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The Palouse Basin Aquifer System is a cross-border, layered groundwater system beneath parts of eastern Washington and northern Idaho. It supplies municipal wells in Pullman, Moscow, and Palouse, but the water is replenished slowly: withdrawals have exceeded natural recharge, contributing to falling water levels. It is not a single underground lake, and its upper and lower aquifers are only weakly connected.
Where is the Palouse Basin Aquifer?
The system lies beneath parts of Whitman County, Washington, and Latah County, Idaho. Its groundwater supports Pullman, Moscow, and Palouse, as well as Washington State University (WSU) and the University of Idaho (UI). The Palouse Basin Aquifer Committee (PBAC), a cooperative body involving local governments, counties, and universities in both states, coordinates monitoring and management. State water agencies participate as non-voting members. PBAC’s aquifer overview describes the local system; it should not be confused with the much larger Columbia Plateau Regional Aquifer System described by the U.S. Geological Survey.
How is the aquifer system layered?
An aquifer is a water-bearing formation of rock or sediment that can store and transmit groundwater. The Palouse Basin is a complex arrangement of basalt and sediment layers, not one open underground reservoir. PBAC identifies two major aquifers:
| Feature | Upper aquifer | Lower aquifer |
|---|---|---|
| Connection between layers | Connection to the lower aquifer is generally minor. | Connection to the upper aquifer is generally minor. |
| Water-level difference | PBAC says the difference in water level, or hydraulic head, between them is generally greater than 100 feet. | |
| Typical wells | Most domestic wells are typically completed here. | Municipal wells serving Pullman, Moscow, and Palouse draw from here. |
These are broad system-level descriptions; individual wells and local geology can vary. For wider context, USGS describes Columbia River Basalt Group units and overlying basin-fill sediments as primary aquifers in the regional system, but that regional framework is not a substitute for PBAC’s more specific account of the Palouse Basin.
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Where does groundwater come from?
Groundwater recharge occurs when water moves down through soil and geologic layers into water-bearing formations. In the Palouse Basin, those layers constrain where and how water can move. PBAC says most recharge occurs near the base of Moscow Mountain, between sediment layers. The precise recharge pathways and locations are still being investigated, so this description should not be taken as a complete map of recharge or as a single basin-wide recharge-volume estimate. PBAC’s recharge information outlines its current account.
Why are water levels declining?
When pumping persistently removes more groundwater than natural processes replenish, the amount stored in the system falls and water levels decline. PBAC says that is the Palouse Basin’s situation: more water is pumped out than enters through natural recharge, and local geology makes replenishment difficult. PBAC’s aquifer system page summarizes the imbalance.
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PBAC currently reports a decline rate of 0.7 feet per year, compared with a rate above 1 foot per year in the 1990s. It also reports that over the past 30 years per-capita water use fell 30% in Pullman/WSU and 43% in Moscow/UI. Those reductions helped slow the decline, but PBAC says current use remains above recharge. These are PBAC’s reported figures, not new measurements made for this article. PBAC’s aquifer overview presents the current rate and long-term usage changes.
How much water is being pumped?
PBAC’s 2025 annual report says Pullman, Moscow, Palouse, WSU, and UI together pumped 2.30 billion gallons in 2024. The total excludes private residential wells, so it is not a complete measure of every withdrawal in the basin. PBAC reports that this combined total was 1.7% below the group’s 2.34 billion gallons in 2023 and 19.1% below its 1992 pumping level. The annual reports are available from PBAC.
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- Fast and Accurate Results: Get accurate results in minutes with just 3 steps: 1) Dip the test strip into water for 2 seconds for pads 1-7; dip pad 8 (Ammonia nitrogen) separately for 5 seconds. 2) Remove the strip without shaking off excess water. Hold horizontally for 30 seconds for pads 1-7, wait 180 seconds to read Pad 8 (Ammonia nitrogen). 3) Compare the colors to the chart for up to 99% accuracy and take necessary action
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- Convenient Use: With this kit, you can test your aquarium water at home without the need to send samples to a lab, saving you both time and money. Enjoy peace of mind knowing your aquarium water is balanced and safe without the stress or extra expense. By identifying potential issues, our fish tank testing kit helps prevent hidden water problems and ensures the safety and well-being of your fish
How is the basin monitored and managed?
PBAC implements the Groundwater Management Plan enacted in 1992. Its work includes monitoring groundwater levels and reported annual use, funding research, and maintaining technical information about the basin. Its research archive includes technical studies and a lecture series by University of Idaho emeritus faculty member John H. Bush on the region’s geologic history, groundwater movement, and recharge.
Conservation and efficiency measures have reduced per-person use and helped slow the decline, but reducing demand is not the same as balancing the system: PBAC says withdrawals still exceed natural recharge. Management therefore also involves evaluating ways to supplement municipal supply.
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What supplemental supply is being studied?
In spring 2025, the City of Moscow and PBAC began studying a potential diversion from the Clearwater River. The concept is under evaluation; it is not a built, approved, or operating replacement supply. PBAC describes two possible ways to use treated river water:
- Direct municipal use: Convey treated water into municipal systems, reducing the amount pumped from the aquifer when river water is available.
- Aquifer storage and recovery: Inject treated water into the aquifer for storage and later pumping, potentially shifting some supply from periods of high river flow to periods of higher local demand.
PBAC notes that Clearwater River flows are high in winter and spring, while local water use peaks in summer. Direct use may therefore need storage to better match supply with demand. Storing treated surface water underground also raises water-quality questions. These are issues PBAC identifies for study, not findings from an independent project review. PBAC’s Clearwater River project page describes the proposal.
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- Versatile Use for Freshwater and Ponds: This aquarium water test kit is versatile and suitable for both freshwater and saltwater environments. It can be used in various aquariums, fish tanks, ponds, pools, and shrimp tanks, making it easy to monitor water quality and ensure your filter functions properly. Experts recommend testing your aquarium water at least twice a week to maintain optimal water quality.
- Fast and Accurate Results: Get accurate results in minutes with just 3 steps: 1) Dip the test strip into water for 2 seconds for pads 1-7; dip pad 8 (Ammonia nitrogen) separately for 5 seconds. 2) Remove the strip without shaking off excess water. Hold horizontally for 30 seconds for pads 1-7, wait 180 seconds to read Pad 8 (Ammonia nitrogen). 3) Compare the colors to the chart for up to 99% accuracy and take necessary action.
- Extended Shelf Life: The test strips are packaged in a well-sealed bottle to ensure they remain effective over time. This protects the strips from moisture and other environmental factors, ensuring you have reliable testing materials on hand when needed.
- Convenient Use: With this kit, you can test your aquarium water at home without the need to send samples to a lab, saving you both time and money. Enjoy peace of mind knowing your aquarium water is balanced and safe without the stress or extra expense. By identifying potential issues, our fish tank testing kit helps prevent hidden water problems and ensures the safety and well-being of your fish.
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