Rotten egg smell: sulfur, sulfate and bacteria in local wells

Everybody calls it sulfur water. Our own notes use the phrase too, because that's what people say. But "rotten egg smell" is a symptom, not a diagnosis, and in our records it has three different causes that want three different answers. One of them is a nuisance. One of them is a taste problem. One of them is a health question. Getting them mixed up is how people end up buying equipment that doesn't fix the smell.

How this was built: Figures come from Jones Air & Water's own 57-town water dataset, built from utility consumer confidence reports, USGS and DNR well data, and our own in-home testing, pulled in 2026. Our well notes describe typical regional conditions where a specific home has not been tested, and I've flagged which is which rather than passing estimates off as measurements.

Three causes, one smell

The first is hydrogen sulfide gas, written H2S. Across our well notes it shows up again and again tied to the same condition: deeper, low-oxygen, reducing wells. Our Godfrey, High Ridge, Herculaneum, Gerald and Weldon Spring notes all put the rotten egg smell specifically in the deeper wells rather than the shallow ones. This is the common one, and on its own it is a nuisance rather than a hazard.

The second is sulfate, the dissolved mineral. Sulfate itself is not what you smell. It's what bacteria can turn into what you smell. Our Alton notes put it plainly, that sulfate and bacteria together drive the rotten egg odor, and our Godfrey notes name the mechanism directly, describing the smell as coming from iron and sulfate reducing bacteria.

The third is the bacteria themselves. Our Foley record lists hydrogen sulfide and sulfur bacteria together as a single line item. Our O'Fallon notes describe the rotten egg smell arriving with black slime and sulfur bacteria. Our Gerald notes mention slimy iron bacteria alongside the smell. When there's slime, you are dealing with something living in the well or the plumbing, and that changes the fix.

What sulfate actually measures out at around here

Sulfate has a secondary standard of 250 mg/L, and our records describe what happens as you approach it: a bitter or soda-like taste and a laxative effect. Here is every town in our dataset with a measured sulfate figure.

Notice that the same 207 figure turns up in all five records, and that four of the five are St. Charles County. That is the kind of repetition you see when towns draw on a shared or purchased supply rather than five separate coincidences. None of those numbers is illegal. All of them are close enough to the line that a bitter taste is a reasonable thing to notice, and worth saying out loud: at 207 mg/L, the taste complaint is not in your head.

Our Augusta notes add that the bluff bedrock wells trend harder and higher in sulfate than the river bottom wells, so on a private well the number can run above anything in that table.

The bacteria question, which is the one that matters

This is where I get serious with people. Our Labadie record cites a Missouri survey of 861 private wells that found 56% positive for total coliform and 22% positive for E. coli. More than half, and better than one in five.

That figure appears across many of our town records because it applies to any untested well in this region, not to one town. And it is the reason I do not treat a rotten egg smell as purely cosmetic until I know whether bacteria are part of it. Coliform and E. coli do not smell like anything. You cannot taste them. A well can smell fine and be contaminated, and a well can smell terrible from plain H2S gas and be bacteriologically clean. The smell and the safety are two separate questions and the only thing that links them is that both get answered by the same test visit.

Our notes repeatedly tie bacteria spikes to specific conditions: shallow flood prone wells after high water, cracked casings, and proximity to septic and livestock. Our Foley notes call for post flood disinfection and UV on floodplain wells. Our High Ridge notes tie recurring coliform and E. coli to septic and livestock density.

What the smell does and does not tell you

Depth is the most useful clue in our data. If your well is deep and the smell has been there consistently since you moved in, our notes point toward plain hydrogen sulfide in a reducing aquifer, which is the ordinary nuisance case.

Slime is the clue that changes my mind. Black slime in the toilet tank or on fixtures alongside the smell is what our O'Fallon notes describe with sulfur bacteria, and that is biological rather than geological.

A new or suddenly worse smell after flooding is the one I want to hear about soonest, because our notes connect high water to bacteria, not to a change in the aquifer's chemistry.

And a bitter or soda-like taste without much smell points at sulfate and total dissolved solids rather than at H2S at all. Those are different complaints with different fixes, which is why I ask people to describe taste and smell separately instead of lumping them into "it's bad."

What we actually install, in our own words

Our own Granite City record spells out the typical treatment train for this geology, and I'd put it almost exactly that way myself: whole house iron and sulfur filtration, a softener, UV, and reverse osmosis for drinking water. Our High Ridge notes pair the rotten egg smell with filtration and aeration, and pair the bacteria with UV sanitation. Our St. Clair notes say to lead with UV and R/O where bacteria and nitrate are the risk, and to lead with softener plus filtration where hardness and metals are the risk.

The ordering in those notes is the useful part. UV is for the living things. Filtration and aeration is for the gas and the metals. The softener is for the 15 to 25 plus grains that our well notes say comes along with all of it out here. Reverse osmosis is for what you actually drink. Which of those four you need depends on which of the three causes you've got, and that is a test question, not a catalog question.

One local item that is not about smell at all

Worth separating out because sulfate is in it. Our Labadie record notes coal ash contamination, specifically arsenic, boron, sulfate, cadmium and mercury, from Ameren's unlined ponds there, and records that Ameren has not tested nearby residential wells. If you are on a well near Labadie, that is a sulfate story with nothing to do with rotten egg odor, and it deserves its own test rather than being folded into a smell complaint.

For how hardness stacks up across the towns in this dataset, see the hardest water towns we serve, ranked by the numbers, and for the longer view on wells versus city water in this region, see well water vs. city water: what 30 years of testing taught me.

Frequently Asked Questions

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If your water smells like rotten eggs, I'd rather find out which of the three causes you've got before anybody talks about equipment. I'll test your actual water, include the bacteria question rather than guessing at it, and tell you honestly whether what you have is a nuisance or something that needs handling.

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