Assessment and Alternatives Study for Municipal Well Stations

The City of Spokane owns and operates several high-capacity caisson wells that pump groundwater from the Spokane Valley-Rathdrum Prairie regional aquifer system, which is the sole source of water supply for the Spokane/Coeur d’Alene metropolitan area. GSI has studied four of the City’s eight well stations to evaluate how best to restore lost production capacity, improve operational performance and reliability, and increase long-term resiliency at each location. The options we evaluated included modifications to existing facilities (caissons, casing liners, and/or pumping systems) and installation of vertical wellfield wells to supplement or replace production from the existing caisson wells.

The first phase of each study has included caisson performance tests, downhole inspections, exploratory test borings, water level monitoring, and compilation and evaluation of historical operational data. Using a groundwater flow model (previously developed by GSI personnel), the project team has then evaluated drawdown interference, water-level thresholds, and feasible production capacities for multiple alternatives involving the caissons and new wellfields, including concepts for the design and layout of new wellfields at each well station site.

In addition to preparing detailed technical reports for each well station, the City and the GSI team collaborated on a pictorial “coffee-table” style booklet for the first study, which described the City’s water system and the study’s findings to City staff, policy makers, and the general public. This booklet received an award for communications at the 2022 American Water Works Association (AWWA) Pacific Northwest Section annual conference.

 GSI’s work involved:

• Condition assessments of existing infrastructure.
• Field investigations including in-caisson video surveys and laser-based plumbness and alignment surveys.
• Exploratory drilling and geologic characterization.
• Installing a test well and monitoring well.
• Aquifer testing, water level monitoring, and water quality testing.
• Feasibility analyses and cost estimates for multiple alternatives for modifying existing caisson wells and constructing new vertical wells.
• Preparation of facility concept plan for each well station.
• Collaborating on the development of a booklet about the City's water system and the recent findings.