How hard is the water in Sonoma County, CA?
7.3 grains per gallon
A softener is worth it here.
The median hardness measured in Sonoma County, CA is 125 mg/L as calcium carbonate, which is 7.3 gpg. That is hard on the USGS scale. It comes from 219 samples at 128 sites between 1990 and 2022, 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 4.5 and 10 gpg. That spread crosses a decision line, so some homes here land on a different answer. A $10 test strip on your own tap settles it. Confidence in the county figure: high (how it is measured).
What to do next
At 7.3 gpg a softener is worth getting, mainly to protect the water heater and cut soap use. Get at least two quotes, ask each installer what grain capacity they are fitting, and check it against the sizing table below. Because Sonoma County straddles a decision line, check your own tap with a $10 test strip before you spend anything.
More on sizing a softener for your household.
From 0.4 to 46.2 gpg
The softest single sample on record in Sonoma County was 7.6 mg/L and the hardest 791 mg/L, a 104-fold gap. The top quarter of sites read above 10 gpg and the bottom quarter below 4.5 gpg. So even here some sites draw water soft enough to need nothing at all.
Wells 7.3 gpg, rivers and lakes 7 gpg
219 groundwater samples from 128 wells put the median at 124.5 mg/L. 310 surface samples from 52 sites put it at 119.5 mg/L. In Sonoma County the wells come out 0.3 gpg harder. Groundwater sits in contact with rock for years and picks up calcium and magnesium as it goes, so wells usually read harder. The two sources are close, so the county figure should hold whichever one your tap is on.
What a softener would cost to run
For a household of 3, a 24,000-grain unit covers a week between regenerations. Expect about 159 lb of salt a year (4 bags of 40 lb, about $29), 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 $129 a year.
Sizing at 7.3 gpg
| People | Grains a day | Unit | Salt a year |
|---|---|---|---|
| 1 | 438 | 24,000 | 53 lb |
| 2 | 876 | 24,000 | 107 lb |
| 4 | 1,752 | 24,000 | 213 lb |
| 6 | 2,628 | 24,000 | 320 lb |
Sonoma County against the rest of California
Among the 58 California counties with data, Sonoma County is number 35 of 58 counting from the hardest. Across the whole country it is harder than 40% of counties.
Softest in California: Sierra County at 1.7 gpg. Hardest: Calaveras County at 98.3 gpg. Closest to Sonoma County: San Bernardino County (7.4), Trinity County (7.4), Sacramento County (6.7).