How hard is the water in St. Mary's County, MD?

0.7 grains per gallon

You do not need a softener here.

The median hardness measured in St. Mary's County, MD is 13 mg/L as calcium carbonate, which is 0.7 gpg. That is soft on the USGS scale. It comes from 40 samples at 30 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 0.1 and 2 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

You can skip the softener. At 0.7 gpg the most a softener would fix is the odd water spot. If a salesperson in St. Mary's County says otherwise, ask to see their reading in grains per gallon and compare it with this one.

More on when a softener is not worth it.

From 0.1 to 7.2 gpg

The softest single sample on record in St. Mary's County was 1.2 mg/L and the hardest 124 mg/L, a 103-fold gap. The top quarter of sites read above 2 gpg and the bottom quarter below 0.1 gpg.

Wells 0.7 gpg, rivers and lakes 2.9 gpg

40 groundwater samples from 30 wells put the median at 12.7 mg/L. 16 surface samples from 9 sites put it at 49 mg/L. In St. Mary's County the rivers and lakes come out 2.2 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.

St. Mary's County against the rest of Maryland

Among the 24 Maryland counties with data, St. Mary's County is number 23 of 24 counting from the hardest. Across the whole country it is harder than 4% of counties.

Softest in Maryland: Charles County at 0.3 gpg. Hardest: Washington County at 18.5 gpg. Closest to St. Mary's County: Charles County (0.3), Wicomico County (1.7), Anne Arundel County (2.1).