How hard is the water in Marin County, CA?
3.6 grains per gallon
Optional: a softener helps, but plenty of homes skip it.
The median hardness measured in Marin County, CA is 62 mg/L as calcium carbonate, which is 3.6 gpg. That is moderately hard on the USGS scale. It comes from 8 samples at 6 sites between 1998 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 0.6 and 5.5 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: medium (how it is measured).
What to do next
At 3.6 gpg this is your call. Look for white film on glasses and crust on the shower head: if you see little of either, you can live without one. If you do want one, a 24,000-grain unit is enough, so ask for that size rather than a bigger one. Because Marin County straddles a decision line, check your own tap with a $10 test strip before you spend anything.
More on when a softener is not worth it.
From 0.1 to 7.9 gpg
The softest single sample on record in Marin County was 2.4 mg/L and the hardest 136 mg/L, a 57-fold gap. The top quarter of sites read above 5.5 gpg and the bottom quarter below 0.6 gpg.
Wells 3.6 gpg, rivers and lakes 6.5 gpg
8 groundwater samples from 6 wells put the median at 61.7 mg/L. 114 surface samples from 13 sites put it at 111 mg/L. In Marin County the rivers and lakes come out 2.9 gpg harder. Here the surface water reads harder than the wells, which often points to streams fed by limestone springs or irrigation return flow. 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 79 lb of salt a year (2 bags of 40 lb, about $14), 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 $114 a year.
Sizing at 3.6 gpg
| People | Grains a day | Unit | Salt a year |
|---|---|---|---|
| 1 | 216 | 24,000 | 26 lb |
| 2 | 432 | 24,000 | 53 lb |
| 4 | 864 | 24,000 | 105 lb |
| 6 | 1,296 | 24,000 | 158 lb |
Marin County against the rest of California
Among the 58 California counties with data, Marin County is number 54 of 58 counting from the hardest. Across the whole country it is harder than 25% of counties.
Softest in California: Sierra County at 1.7 gpg. Hardest: Calaveras County at 98.3 gpg. Closest to Marin County: Modoc County (3.9), Inyo County (3.9), Plumas County (4.1).