Renton Water Hardness & Quality Report (2026)

Renton, Washington, at the south end of Lake Washington in King County, draws its municipal water supply from two primary sources managed by the City of Renton Utilities: the Cedar River via Seattle Public Utilities (SPU) wholesale delivery — Cedar River water from Chester Morse Lake (Cedar Lake) in the Cascade Range of King County, treated at the Cedar River Treatment Facility; and local Cedar River Valley Aquifer groundwater wells tapping the shallow alluvial aquifer beneath the Cedar River floodplain in Renton. Water hardness in Renton measures 27.5 mg/L — classified as very soft, one of the softest municipal supplies in Washington.

Renton's exceptionally soft supply reflects two equally soft source waters. The Cedar River originates in the Cascade Range — draining the Oligocene–Miocene Ohanapecosh Formation volcanic sequences and Miocene Columbia River Basalt Group of the central Washington Cascades, supplemented by Quaternary glacial outwash in the upper Cedar valley. The Cedar River Valley Aquifer — a shallow alluvial aquifer recharged directly by the Cedar River at the Renton floodplain — stores and delivers the same very soft Cedar River water with minimal additional mineral acquisition from the thin Quaternary glacial alluvial fill. Both the surface and groundwater components are dominated by Cascadian volcanic-origin water with near-zero calcium contribution.

With hardness at 27.5 mg/L, Renton residents enjoy essentially scale-free water. Soap and shampoo lather with minimal product. Faucet aerators and showerheads essentially never need descaling. Hot water systems remain scale-free indefinitely. City of Renton Utilities and Seattle Public Utilities consistently deliver water meeting all Washington DOH and EPA Safe Drinking Water Act requirements.