Water hardness in St. Charles County, Missouri

City water in St. Charles County ranges from about 5 to 17.3 grains per gallon, depending on who supplies your street. Most of it is hard or very hard on the U.S. Geological Survey scale. The numbers below come from each utility's own 2025 water quality report, except Missouri American Water's, which we worked out from the minerals in its report.

What the numbers mean

U.S. Geological Survey hardness classes

U.S. Geological Survey classes, converted to grains per gallon (17.1 mg/L per grain).

Soft

0 to 3.5

0 to 60 mg/L

Moderately hard

3.5 to 7

61 to 120 mg/L

Hard

7 to 10.5

121 to 180 mg/L

Very hard

over 10.5

over 180 mg/L

Most of St. Charles County

Soft, 0 to 3.5. 0 to 60 mg/L.
Moderately hard, 3.5 to 7. 61 to 120 mg/L.
Hard, 7 to 10.5. 121 to 180 mg/L.
Very hard, over 10.5. over 180 mg/L.

Hardness is the amount of calcium and magnesium dissolved in water. Utilities report it in milligrams per liter. Water treatment uses grains per gallon. One grain per gallon is 17.1 milligrams per liter.

By provider

Wentzville city wells

17.3 grains per gallon

296 mg/L. Very hard.

One sample, taken 3/20/2023.

Sodium 44.5 mg/L. Total dissolved solids 414 mg/L.

City of Wentzville 2025 water quality report

City of O'Fallon water system

7.2 to 16.5 grains per gallon

123 to 283 mg/L. Hard to very hard.

Range across the city's sources. It depends on which source feeds your street.

The city system serves about 32,500 people. Other O'Fallon homes are on PWSD 2 or Missouri American Water.

City of O'Fallon 2025 water quality report

Public Water Supply District 2 of St. Charles County (PWSD 2)

8.3 to 10.8 grains per gallon

142 to 184 mg/L. Hard to very hard.

Range across the district's sources.

Serves about 100,000 people. It also sells water to Wentzville and O'Fallon.

PWSD 2 2025 water quality report

City of St. Peters

10.1 grains per gallon

173 mg/L. Hard.

As printed in the 2025 report.

Sodium 75.3 mg/L. Total dissolved solids 482 mg/L.

City of St. Peters 2025 water quality report

City of St. Charles

8.9 grains per gallon

152 mg/L. Hard.

As printed in the 2025 report.

Sodium 15.1 mg/L. Total dissolved solids 212 mg/L.

City of St. Charles 2025 water quality report

Missouri American Water (St. Louis and St. Charles counties)

about 5 to 7 grains per gallon

about 83 to 119 mg/L. Moderately hard.

Our calculation. The report prints no hardness number, so we worked it out from the calcium and magnesium it lists.

Serves parts of O'Fallon and Dardenne Prairie.

Missouri American Water 2025 water quality report

By town

Wentzville
Wentzville homes get water from the city's own wells, from water the city buys from PWSD 2, or a mix of the two. Which one reaches your tap depends on your street.
O'Fallon
O'Fallon has three water providers. The city system serves about 32,500 people, and other homes are on PWSD 2 or Missouri American Water. Your water bill names yours.
St. Peters
The City of St. Peters runs its own water system, which serves about 52,500 people.
St. Charles
The City of St. Charles runs its own water system, which serves about 73,000 people.
Lake Saint Louis
Lake Saint Louis does not run its own water system. The city's map of water providers shows PWSD 2 serving nearly all of the city, and the City of O'Fallon and the City of Wentzville serving a few small areas. Your water bill names your provider.
Dardenne Prairie
Dardenne Prairie homes get PWSD 2 or Missouri American Water, depending on the address. The city's utilities page says so.

Why your tap may differ

Several towns here get water from more than one source, and utilities blend and switch sources through the year. The reports give a range or a single sample, not a reading for your street. That is why we test at your tap before recommending anything.

Figures are from each utility's 2025 water quality report as published on the Missouri Department of Natural Resources website, read on October 27, 2026. Most hardness results are 2023 samples reprinted in the 2025 reports. Last reviewed November 10, 2026.