Jefferson County water by the numbers: what the Ozark aquifer does to a well

I've tested water in Jefferson County since before some of these subdivisions existed, and the question I get asked most out here isn't "is my water hard." It's "why does my neighbor's water look so different from mine when we're two miles apart." The honest answer is almost always the same word: aquifer. So this time I pulled every Jefferson County town in our own dataset and laid the real numbers side by side.

How this table was built: This pulls all 13 Jefferson 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. The "Private wells nearby" column is the specific range our dataset's own well notes give for that town. Where our notes don't give a specific gpg range for private wells there, I left it blank instead of guessing.

The pattern nobody notices until you look at all 13 at once

Look at the spread on the public-system column: 5.3 grains in Festus, 21.0 in De Soto, both the same county. Averaged straight across all 13 town public-system numbers, not weighted by population, Jefferson County lands at roughly 13.3 gpg, itself in the Water Quality Association's "very hard" range. Seven of these 13 towns carry our F grade, three carry a C, and three carry a B. Nothing sits in the flat middle.

Here's the part that actually matters for a homeowner: the three lowest public numbers, Festus, Herculaneum and High Ridge, all get there the same way. Festus runs chlorinated Mississippi River water. Herculaneum's is described in our own notes as "chlorinated, river-influenced groundwater." High Ridge draws partly from the Big River surface supply. A public system blending in river water can post a soft-looking number even in a county sitting on hard bedrock. But that number describes the treated water coming through the city's pipe, not the well two streets over. Our own well notes for all three of those towns say the same thing: private wells there tapping the deeper bedrock still run an estimated 15 to 25-plus grains, because they're pulling straight from the same Ozark-area limestone and dolomite that makes De Soto, Hillsboro and Pevely so hard on the public side.

Iron, sulfur, and the one thing nobody thinks to test for

Almost every well note in this county mentions the same three nuisance issues: iron and manganese causing orange-brown or black staining and a metallic taste, hydrogen sulfide giving well water that rotten-egg smell, and hardness itself caking scale onto faucets, showerheads and water heater elements. That part is consistent enough that I can usually guess it before I open the test kit.

Radium is the one that doesn't announce itself. Our dataset describes Crystal City as sitting over "the deep Ozark/Potosi-Eminence aquifer, one of the most radium-prone aquifers in the country," where deep private wells routinely hit or exceed the federal 5 pCi/L limit. It's not just a Crystal City story. De Soto's own city wells already measure combined radium at 2.5 pCi/L, Pevely's at 1.6 pCi/L, and Hillsboro's at 2.1 pCi/L, described in our notes as "far over the EWG health guideline" even though all three are legally compliant. Radium doesn't change the taste, smell or hardness of your water. It takes a dedicated radiological test to find it, which is exactly why it's the easiest thing to miss on a well that's never had one.

Lead and the Southwest Jefferson County Mining Superfund

Seven of the 13 towns on this list, Byrnes Mill, Cedar Hill, De Soto, Festus, Hillsboro, House Springs and High Ridge, sit inside or next to the Southwest Jefferson County Mining Superfund Site, where historic lead-belt mining waste reached the same Ozark groundwater private wells draw from. EPA tested 1,414 properties across that program and found 147 of them, 10.4%, over the health limit for lead, cadmium, arsenic or barium. Festus has its own specific number on record too: 1,300-plus wells sampled locally, with 130-plus exceeding the 15 ppb lead action level. That's a documented, government-confirmed contaminant, not a scare number I'm making up to sell a filter.

What I actually recommend if you're on a Jefferson County well

Don't just test for hardness. A basic hardness test tells you whether you need a softener, but it tells you nothing about iron, sulfur, bacteria, radium or lead sitting in the same water. If you're anywhere in this county, especially in one of the seven towns tied to the mining Superfund site, I'd add a full panel: hardness, iron/manganese, coliform and E. coli, and a radiological test for radium. If lead comes back elevated, or if you're on the deep bedrock aquifer where radium shows up, reverse osmosis at the point of use is the proven fix for both. Softening handles the scale; it doesn't touch either of those on its own.

For the well-water lessons I've picked up across 30 years of testing this whole region, not just Jefferson County, see well water vs. city water: what 30 years of testing taught me. To see how these 13 towns stack up against the rest of our 57-town service area, see the hardest water towns we serve, ranked by the numbers. And for a look at how the same radium question plays out in a different county, see radium in Lincoln County wells: what the EPA numbers actually say.

Frequently Asked Questions

Schedule a Free Water Test With Jones Air & Water

If you're on a private well anywhere in Jefferson County, I'll test it in person, at your sink, for hardness, iron, sulfur, bacteria and radium, and tell you honestly what it means for your home.

Schedule your free water test or see the reverse osmosis systems we install for radium, lead and other dissolved contaminants.