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Water Treatment Plant


To ensure reliable and safe drinking water for decades to come, Issaquah is now planning for a new, centralized water treatment plant.

Currently, Issaquah’s water comes from two separate sources:
  • Groundwater: In general, residents in Squak Mountain, Olde Town and Issaquah Valley have always received water from a complex system of groundwater wells that pump from the Issaquah Aquifer.
  • Regional water: Starting in the late 1990s, Issaquah started to purchase regional water from Cascade Water Alliance to accommodate new growth. Today, Issaquah Highlands, as well as Lakemont, Montreux and South Cove, all receive regional water.

Meanwhile, the Talus community can receive either groundwater or regional water, depending on multiple factors (such as a hot weather season).

Why Build a Plant Now?
In 2018, the City Council adopted a Water System Plan Update that recommends building a new treatment plant south of the Interstate 90 corridor in Issaquah Valley.

Issaquah’s population is estimated to grow from 37,000 in 2017 to approximately 50,000 by 2027, with much of that growth focused in the Central Issaquah Area along I-90. 

Due to that projected growth, the City’s existing groundwater supply will not meet the demand in Issaquah Valley as soon as 2021, and regional water will be needed in areas historically served only by groundwater. Status quo isn’t an option.

The centralized plant would: 
  • Enable the blending of groundwater and regional water.
  • Address differing water qualities in each groundwater well.
  • Deliver consistent water quality between regional and groundwater.
  • Allow Issaquah to be more responsive to new/emerging water quality regulations. 

The proposed treatment plant design would follow regional water quality protocols, which includes fluoridation. 

Currently, about two-thirds of Issaquah residents receive fluoridated water, while one-third receive unfluoridated water. See Issaquah’s fluoridation map for further details.

Please note:
  • Some Issaquah residents are not City water customers, but instead are served by other systems.
  • No matter if Talus customers are receiving groundwater or regional water, it is always fluoridated.

As the water supplies would be blended in the proposed treatment plant, all Issaquah water customers would receive fluoridated water. This follows the lead of regional water providers, public health agency recommendations and best practices for water treatment.

Should best-available science change concerning fluoride – and the regional supply is no longer fluoridated – the treatment plant would be built with flexibility to also implement that change. 

Learn more about fluoride via Seattle Public Utilities, which started adding fluoride to its water in 1970 (a full history is also available online).

Get Involved
  • Email the City Council.
  • Provide comments or questions via our online tool.
  • Provide feedback to the City Council during public testimony at a future study session (timing to be determined).
  • Sign up for future updates via Notify Me (subscribe to the City Hall category).

Open House Recap
An open house was held on Aug. 7, 2019. In total, eight people attended. Here's what we heard:
  • Five people were anti-fluoride in the water, and preferred to have it removed. 
  • Two of the attendees were OK with having fluoride remain in the water — one lived in a fluoridated area, the other lived in an area that currently is not fluoridated. 
  • Some of the questions that were asked were: - Why now? - Where would the plant be located? - How much fluoride would be in the water once Issaquah’s water mixed with Cascade Water Alliance? - How much is this going to cost me?


Issaquah Contacts

Staff Liaison

Robert York
Utilities Engineering Manager