St. Charles County town by town: hardness grades from our own testing data
St. Charles County is where I've done more service calls than anywhere else in our territory, and it's also the most consistent county in our whole dataset, for better and for worse. Most of these 14 towns land within a couple grains of each other. Only one breaks the pattern, and it breaks it with something more interesting than hardness. Here's every town, with the real numbers next to each one.
How this table was built: This pulls all 14 St. Charles County, Missouri towns in Jones Air & Water's own 57-town water dataset, built from utility consumer confidence reports, DNR and EPA well data, and our own in-home testing, pulled in 2026. Towns are sorted by public-system hardness in grains per gallon (gpg), high to low.
| Town | Hardness (gpg) | Grade |
|---|---|---|
| Flint Hill | 16.7 | F |
| Wentzville | 14.0 | D |
| Augusta | 10.8 | D |
| Lake St. Louis | 10.8 | D |
| New Melle | 10.8 | D |
| Portage Des Sioux | 10.8 | D |
| Cottleville | 10.7 | D |
| Foristell | 10.7 | D |
| Dardenne Prairie | 10.0 | D |
| Defiance | 10.0 | D |
| Weldon Spring | 9.6 | C |
| St. Peters | 9.0 | C |
| St. Charles | 8.9 | C |
| O'Fallon | 8.0 | C |
The one outlier: Flint Hill
Every other town in this county grades C or D. Flint Hill is the only F, and hardness isn't even the main reason. Our dataset lists Flint Hill's village well at 16.7 gpg, well above the rest of the county, but the bigger issue is what's riding along with it: naturally occurring radium measured at 2.5 pCi/L in that village well, roughly 50 times the EWG health guideline, plus gross alpha radioactivity at 7 pCi/L. Both numbers are legally compliant. Neither one is something you'd catch with a basic hardness test. Our own notes on Flint Hill's surrounding wells say private wells in that part of the county "likely" carry similar radium, since they tap the same limestone and dolomite bedrock as the village well.
Why most of the county clusters so tight
Set Flint Hill and Wentzville aside and the other 12 towns all sit between 8.0 and 10.8 gpg, every one graded C or D. Averaged straight across all 14 town numbers, not weighted by population, St. Charles County comes out to roughly 10.8 gpg. That's a much tighter, gentler spread than what we found next door in Jefferson County, where the same kind of table ran from 5.3 to 21.0 gpg. Most public systems here blend purchased Missouri River water with local wells, which flattens out the extremes you get on straight bedrock groundwater. It also means most of these towns land squarely on the Water Quality Association's "very hard" line rather than blowing past it.
What ties nearly half these towns together: Weldon Spring
Six of the fourteen towns in this county's dataset entries specifically flag the old Weldon Spring uranium site as a documented reason for nearby well owners to test: Augusta, Lake St. Louis, O'Fallon, St. Charles, St. Peters, and Weldon Spring itself. Weldon Spring is the site of the old Mallinckrodt uranium plant, a $1 billion, 16-year federal cleanup, and our dataset notes that documented uranium, radium and nitrate leaked more than 100 feet into groundwater there, with Burgermeister Spring still testing above background today. Missouri regulators said in 2021 that the groundwater situation there "isn't improving," a detail specifically flagged in our own O'Fallon well notes. The city taps in this county test compliant. This is specifically a private-well story for homes in the western and southwestern part of the county, near Defiance, Augusta and Weldon Spring itself.
What your private well number probably is, even if the town's isn't
Almost every well note attached to these 14 towns describes the same thing: private wells here commonly run 15 to 25-plus grains, noticeably harder than any of the treated public numbers in the table above, because they're pulling straight from the Missouri and Mississippi River alluvial aquifer or the deeper limestone and dolomite underneath it. Iron and manganese staining, that rotten-egg hydrogen sulfide smell, and nitrate from ag and septic runoff show up again and again across the county's well notes. Our Dardenne Prairie entry cites a statewide Missouri survey that found 56% of sampled private wells positive for total coliform bacteria and 22% for E. coli, a figure that applies to any untested well in this county, not just Dardenne Prairie's.
What I actually recommend for a St. Charles County well
If you're on city water anywhere in this county, hardness is your main daily problem, and a softener sized to your actual number handles it. If you're on a private well, don't stop at hardness. I'd test for iron and manganese, coliform and E. coli bacteria, and nitrate everywhere in the county, and I'd specifically add a radium test if you're anywhere near Flint Hill, Defiance, Augusta or Weldon Spring. Reverse osmosis is the standard fix once a test actually confirms radium or nitrate, not before.
For how this county compares to the rest of our service area, see the hardest water towns we serve, ranked by the numbers, and for the same kind of county breakdown next door, see Jefferson County water by the numbers. For more on what 30 years of testing this whole region has taught me, see well water vs. city water: what 30 years of testing taught me.
Frequently Asked Questions
Why is Flint Hill the only F-grade town in St. Charles County?
Is Weldon Spring's water actually contaminated today?
Schedule a Free Water Test With Jones Air & Water
Whether you're on one of these 14 town systems or a private well anywhere in St. Charles County, I'll test your actual water at your sink and tell you honestly what it means for your home.
Schedule your free water test or see the water softener systems we install for hardness in this exact range.