Chicago draws its municipal water from Lake Michigan, one of the largest freshwater sources in the world, producing a water profile that's genuinely moderate across most measures — harder and more alkaline than the soft western American cities on this site, but nowhere near the extreme hardness of Burton-on-Trent or Dortmund's historic profiles.
Great Lakes water, moderate but not neutral
The figures for Chicago's water: calcium 40 ppm, magnesium 10 ppm, sodium 8 ppm, sulfate 25 ppm, chloride 15 ppm, bicarbonate 115 ppm. The bicarbonate figure is the one worth paying attention to here — meaningfully higher than the soft West Coast profiles on this site, giving Chicago's water real alkalinity and buffering capacity without the accompanying high calcium some of the more mineral-dense historic European profiles carry alongside their own bicarbonate. That combination (moderate calcium, meaningfully higher bicarbonate) can leave mash pH running a touch high for a very pale grain bill unless corrected, a genuinely different practical consideration than a brewer working from Denver's or Portland's much softer, lower-bicarbonate water would need to think about.
A brewing history shaped more by industry than by any single mineral
Chicago's brewing history is substantial — the city was a major American brewing centre through the 19th and early 20th centuries, home to large-scale industrial lager production that helped establish the adjunct-lager tradition covered elsewhere on this site — but that history traces more directly to the city's transportation infrastructure, immigrant brewing labor, and industrial capacity than to any single defining mineral in its water the way Burton's sulfate or Pilsen's softness drove those cities' specific styles.
Using this profile
Because bicarbonate runs meaningfully higher here than calcium, a brewer targeting a very pale, delicate lager style from this water may need an acid addition (or a calcium-boosting salt like gypsum or calcium chloride, which also helps counteract excess alkalinity) to keep mash pH from running high, while a brewer targeting an amber or dark style can often use this water closer to as-is; see the International Pale Lager and American Amber Lager style guidelines for two directions this water could reasonably be built toward. Municipal supplies shift over time, so treat this as a general reference rather than a live feed and check your own Chicago tap water before brewing.