Water Hardness in Chicago

Chicago's tap water is hard, at approximately 153 mg/L of calcium carbonate — roughly 8 to 9 grains per gallon. The entire city draws from Lake Michigan, pulled in through intake cribs sited two miles offshore and treated at two facilities: the James W. Jardine Water Purification Plant, which serves the northern, downtown and western parts of the city plus many suburbs, and the Eugene Sawyer plant serving the south. Jardine is the largest water filtration plant in the world by capacity, processing well over a billion gallons a day.

The hardness is geological, not a treatment artefact. Lake Michigan sits in a basin ringed by dolomite and limestone bedrock — the Niagara Escarpment runs along its western edge — and water in contact with that rock dissolves calcium and magnesium carbonate over time. Chicago's treatment process is built to remove particulates, pathogens and taste-and-odour compounds; it is not a softening process, and dissolved minerals pass through the plants essentially untouched. This is why the world's largest filtration plant still produces hard water.

One practical advantage of a single-source system is consistency. Unlike cities blending rivers, reservoirs and groundwater, Chicago's hardness does not swing much between neighbourhoods, and seasonal variation is modest — driven by lake conditions rather than by which source is running. A reading taken in Rogers Park and one taken in Beverly will be close.

At this level, expect visible scale on kettles and shower heads within a few months, dishwashers performing best on a mid-to-high salt setting, and water heaters benefiting from periodic flushing. It is meaningfully harder than Toronto, which draws from the same Great Lakes system but from Lake Ontario, and far harder than Seattle or Portland. It is not, however, in the same category as the Southwest: San Antonio and Las Vegas are roughly double.