Quick Answer: St. Louis water is hard, ranging from 8 to 12 grains per gallon (gpg), sourced from the Missouri and Mississippi Rivers. Both rivers carry substantial industrial and agricultural contamination from upstream states. EWG data shows multiple contaminants of concern in St. Louis water above health-protective limits. A water softener is recommended for scale protection; a reverse osmosis system for drinking water addresses industrial chemicals.
Is St. Louis Water Hard or Soft?
St. Louis Water Hardness Data
According to Missouri American Water (American Water subsidiary) annual water quality reports, St. Louis water hardness ranges from 8 to 12 grains per gallon (gpg) — classified as hard. The city uses chloramine as its primary disinfectant, which requires catalytic carbon filters (not standard activated carbon) for effective removal. See the home water hardness test guide to verify your specific tap's hardness level.
Where Does St. Louis Get Its Water?
St. Louis draws its drinking water from Missouri River and Mississippi River, managed by Missouri American Water (American Water subsidiary).
Missouri American Water serves the greater St. Louis area drawing primarily from the Missouri River at the Howard Bend Intake (west of St. Louis) and the Mississippi River at the Florissant Intake. Both rivers carry contamination from extensive upstream agricultural and industrial activity. Water is treated at the Howard Bend Water Treatment Plant (one of the largest in Missouri) and the Florissant Water Treatment Plant, using conventional treatment with ozonation, granular activated carbon, and chloramine disinfection. St. Louis has been investing in treatment upgrades to address emerging contaminants.
What Contaminants Are in St. Louis Water?
According to Missouri American Water (American Water subsidiary) annual water quality reports and independent EWG Tap Water Database analysis, the primary concerns in St. Louis drinking water include:
- Disinfection Byproducts: St. Louis uses chloramine to disinfect water. When chloramine reacts with natural organic matter, it forms trihalomethanes (TTHMs) and haloacetic acids (HAA5) — compounds linked to increased cancer risk with long-term exposure.
- Key Concerns: Industrial chemical contamination from both Missouri and Mississippi Rivers, hexavalent chromium, PFAS, disinfection byproducts, hard water scale, and agricultural chemical runoff.
- Agricultural and Urban Runoff: Depending on watershed proximity to farmland or industry, nitrates, pesticides, and industrial chemicals may be present at low levels.
Hard Water Effects in St. Louis
At 8–12 gpg, St. Louis water causes significant scale buildup in water heaters, pipes, dishwashers, and faucets. Limescale accumulation reduces water heater efficiency by up to 48% over time and shortens appliance lifespan considerably.
The EPA estimates hard water costs US households $800–$1,500 per year in excess energy, detergent use, and appliance wear. For St. Louis residents with hard water, investing in a quality water softener typically pays for itself within 2–4 years.
Best Water Treatment Solutions for St. Louis Homes
1. Water Softener — Yes
Yes — at 8–12 gpg, St. Louis has hard water that significantly impacts appliances and plumbing; a water softener is recommended. A traditional salt-based ion-exchange water softener is the most effective solution for St. Louis's hard water. Look for a softener sized for your household (grain capacity based on water hardness × daily usage). See our top water softener picks for 2026.
2. Whole-Home Carbon Filtration
A whole-home carbon filter removes chloramine, disinfection byproducts (TTHMs, HAA5), chlorine taste and odor, and many industrial chemicals before water enters your home's plumbing. For chloramine-treated water like St. Louis's, use a catalytic carbon filter — standard activated carbon removes chloramine much less effectively.
3. Reverse Osmosis System (Drinking Water)
For drinking and cooking water, a reverse osmosis (RO) system under the kitchen sink is the most comprehensive solution. RO removes dissolved minerals to near-zero levels, plus filters out PFAS, nitrates, heavy metals, radium, disinfection byproducts, and most other contaminants of concern in St. Louis water. Look for NSF/ANSI 58-certified systems. See our guide on water treatment options for 2026.
St. Louis Water Hardness vs. Other Major Cities
| City | Hardness (gpg) | Classification |
|---|---|---|
| Las Vegas, NV | 16–18 | Extremely Hard |
| Phoenix, AZ | ~16 | Extremely Hard |
| Dallas, TX | ~14 | Very Hard |
| St. Louis, MO | 8–12 | Hard |
| Chicago, IL | ~8.2 | Hard |
| Ottawa, ON | 2.5–5 | Soft to Moderately Soft |
| Seattle, WA | ~1.2 | Soft |
How to Test Your Water Hardness at Home
You can verify St. Louis's water hardness at your specific tap using these simple methods:
- Test strips: Dip a water hardness test strip in a glass of cold tap water. Results appear in seconds. Accuracy: ±1–2 gpg. Inexpensive and widely available.
- The soap test: Fill a clear bottle halfway with tap water, add 10 drops of pure liquid castile soap, and shake vigorously. Abundant, persistent suds = soft water. Milky, soapy film with few suds = hard water.
- Visual check: White crusty deposits inside your toilet tank, on showerheads, or around faucet bases are limescale — a reliable sign of hard water above ~7 gpg.
- Lab test: For precise results, send a water sample to a certified lab. This also tests for contaminants beyond hardness. See our full home water testing guide.