Hard water in Duncan, OK

Tap water hardness
290mg/L
17.0gpg
Very hardCounty estimate

County estimate from 231 water samples in Stephens County · July 2026

OK typical
152 mg/L
EPA health violations since 2016
44
Softener for 3 people
32k grain
Data
County estimate

Water samples from around Duncan put hardness at about 290 mg/L (17.0 gpg), which would be very hard on the USGS scale. Treat this as a rough guide: it is the median of 231 samples taken across Stephens County, mostly wells and streams, not a test of Duncan's treated tap water. One more reason for caution: EPA lists Duncan's water system as using surface water, which is often softer than the wells behind the county figure.

Duncan on the hardness scale, with nearby cities
060120180240300360+OK typical 152Moore: 94 mg/LMoore 94Norman: 148 mg/LNorman 148Lawton: 152 mg/LLawton 152Newcastle: 157 mg/LNewcastle 157Yukon: 157 mg/LYukon 157Oklahoma City: 157 mg/LOklahoma City 157Bethany: 221 mg/LBethany 221Ardmore: 242 mg/LArdmore 242Mustang: 463 mg/LMustang 463El Reno: 463 mg/LEl Reno 463Duncan 290mg/L as calcium carbonate060120180240300360+OK 152Moore: 94 mg/LMoore 94Norman: 148 mg/LNorman 148Lawton: 152 mg/LLawton 152Newcastle: 157 mg/LNewcastle 157Ardmore: 242 mg/LArdmore 242Mustang: 463 mg/LMustang 463290 mg/Lmg/L as calcium carbonate
Soft 0–60 mg/LModerately hard 61–120 mg/LHard 121–180 mg/LVery hard 181+ mg/L

At 290 mg/L, Duncan is 138 mg/L above the typical Oklahoma figure of 152. Of the 10 nearest cities we track, 8 have softer water and 2 have harder. The range nearby runs from 94 mg/L in Moore to 463 in Mustang.

What 290 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 290 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.

Softener size for Duncan

3 people × 75 gallons a day × 17.0 gpg = 3,816 grains a day. A 32,000-grain unit regenerates about every 8 days.

Size it for your household

EPA record for Duncan's water system

Duncan Public Utilities Authority · PWSID OK1010809

44 health-based violations since 2016

Serves about 23,000 people from surface water.

Health-based
44
Monitoring or reporting
11
Open now
0

EPA lists this system as serving Duncan.

20162017201820192020202120222023202420252026Total organic carbon removalTotal trihalomethanes (TTHM)Haloacetic acids (HAA5)Surface Water Treatment Rule'16'18'20'22'24'26Total organic carbon removalTotal trihalomethanes (TTHM)Haloacetic acids (HAA5)Surface Water Treatment Rule
Each dot is one health-based violation, placed at the start of its compliance period.
Contaminant or ruleViolationsPeriodStatus
Total organic carbon removaltreatment requirement52020–2024Resolved
Total trihalomethanes (TTHM)a disinfection byproduct302016–2023Resolved
Haloacetic acids (HAA5)a disinfection byproduct42016–2017Resolved
Surface Water Treatment Rulefiltration and disinfection12016Resolved

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 Duncan water hard?

Yes. It measures about 290 mg/L (17.0 grains per gallon), which the USGS classes as very hard.

What is the water hardness in Duncan in grains per gallon?

About 17.0 gpg. One grain per gallon equals 17.1 mg/L, so 290 mg/L divided by 17.1 gives 17.0.

What size water softener do I need in Duncan?

For 3 people using 75 gallons each per day, 17.0 gpg works out to about 3,816 grains a day. A 32,000-grain softener covers that with a regeneration about every 8 days.

Has the Duncan water system had EPA violations?

EPA records show 44 health-based violations for Duncan Public Utilities Authority since 2016, involving Total organic carbon removal, Total trihalomethanes (TTHM), Haloacetic acids (HAA5). There were also 11 monitoring or reporting violations, which are about late or missing tests and notices.

Nearby cities

All 34 cities in Oklahoma

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.