If you moved to Spokane from a soft-water area, the single biggest drinking water surprise is the hardness. Spokane runs 7–10+ gpg. Your water heater, your dishwasher, your showerheads, and your skin will all behave differently. The reason sits underneath the Spokane Valley and extends into northern Idaho: the Spokane Valley–Rathdrum Prairie Aquifer, a 370-square-mile glacial aquifer that's one of the fastest-flowing in the United States and one of the most productive drinking water sources in the Northwest. This article covers what that geology means for your tap water and what you need to know about water softeners in eastern Washington.
The Aquifer
The Spokane Valley–Rathdrum Prairie Aquifer (SVRP) covers approximately 370 square miles, eastern Washington plus northern Idaho. The Idaho portion (often called the Rathdrum Prairie Aquifer or RPA) is about 250 square miles, stretching from Lake Pend Oreille south to Coeur d'Alene and Post Falls, and west to the Washington state line.
EPA Sole Source Aquifer designation: 1978. Because it's the primary drinking water source for the region with no practical alternative, any contamination or depletion is a major public health concern.
Population served: More than 500,000 people across Spokane County, Washington and Kootenai County, Idaho, including Spokane, Spokane Valley, Coeur d'Alene, and Post Falls.
The Geology
The SVRP is made up of glacial outwash gravel, cobbles, and boulders deposited by the Missoula Floods approximately 8,000 to 18,000 years ago. The coarse, open sediment structure is why the aquifer is so productive, and why it's one of the fastest-flowing in the United States, with groundwater moving up to 60 feet per day in some zones. For comparison, water in most aquifers moves feet per year.
Total aquifer storage is estimated at approximately 10 trillion gallons.
Why the Water Is Hard
Glacial outwash sediments are mineral-rich, calcium, magnesium, and other ions dissolve into groundwater as it moves through the permeable gravels. Typical Spokane municipal water hardness: 120–200 ppm (approximately 7–12 gpg). Verify against the current City of Spokane Water Department Consumer Confidence Report for your specific service area, Spokane typically runs about 140–160 ppm.
Hard Water Across Eastern WA
Mineral-rich glacial aquifers supply Spokane, Pullman and the Tri-Cities, and hardness of 7–12+ gpg is normal across Eastern WA. That changes everything about home water treatment. If you moved here from a soft-water area and never had a water softener, you will likely benefit from one.
Water Softeners and Septic Systems in Washington
Washington does not have a statewide softener discharge statute, unlike California, where certain regions (Santa Clarita, specific municipalities) have banned salt-based softeners outright. In WA, the rules are county-level through local health departments.
General WQA and NOWRA (National Onsite Wastewater Recycling Association) guidelines indicate that demand-initiated softeners are generally safe for properly designed septic systems. Demand-initiated softeners regenerate only when needed based on actual water usage, producing far less brine than timer-based systems. Timer-based softeners that regenerate on a fixed schedule regardless of use can overload septic systems.
If you're on septic in Spokane County, Kootenai County, or surrounding areas:
- Choose a demand-initiated (metered) softener, not a timer-based system
- Verify current local county health rules before installation, rules occasionally change
- Size the softener for actual household usage, not maximum possible demand
- Consider potassium chloride salt rather than sodium chloride if sodium loading to septic is a concern
Iron, Manganese, and Other Local Concerns
Some SVRP-sourced wells produce water with elevated iron and manganese. Municipal Spokane water is typically treated for these; private wells in the area may need iron/manganese removal in addition to softening. Test your well to know.
What to Install for a Spokane/CdA Home
- Water softener, sized for household demand, demand-initiated, salt or potassium chloride based on septic considerations
- Whole-house carbon filter, removes chlorine from municipal water; improves taste and shower experience
- Under-sink RO, for drinking water quality
- Iron/manganese treatment, only if your specific well or service area requires it
Free In-Home Water Assessment
Water Guys serves the Spokane, Spokane Valley, Coeur d'Alene, Post Falls, and greater Rathdrum Prairie Aquifer region. We test your specific water, size systems for your household, and handle installation. Free assessment, call or contact us online.