If the water in your Lake St. Louis home smells like rotten eggs, the usual cause is hydrogen sulfide, a gas that dissolves in water and can be noticed at very low levels. It is a naturally occurring compound. It does not usually change the color of the water, so the smell is often the only clue you have.
Homes in the Lake St. Louis area draw water from a mix of river-influenced sources and groundwater. Municipal supply is treated before it reaches your tap, so a sulfur smell in town water is less common and is worth reporting to your water provider. Private wells are different. Deeper bedrock wells in the region are the ones most often linked with hydrogen sulfide, while shallow river-bottom wells more often show iron and manganese staining.
Check Where the Smell Shows Up
Before you test, narrow down where the smell is coming from. Run the cold tap at a bathroom sink for a minute, then smell the water. Next, run the hot tap at the same sink. If only the hot side smells, the source may be bacteria in the water heater rather than the well or the supply line.
Also notice whether the smell appears after the water has sat unused, such as after a trip or a long quiet stretch. A smell that clears after the tap runs for a while points toward standing water in the plumbing.
Other Causes to Rule Out
- Drain traps that have dried out, letting sewer gas into the room
- Water heater anode rods reacting with sulfur-loving bacteria
- A well that was recently serviced or disinfected and has not been fully flushed
Test Steps for a Private Well
If you have a private well in or around Lake St. Louis and the smell is in the cold water, test before you commit to any treatment. A certified lab can measure hydrogen sulfide and other parameters. Many labs offer a package that also covers coliform bacteria, nitrates, and basic chemistry. Ask the lab which tests are included and how samples should be collected.
- Collect the sample in the container the lab provides, following its sampling instructions exactly.
- Label the sample with the date, the time, and the tap used, such as an untreated kitchen cold tap.
- Keep the sample cool and deliver it to the lab within the window the lab specifies.
Reading the Results
A lab report usually lists each test with its result and a reference level. If the smell is your main concern, look at the hydrogen sulfide result first. Then check the bacteria results, because a sulfur smell alone does not tell you whether the water is suitable for drinking and cooking. If a bacteria result is positive, contact a qualified water professional or your county health office before using that water for drinking or cooking.
Radium and other radionuclides occur naturally in some deep eastern Missouri bedrock aquifers, and the former Weldon Spring cleanup site sits within the broader service area. Current municipal radium readings are within limits, but a private well draws from its own source. If your well is deep, including radium in your testing is a reasonable step.
Treatment Comes After Testing
Once you know what is in the water, you can talk about treatment options. Treatment for hydrogen sulfide depends on the level found and on whether iron is also present. Bring your lab report to any water treatment conversation so the discussion starts from your own results rather than a general guess. Keep every report, dated, in one folder.
Keep Notes Between Tests
- Record when the smell is strongest, such as after rain or after the water has been idle.
- Note any staining, dark deposits, or changes that followed well or pump work.
- Write down any recent work on the well, pump, or pressure tank.
These notes help whoever reads your results and make a repeat test more useful. If you are unsure what a report means, a certified lab or your county health office can explain it in plain terms.