Rock Island Water Hardness & Quality Report (2026)

Rock Island, Illinois, in Rock Island County — a major Quad Cities Illinois city on the Mississippi River, home of the Rock Island Arsenal (a major US Army weapons and equipment manufacturing facility on Arsenal Island in the Mississippi River — one of the nation's largest military arsenals and a National Historic Landmark), a historic Mississippi River city (Rock Island was a major river crossing and railroad hub in 19th-century America — the Rock Island Railroad was one of the first to bridge the Mississippi River in 1856), and a diverse Quad Cities industrial community — draws its municipal water supply from the Mississippi River via the City of Rock Island Water Division. Water hardness in Rock Island measures 157 mg/L — classified as hard.

Rock Island's moderate-to-hard supply reflects the Quad Cities Mississippi River watershed's calcareous geology. The Mississippi River at Rock Island–Quad Cities carries water from: the Upper Mississippi calcareous drainage (Minnesota, Iowa, and Wisconsin Ordovician Prairie du Chien Dolomite and Silurian Niagara Dolomite — the highly calcareous Upper Midwest carbonate drainage); the Wisconsin glacial meltwater calcareous till plain of Iowa and Illinois; and the Rock Island Arch calcareous Silurian dolomite bedrock. Rock Island's Mississippi River treatment produces the moderate 157 mg/L.

At 157 mg/L, Rock Island residents face moderate-to-hard scale challenges. Monthly descaling with citric acid solution is recommended. City of Rock Island Water Division consistently delivers water meeting all Illinois EPA and federal EPA Safe Drinking Water Act requirements.