Hard water in Charter Oak, CA
Reported by GSWC-San Dimas · September 2025
- CA typical
- 121 mg/L
- EPA health violations since 2016
- 0
- Softener for 3 people
- 32k grain
- Data
- Utility report
Tap water in Charter Oak has about 275 mg/L of hardness, which is 16.1 grains per gallon. On the USGS scale that counts as very hard. The number comes from GSWC-San Dimas, dated September 2025.
Charter Oak sits 154 mg/L above the typical California figure of 121. None of the 10 nearest cities we track has harder water. The range nearby runs from 112 mg/L in Citrus to 275 in San Dimas.
What 275 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 275 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 × 16.1 gpg = 3,618 grains a day. A 32,000-grain unit regenerates about every 8 days.
EPA record for Charter Oak's water system
No health-based violations since 2016
Serves about 56,564 people from surface water.
- Health-based
- 0
- Monitoring or reporting
- 0
- Open now
- 0
TapWaterData ties Charter Oak to this system. EPA's service-area records do not name Charter Oak, 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 Charter Oak water hard?
Yes. It measures about 275 mg/L (16.1 grains per gallon), which the USGS classes as very hard.
What is the water hardness in Charter Oak in grains per gallon?
About 16.1 gpg. One grain per gallon equals 17.1 mg/L, so 275 mg/L divided by 17.1 gives 16.1.
What size water softener do I need in Charter Oak?
For 3 people using 75 gallons each per day, 16.1 gpg works out to about 3,618 grains a day. A 32,000-grain softener covers that with a regeneration about every 8 days.
Has the Charter Oak water system had EPA violations?
EPA records show no health-based violations for GSWC-San Dimas since 2016.
Nearby cities
- Covina167 mg/LHard
- Citrus112 mg/LModerately hard
- San Dimas275 mg/LVery hard
- Glendora122 mg/LHard
- Vincent112 mg/LModerately hard
- Azusa116 mg/LModerately hard
- West Covina170 mg/LHard
- Walnut210 mg/LVery hard
- La Verne231 mg/LVery hard
- Valinda223 mg/LVery hard
Hardness data: TapWaterData US Water Hardness (utility report, September 29, 2025), CC BY 4.0. Population: US Census Bureau. Violations: EPA ECHO. How we use them.