Johnston Water Hardness & Quality Report (2026)

Johnston, Iowa is served by the Johnston Municipal Water Supply, which purchases treated surface water from Des Moines Water Works (DMWW). DMWW operates treatment plants that draw raw water from the Des Moines River, treating it to meet federal and state drinking water standards before distributing to Johnston and other communities. Johnston's water system serves approximately 24,000 residents in Polk County, Iowa. Treatment includes a multi-barrier approach with source-water monitoring, riverbank filtration, and advanced processes; disinfection is achieved using hypochlorite.

The Des Moines River watershed drains a broad agricultural region underlain by Pleistocene glacial drift and alluvial sediments. Beneath these deposits lie Paleozoic carbonate formations — limestone and dolomite — which are highly soluble and rich in calcium and magnesium. As river water moves through and over these rocks, it acquires dissolved minerals, producing a hard supply typical of much of central Iowa. Agricultural runoff and natural mineral weathering further contribute to the elevated mineral content of the treated tap water.

Hard water causes visible scale buildup on fixtures, showerheads, and inside water heaters, kettles, and dishwashers over time. Appliances that heat water are most affected and may require more frequent descaling. Using vinegar or citric acid solutions periodically helps manage scale, and many households choose to install water softeners to reduce spotting on glassware. DMWW monitors for emerging contaminants such as cyanotoxins, PFAS, and pharmaceuticals; recent data indicate detection of total trihalomethanes and chromium-6 above health-based guidelines, underscoring the value of point-of-use filtration for sensitive users.