Janesville Water Hardness & Quality Report (2026)

Janesville, Wisconsin, the Rock County seat on the Rock River — the hometown of Congressman and Speaker Paul Ryan, a General Motors city until 2009, and the largest city in southern Wisconsin — draws its municipal water supply from the Rock River via the City of Janesville Water Utility, treating Rock River water at the Janesville Water Treatment Plant. The Rock River in southern Wisconsin drains the Driftless and drift-covered terrain of south-central Wisconsin. Water hardness in Janesville reaches 258.5 mg/L — classified as very hard, among the highest in Wisconsin.

Janesville's very hard supply reflects the Rock River watershed's exceptional Southern Wisconsin dolomite geology. The Rock River above Janesville drains: the Silurian Niagaran Dolomite (one of the most reactive dolomite formations in the Midwest — the same formation underlying Niagara Falls; the Niagaran in southern Wisconsin is extensively exposed in the Niagara Escarpment and in buried river valley bedrock throughout Rock County); the Ordovician Galena Dolomite (a massive calcareous pure dolomite — the primary ore-hosting formation of the Iowa–Wisconsin lead-zinc mining district); and the Ordovician Maquoketa Shale (calcareous shale with some limestone interbeds). The combination of the highly reactive Niagaran and Galena dolomites in the Rock River basin produces the very hard 258.5 mg/L supply.

At 258.5 mg/L, Janesville residents face significant hard water challenges. Scale deposits form rapidly on all fixtures — monthly descaling with citric acid solution is essential maintenance. City of Janesville Water Utility consistently delivers water meeting all Wisconsin DNR and EPA Safe Drinking Water Act requirements.