Redmond Water Hardness & Quality Report (2026)

The City of Redmond Water System serves over 70,000 residents across Redmond and unincorporated areas including Redmond Ridge and Trilogy in King County, Washington. The utility operates two distinct supplies: groundwater from Redmond's shallow municipal aquifer east of the Sammamish River and Lake Sammamish, and surface water from Seattle's Tolt Reservoir in the Cascade Mountains west of the Sammamish River. Groundwater supplies approximately 40% of the city's drinking water — municipal wells pumped over 860 million gallons in 2024 — with both sources blended during summer peak demand.

Redmond's groundwater aquifer lies within the Puget Sound Lowland, shaped by Pleistocene glaciation. Ancient glacial deposits — sand, gravel, and till — contain abundant calcium and magnesium minerals; the shallow aquifer (5–15 feet below surface) is recharged primarily by rainwater infiltration. The Tolt River Watershed supplies surface water flowing through mineral-rich Cascade geology. As groundwater percolates through glacial sediments and carbonate-bearing bedrock, limestone and dolomite dissolve to create a moderately hard supply, while the Tolt Reservoir yields very soft water.

Residents on the groundwater system (east side) experience scale buildup on fixtures, appliances, and inside water heaters and pipes, with reduced soap lathering compared to those on the soft Tolt surface water system (west side). Water softening is commonly recommended for groundwater users to extend appliance lifespan and improve detergent efficiency. The utility adds fluoride to groundwater at 0.7–0.8 ppm; the aquifer is classified as highly susceptible to surface pollutants due to its shallow depth, with Critical Aquifer Recharge Areas (CARA I and CARA II) providing special protection.