How hard is the water in Cook County, IL?
34.8 grains per gallon
A softener will pay its way here.
The median hardness measured in Cook County, IL is 596 mg/L as calcium carbonate, which is 34.8 gpg. That is very hard on the USGS scale. It comes from 86 samples at 67 sites between 1990 and 2021, drawn from groundwater wells, which is what most taps and private wells in the county draw on.
How much it varies here
Half the sites came in between 21.3 and 49.6 gpg. The whole middle of that range gives the same answer, so a test of your own tap is unlikely to change it. Confidence in the county figure: high (how it is measured).
What to do next
At 34.8 gpg a softener pays its way here. Before you buy, check the quoted unit's grain capacity against the sizing table below, and ask whether it regenerates on demand, which uses less salt than a timer. Call your utility for its own report if you are on city water.
More on sizing a softener for your household.
From 2.0 to 268.1 gpg
The softest single sample on record in Cook County was 34.3 mg/L and the hardest 4,590 mg/L, a 134-fold gap. The top quarter of sites read above 49.6 gpg and the bottom quarter below 21.3 gpg. So even here some sites draw water soft enough to need nothing at all.
Wells 34.8 gpg, rivers and lakes 15.2 gpg
86 groundwater samples from 67 wells put the median at 596 mg/L. 1,065 surface samples from 62 sites put it at 260.5 mg/L. In Cook County the wells come out 19.6 gpg harder. Groundwater sits in contact with rock for years and picks up calcium and magnesium as it goes, so wells usually read harder. A utility here that draws from the river can deliver noticeably different water from a private well next door.
What a softener would cost to run
For a household of 3, a 48,000-grain unit covers a week between regenerations. Expect about 760 lb of salt a year (20 bags of 40 lb, about $137), and roughly 2,346 gallons of regeneration water. Installed, most units run $1,000 to $4,500; spread over 15 years with salt, that is about $237 a year.
Sizing at 34.8 gpg
| People | Grains a day | Unit | Salt a year |
|---|---|---|---|
| 1 | 2,088 | 24,000 | 254 lb |
| 2 | 4,176 | 32,000 | 508 lb |
| 4 | 8,352 | 64,000 | 1,016 lb |
| 6 | 12,528 | twin tank | 1,524 lb |
Cook County against the rest of Illinois
Among the 102 Illinois counties with data, Cook County is the hardest. Across the whole country it is harder than 96% of counties.
Softest in Illinois: Pope County at 4.3 gpg. Closest to Cook County: Perry County (30.8), Madison County (28.7), Kane County (25.7).