Hard water in Cottleville, MO
Reported by ST Charles County PWSD 2 · October 2025
- MO typical
- 121 mg/L
- EPA health violations since 2016
- 0
- Softener for 3 people
- 24k grain
- Data
- Utility report
Cottleville water measures 120 mg/L (7.0 gpg) of hardness, hard by USGS standards. The number comes from ST Charles County PWSD 2, dated October 2025. Reports for this system disagree, with values from 120 to 206 mg/L.
At 120 mg/L, Cottleville is close to the typical Missouri figure of 121 mg/L. Of the 10 nearest cities we track, 1 has softer water and 9 have harder. The range nearby runs from 79 mg/L in Eureka to 263 in Des Peres.
What 120 mg/L means at home
Expect white crust on faucets and showerheads, plus spots on dishes that come out of the dishwasher.
Kettles collect scale quickly. Water heaters build up sediment at the bottom of the tank, which makes them work harder over time.
Soap and detergent need a little more to lather, and soap scum shows up in the tub.
Hardness is not a health problem, and EPA sets no limit for it. It is a matter of scale and soap.
What to do about it
At 120 mg/L most of the trouble is scale, and only a few things deal with it.
Ion-exchange water softener
The standard whole-house fix. It swaps calcium and magnesium for sodium or potassium and needs salt plus a drain. Size it with the calculator below.
Salt-free conditioner
No salt and no drain line. It changes how minerals form scale instead of removing them, so water still tests hard.
Reverse osmosis or carbon filter
Under-sink reverse osmosis removes minerals from drinking water only. Plain carbon filters improve taste but do not soften.
3 people × 75 gallons a day × 7.0 gpg = 1,586 grains a day. A 24,000-grain unit regenerates about every 15 days.
EPA record for Cottleville's water system
No health-based violations since 2016
Serves about 100,587 people from surface water.
- Health-based
- 0
- Monitoring or reporting
- 2
- Open now
- 0
TapWaterData ties Cottleville to this system. EPA's service-area records do not name Cottleville, so check the utility printed on your water bill.
Health-based means a contaminant above its legal limit or a missed treatment step. Monitoring and reporting violations are late or missing tests and notices. Source: EPA ECHO, 2026Q3 export, violations with periods starting 2016 or later. Full EPA record.
Questions
Is Cottleville water hard?
Yes. It measures about 120 mg/L (7.0 grains per gallon), which the USGS classes as hard.
What is the water hardness in Cottleville in grains per gallon?
About 7.0 gpg. One grain per gallon equals 17.1 mg/L, so 120 mg/L divided by 17.1 gives 7.0.
What size water softener do I need in Cottleville?
For 3 people using 75 gallons each per day, 7.0 gpg works out to about 1,586 grains a day. A 24,000-grain softener covers that with a regeneration about every 15 days.
Has the Cottleville water system had EPA violations?
EPA records show no health-based violations for ST Charles County PWSD 2 since 2016. There were also 2 monitoring or reporting violations, which are about late or missing tests and notices.
Nearby cities
- St. Peters168 mg/LHard
- Lake St. Louis123 mg/LHard
- Chesterfield121 mg/LHard
- Maryland Heights121 mg/LHard
- Ballwin242 mg/LVery hard
- Town and Country234 mg/LVery hard
- Des Peres263 mg/LVery hard
- Ladue155 mg/LHard
- Kirkwood130 mg/LHard
- Eureka79 mg/LModerately hard
Hardness data: TapWaterData US Water Hardness (utility report, October 7, 2025), CC BY 4.0. Population: US Census Bureau. Violations: EPA ECHO. How we use them.