In Illinois, mains water is delivered by local public water systems, regulated by the Illinois EPA, Bureau of Water under the US EPA Safe Drinking Water Act. Typical hardness: Hard to very hard; ~137 to 240 ppm typical, Lake Michigan (Chicago) ~130 to 150 ppm. For a specific ZIP, see the EWG Tap Water Database.
Sources: Illinois EPA, Bureau of Water, EWG Tap Water Database