Official Government Website

Rathdrum Prairie Aquifer

The Spokane Valley – Rathdrum Prairie (SVRP) aquifer is the primary source of water for most people in Spokane County, Washington, and Kootenai County, Idaho. It covers about 370 square miles in northern Idaho and eastern Washington and is composed of gravels, cobbles, and boulders deposited from Ice Age floods.

Discovered in 1895, this aquifer has become one of the most important resources in the region, supplying drinking water to more than 500,000 people.

This accordion will not appear on the screen

The boundary of the Rathdrum Prairie Aquifer is defined differently among various government agencies. DEQ has classified the RPA as a sensitive resource aquifer, following the boundary defined by the US Environmental Protection Agency. Because of this classification, all activities that could impact the water quality of the RPA must be carried out so they maintain or improve the existing quality of the ground water.

Early inhabitants of the Rathdrum Prairie include the Kootenai Tribe of Idaho and the Coeur d’Alene Indians, followed by settlers, miners, and farmers.

In the early 1900s, a majority of drinking or irrigation water on the Rathdrum Prairie was derived from local streams, rivers, and lakes. Today, nearly all the water used for drinking and irrigation comes from ground water, including the Rathdrum Prairie Aquifer.

In 2002, Newport Generation, Cogentrix Energy, and Avista Utilities applied for water rights to drill wells on the Rathdrum Prairie to obtain ground water for proposed cogeneration plants. About 18 million gallons of ground water per day would have been used in the plants. Significant concern was expressed about using so much water and whether the aquifer could sustain such use. The water rights were granted for one of the proposed projects that would use only a small portion of the 18 million gallons per day and the other projects were eventually denied.

Population growth, evolving water uses, and land conversion continues to present new challenges for the RPA, particularly the potential for ground water contamination from septic discharge, urban runoff, and industrial chemicals.

The population in the area over the RPA is projected to potentially grow to 400,000 by the year 2060, which could impact water quality. DEQ completed a yearlong water quality investigation, which included sampling numerous water wells in the RPA and along with the peripheral lakes. General results indicate that the drinking water quality is very good.

  • Nitrate-Nitrogen: All the water samples obtained and analyzed had concentrations well below the drinking water standards of 10 milligrams per liter.
  • Arsenic and Uranium: Arsenic analytical results indicated concentrations were below the allowed maximum contaminant level of 10 micrograms per liter in all wells.
  • CFC’s and SF6: Age dating of the RPA ground water showed mixed results. The chlorofluorocarbons indicated either very high concentrations or very low concentrations, most likely reflecting chemical changes that take place when water is recharged through the thick unsaturated zone or from the peripheral lakes.
  • Deuterium and Oxygen-18: The 2H and 18O analytical results from many of the ground water wells reflect ratios of the two isotopes that are very similar to the upgradient surface water bodies indicating significant recharge from the lake.

Sole Source Designation – In the 1970s, area residents recognized that their unconfined aquifer could easily become contaminated. The highly permeable flood deposits, together with very thin topsoil layers in many locations, make the SVRP aquifer highly susceptible to pollution. Area residents petitioned the US Environmental Protection Agency (EPA) to designate the SVRP aquifer as a “sole source aquifer”.

EPA agreed and granted this designation in 1978. It was the second aquifer in the nation to receive this special designation. The sole source designation increased public awareness for SVRP aquifer and supported the development of special management practices such as reducing the number of septic systems and pretreating stormwater over the SVRP aquifer by local agencies.

Presently, SVRP aquifer protection efforts are managed cooperatively by Spokane County, local cities, the Department of Ecology and utilities in Washington, and by the Department of Environmental Quality, Panhandle Health District, and local cities and counties in Idaho.

Hydrogeological Study – Increased demand for ground water and sparse knowledge about the aquifer prompted a call for a hydrogeologic study of the Rathdrum-Spokane Aquifer. In 2003, Congress appropriated $500,000 for the first year of the study. The project was completed in 2007 and was the most comprehensive hydrogeologic study to date of the Rathdrum-Spokane Aquifer.

In November 2006, Kootenai County residents voted to form the Rathdrum Prairie Aquifer Protection District, which encompasses the area over the RPA and the surrounding upland areas that contribute water to the aquifer. Residents of these areas pay approximately $6 per household and $12 per business to fund aquifer protection activities.

Idaho Comprehensive Aquifer Planning and Management Program – To address long-term management strategies and provide information for managing ground and surface water, the Idaho Comprehensive Aquifer Planning and Management Program (CAMP) was created by the Idaho Department of Water Resources (IDWR).

The purpose of CAMP is to avoid future conflicts over water resources, prioritize state investments in water resources, and find ways to decrease the difference between future water needs and available supply. A CAMP report forecasts Rathdrum Prairie water use over the next 50 years for three levels of population and addresses water conservation methods to manage an increase in water use.

Weather conditions – temperature, rain, humidity, and wind speed—can have a big influence on how much water is needed for irrigation and available to recharge the water in the aquifer. The Bureau of Reclamation’s AgriMet station on the Rathdrum Prairie collects weather data for use in water quantity studies but also reports the amount of water needed each day to irrigate lawns or crops to help conserve water. To learn more about the Rathdrum Prairie AgriMet station and current or historical weather conditions, visit the BOR AgriMet website.

ver: 3.4.0 | last updated:
Jump back to top of page button