Longmont Water Hardness & Quality Report (2026)

Longmont, Colorado, in Boulder County at the base of the Front Range, draws its municipal water supply through the City of Longmont Water Utility, sourcing from two primary systems: Button Rock Reservoir (also called Longmont Reservoir) on North St. Vrain Creek in Larimer County, impounding St. Vrain Creek drainage from the Rocky Mountain National Park–Front Range zone, treated at the Button Rock Water Treatment Plant; and Colorado–Big Thompson (C-BT) Project water delivered via Carter Lake and Lake Loveland through the Northern Colorado Water Conservancy District (Northern Water) infrastructure. Water hardness measures 109 mg/L — classified as moderately hard.

Longmont's moderate hardness reflects the blended supply of St. Vrain Creek mountain water and C-BT transmountain water. The St. Vrain Creek headwaters drain the Front Range crystalline terrain — the Precambrian Longs Peak granite and related granodiorite plutons of the Boulder–Estes Park zone — inherently calcium-poor. The C-BT water (western slope Colorado River origin, from the Granby Reservoir and Shadow Mountain zone on the Fraser River) adds additional soft mountain water. The blend's moderate hardness at 109 mg/L reflects modest mineral acquisition through the St. Vrain Creek valley soils and the distribution system, rather than highly calcareous bedrock.

With hardness at 109 mg/L, Longmont residents encounter moderate scale accumulation. Faucet aerators and showerheads develop deposits after several months — monthly cleaning with citric acid solution is practical maintenance. Dishwashers produce cleaner glassware with rinse-aid. City of Longmont Water Utility consistently delivers water meeting all Colorado CDPHE and EPA Safe Drinking Water Act requirements.