How hard is the water in St. Mary Parish, LA?
8.2 grains per gallon
A softener is worth it here.
The median hardness measured in St. Mary Parish, LA is 141 mg/L as calcium carbonate, which is 8.2 gpg. That is hard on the USGS scale. It comes from 1,701 samples at 22 sites between 1990 and 2024, drawn from every sampling site in the county, wells and surface water together.
How much it varies here
Half the sites came in between 6.3 and 10.3 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 8.2 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 St. Mary Parish 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 4.7 to 51.7 gpg
The softest single sample on record in St. Mary Parish was 80 mg/L and the hardest 885 mg/L, an 11-fold gap. The top quarter of sites read above 10.3 gpg and the bottom quarter below 6.3 gpg.
Wells 19.4 gpg, rivers and lakes 7.6 gpg
1 groundwater samples from 1 wells put the median at 332 mg/L. 1,667 surface samples from 20 sites put it at 130 mg/L. In St. Mary Parish the wells come out 11.8 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 24,000-grain unit covers a week between regenerations. Expect about 180 lb of salt a year (5 bags of 40 lb, about $32), 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 $132 a year.
Sizing at 8.2 gpg
| People | Grains a day | Unit | Salt a year |
|---|---|---|---|
| 1 | 492 | 24,000 | 60 lb |
| 2 | 984 | 24,000 | 120 lb |
| 4 | 1,968 | 24,000 | 239 lb |
| 6 | 2,952 | 24,000 | 359 lb |
St. Mary Parish against the rest of Louisiana
Among the 64 Louisiana counties with data, St. Mary Parish is number 30 of 64 counting from the hardest. Across the whole country it is harder than 44% of counties.
Softest in Louisiana: Lincoln Parish at 0.1 gpg. Hardest: St. Bernard Parish at 53.9 gpg. Closest to St. Mary Parish: St. John the Baptist Parish (8.2), Terrebonne Parish (8.5), St. James Parish (8.6).