/3 of this from wells and the rest from canals. This extensive irrigation system is the primary reason that Idaho has the highest per capita water consumption in the U.S.

The Snake River aquifer is a complex system, with multiple layers of high permeability. It discharges 8 million acre feet of water per year in the famous Thousand Springs area on the north wall of the Snake River canyon from Twin Falls to Hagerman. Most of the commercially produced trout in the United States are grown there.

The Snake River Plain is underlain by fractured and rubbly basalt lava flows, which form a highly permeable aquifer. Interbeds between the basalt layers are mainly sand, silt and clay, with smaller amounts of volcanic ash. Within basalts, permeable zones are mainly the tops and bottoms of lava flows, with columnar jointing in between providing slower vertical transmission of water. Rhyolite that underlies the basalt does not have high permeability, as many of the pore spaces are filled with chemical precipitates.

Water which falls mainly as snow in the mountains north and east of the eastern Snake River Plain is absorbed into the basalt in many places along the northern margin of the plain. The most obvious is the Sinks of the Big Lost River east of Howe, where waters of the Big and Little Lost Rivers, and Birch Creek sink into the lava plateau.

Dams were put on the Snake River at Milner, Minidoka, and American Falls in the early 1900s. The extensive irrigation essentially recycles the Snake River water, drawing it out of the river, and returning it to the river or the aquifer downstream. When the amount of irrigation on the Snake River Plain is reduced, output from the springs goes down also. The level of the aquifer rose dramatically after initiation of irrigation, but the level stabilized after a new equilibrium was reached. In the last 25 years, the level has begun to fall slowly.

Fundamentally, the Snake River Plain Aquifer is so large, with so much water running through it, and with residence times on the order of 100s of years, that it will be hard for man's efforts to change it much. Point sources of pollution certainly exist, but the dilution factor prevents them from becoming regional problems.

Water quality of the Snake River Plain aquifer is adversely affected by several human activities, most importantly agriculture. Runoff from fertilizer, feedlots, and potato processing plants has produced local acute pollution of the aquifer.

Another potential source of pollution is the Idaho National Engineering and Environmental Laboratory. Voluminous and expensive monitoring programs are being conducted to determine the extent of INEEL-caused pollution. The bottom line appears to be that the dry climate on the Snake River Plain combined with the huge volume of water in the Snake River Plain aquifer act to limit the amount of radionuclides that have reached the aquifer and then to dilute them below detection limits.

Idaho’s major aquifers have been prioritized based on their vulnerability to pollution by the Idaho Department of Health and Welfare, the Idaho Division of Environmental Quality, and the Idaho Department of Water Resources. Aquifers are vulnerable where groundwater is shallow or where soils are thin or very permeable. Also, the potential for contamination is greater where considerable water is applied to the land surface from precipitation or irrigation water and where population density and intensity of groundwater use are greatest.

The ranking of Idaho's most important aquifers from most to least vulnerable is shown in Figure B and is as follows:

1.Boise Valley
2.Snake River Plain
3.Rathdrum Prairie
4.Marsh Creek/Lower Portneuf
5.Salmon Falls Creek/Rock Creek
6.Payette Valley
7.Coeur d'Alene River Valley
8.Mountain Home Plateau
9.Moscow Basin
10.Clearwater Uplands and Plateau
11.Goose Creek/Golden Valley