The Spokane Valley - Rathdrum Prairie Aquifer System

Dr. John P. Buchanan
Professor of Geology
Eastern Washington University

This article was published in the July 31 - August 6, 1996 issue of The Inlander, the inland northwest's weekly news and arts authority.

The Spokane Valley - Rathdrum Prairie aquifer system serves dozens of communities (about 700,000 people) in both Idaho and Washington. It lies beneath the relatively flat land between the mountains on either side, covering an area of more than 325 square miles, extending from the southern part of Lake Pende Oreille to the Little Spokane River (see graphic). The water in the aquifer, groundwater, enters the system in Idaho through rainfall and as Lake Coeur d'Alene and the other lakes around the Rathdrum Prairie slowly leak into the ground and replenishes or recharges the system. The groundwater then flows from east to west, from Idaho to Washington, and knows no arbitrary political boundary - a molecule of groundwater will not stop (or yield) at the state line. Eventually the groundwater exits the aquifer along the Little Spokane River, discharging through numerous springs and seeps. An adventurous drop of rain, then, falling on the northern Rathdrum Prairie, will percolate into the ground and slowly move through the underground system and eventually emerge to the surface again, perhaps at Griffiths Spring at the Spokane Fish Hatchery, decades after it began its travel. However, that water droplet could have its journey interupted, captured along its course by one of more than 180 large wells that pump water from this system, on which many of us get all of our drinking water. Because of this dependence by the communities on the aquifer as our largest source of fresh, clean water, it was designated a "sole source aquifer" by the Environmental Protection Agency in the 1970s, special status afforded to select aquifer systems nationwide.

For most people it is difficult to envision what the aquifer physically looks like - it is not an "underground river" as so many believe. Try to imagine a long bathtub that is open at each end, and filled with sand and gravel to the brim. Now fill the tub with water, allowing it to enter at one end and flow out the other, but only saturating the lower portion of the sandy material. That, essentially, is a model of the aquifer system on which so many depend. Furthermore, the bathtub is not even along the bottom, however, and is deeper in some places and shallower in others. For example, the aquifer is more than 500 feet thick in the area around Opportunity, while the Walk-in-the-Wild Zoo site sits atop a bedrock hill that emerges from the center of the aquifer. Generally, the water table lies just beneath the surface, less than 100 feet down in the center part of the Spokane Valley, yet is more than 300 feet deep in the northern Rathdrum Prairie. Yet the flow of groundwater, on average about 30 feet per day, is still from east to west. Remember our slow-moving but adventurous water droplet? It would take about seven years for it to travel uninterupted from the state line to the eastern city limits of Spokane near Havana Street.

Because we can't see the aquifer in a physical sense, scientists use an array of both simple and high-tech gear to visualize this hidden body of freshwater: it's much like trying to describe the contents of a box without opening it. In the past, much data came from drilling wells into the ground to obtain water. But since the water table was relatively shallow, the wells did not penetrate the full thickness of the sand and gravel to the bottom of the aquifer. Recently, more sophisticated seismic reflection profiling techniques have been used to examine the deeper parts of the aquifer and better define the shape of its bottom. Using these data, and computers to construct mathematical models of the aquifer, gives us the best ability to understand this hidden resource.

An important debate is finally emerging: what is the quantity of groundwater available in the aquifer that we have to use? It is simple to gauge the flow in a river, where you can easily measure the shape of the channel and the velocity of flow. But again, the aquifer is a subterranean body of slowly moving groundwater, beyond the scope of direct examination and measurement. Recent technical studies by consultants to the City of Spokane, by Eastern Washington University and the Idaho Department of Environmental Quality, all have shown that the flow of groundwater across the state line is less than the original estimates made in the 1960s. The actual flow of water probably hasn't changed in this time, just that our estimates are getting better. At present we are at a point where water use is about equal to the water available. If we pump more groundwater than is available (essentially mining the aquifer), the water table will lower to deeper levels requiring deeper wells, flow in springs and seeps along the Little Spokane River will diminish or disappear, and flow in the Spokane River may change as it begins to lose more water underground.

Water quality is generally good to very good in the aquifer, far better than water in the Spokane River. However, time progressive degradation of water quality is an inevitable consequence of continued development over the aquifer surface and along its margins. Old landfills, industry, large urban and suburban developments, the highway and rail transportation corridors on top of the aquifer surface all serve as potential sources of groundwater contamination, now and into the future. Today, water quality is poorest in pockets along the aquifer margin and in areas of intense development. But in some areas, the water quality has improved in the past 20 years, as sewering begins to have a positive affect.

In sum, we have been studying the aquifer for more than 50 years, and have been depending on it much longer as a source of fresh, clean water. We will continue to depend on the system to sustain projected growth that will undoubtedly occur into the future. But remember: it is not infinite and it is not physically protected from careless and inadvertent contamination.