2025 MODFlow Study

On behalf of the Spokane Aquifer Joint Board (SAJB), GSI Water Solutions, Inc. (GSI) completed updated wellhead protection delineations for drinking water supply wells drawing from the Spokane Valley–Rathdrum Prairie (SVRP) Aquifer. Using SAJB’s updated regional groundwater flow model, GSI delineated groundwater capture zones for SAJB-member wells to support implementation of Washington State’s Wellhead Protection (WHP) Program.

The SVRP Aquifer underlies approximately 370 square miles across Spokane County, Washington, and Kootenai and Bonner Counties, Idaho, and serves as the sole source of drinking water for the region. SAJB’s 21 member agencies collectively own and operate 152 wells across 103 wellfields in Spokane County, supplying residential, commercial, and industrial water users. Due to this reliance, the SVRP Aquifer was designated a Sole Source Aquifer by the U.S. Environmental Protection Agency in 1978.

For each SAJB-member well, GSI delineated groundwater capture zones corresponding to 1-, 5-, and 10-year travel times, as required under Washington Department of Health (DOH) WHP program rules. In addition, the project supports the delineation of Special Wellhead Protection Areas (SWPAs), which provide water purveyors with greater flexibility to develop targeted management, pollution prevention, and risk-reduction strategies within optimally sized protection areas.

This project was funded through a combination of SAJB member contributions and a grant from the Washington State Department of Health.

Final Reports

1. Update of Groundwater Capture Zones for Wellhead Protection Program
   • Technical Memorandum
   • Tables
   • Figures
   • Attachment A: Process for Developing Time Durations for Special Wellhead Protection Areas
   • Attachment B: Capture Zone Maps for Each SAJB Member
     • Carnhope Irrigation District No. 7
     • Consolidated Irrigation District No. 19
     • East Spokane Water District No. 1
     • Hutchinson Irrigation District No. 16
     • Honeywell Electronic Materials
     • Irvin Water District No. 6
     • Kaiser Aluminum Trentwood Works
     • Liberty Lake Sewer & Water District No. 1
     • Modern Electric Water Company
     • Model Irrigation District No. 18
     • City of Millwood
     • Moab Irrigation District No. 20
     • North Spokane Irrigation District No. 8
     • Orchard Irrigation District No. 6
     • Pasadena Park Irrigation District No. 17
     • Spokane Business & Industrial Park
     • City of Spokane
     • Spokane County Water District No. 3
     • Trentwood Irrigation District No. 3
     • Vera Water & Power
     • Whitworth Water District No. 2
2. Climate Impact Analysis
3. Model Development