How hard is the water in Hawaii County, HI?
4 grains per gallon
Optional: a softener helps, but plenty of homes skip it.
The median hardness measured in Hawaii County, HI is 69 mg/L as calcium carbonate, which is 4 gpg. That is moderately hard on the USGS scale. It comes from 56 samples at 25 sites between 1992 and 2013, 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 2.2 and 67.2 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 4 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 Hawaii 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 1.3 to 171.2 gpg
The softest single sample on record in Hawaii County was 23.1 mg/L and the hardest 2,930 mg/L, a 127-fold gap. The top quarter of sites read above 67.2 gpg and the bottom quarter below 2.2 gpg.
Wells 4 gpg, rivers and lakes 1.3 gpg
56 groundwater samples from 25 wells put the median at 68.9 mg/L. 22 surface samples from 9 sites put it at 21.7 mg/L. In Hawaii County the wells come out 2.7 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 88 lb of salt a year (3 bags of 40 lb, about $16), 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 $116 a year.
Sizing at 4 gpg
| People | Grains a day | Unit | Salt a year |
|---|---|---|---|
| 1 | 240 | 24,000 | 29 lb |
| 2 | 480 | 24,000 | 58 lb |
| 4 | 960 | 24,000 | 117 lb |
| 6 | 1,440 | 24,000 | 175 lb |
Hawaii County against the rest of Hawaii
Among the 5 Hawaii counties with data, Hawaii County is number 2 of 5 counting from the hardest. Across the whole country it is harder than 26% of counties.
Softest in Hawaii: Kalawao County at 2.1 gpg. Hardest: Maui County at 24.7 gpg. Closest to Hawaii County: Honolulu County (3.6), Kauai County (2.6), Kalawao County (2.1).