Hard water in Arnold, MO
County estimate from 213 water samples in Jefferson County · July 2026
- MO typical
- 121 mg/L
- Softener for 3 people
- 24k grain
- Data
- County estimate
Water samples from around Arnold put hardness at about 212 mg/L (12.4 gpg), which would be very hard on the USGS scale. Treat this as a rough guide: it is the median of 213 samples taken across Jefferson County, mostly wells and streams, not a test of Arnold's treated tap water.
Arnold sits 91 mg/L above the typical Missouri figure of 121. Of the 10 nearest cities we track, 4 have softer water and 6 have harder. The range nearby runs from 79 mg/L in Eureka to 346 in Herculaneum.
What 212 mg/L means at home
Scale forms fast here. Kettles and coffee makers need regular descaling, and showerheads clog unless you clean them.
Tank water heaters collect mineral sediment that insulates the water from the burner, so flushing the tank matters more than usual.
Dishes and glass shower doors spot easily. Soap scum and a dull film on hair or laundry are common complaints.
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 212 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 × 12.4 gpg = 2,790 grains a day. A 24,000-grain unit regenerates about every 8 days.
EPA record for Arnold's water system
We could not match Arnold to a community water system in EPA's records. Your water bill or the yearly Consumer Confidence Report names the system and its ID.
Questions
Is Arnold water hard?
Yes. It measures about 212 mg/L (12.4 grains per gallon), which the USGS classes as very hard.
What is the water hardness in Arnold in grains per gallon?
About 12.4 gpg. One grain per gallon equals 17.1 mg/L, so 212 mg/L divided by 17.1 gives 12.4.
What size water softener do I need in Arnold?
For 3 people using 75 gallons each per day, 12.4 gpg works out to about 2,790 grains a day. A 24,000-grain softener covers that with a regeneration about every 8 days.
Nearby cities
- Murphy104 mg/LModerately hard
- Pevely308 mg/LVery hard
- Kirkwood130 mg/LHard
- Herculaneum346 mg/LVery hard
- Des Peres263 mg/LVery hard
- Festus247 mg/LVery hard
- Ladue155 mg/LHard
- Ballwin242 mg/LVery hard
- Town and Country234 mg/LVery hard
- Eureka79 mg/LModerately hard
Hardness data: TapWaterData US Water Hardness (county estimate, July 4, 2026), CC BY 4.0. Population: US Census Bureau. Violations: EPA ECHO. How we use them.