If you turn on a tap in your Lake St. Louis home and catch a smell like rotten eggs, the first thing to know is that the odor usually comes from a gas in the water or from a part of the plumbing, not from a problem with the whole neighborhood. Homes on the public system get treated water from the utility, so a sulfur smell that shows up in one house is more often tied to a private well, a pressure tank, a water heater, or a supply line. If your home runs on a well, this guide walks through a simple order of checks.
Find out which line carries the smell
Run the cold tap at a bathroom sink, then run the hot tap at the same sink. Note which one smells, and whether the smell is strong at both or only one. Repeat the check in the kitchen, the laundry, and any outdoor hose bib.
- Smell in cold water only: the likely places to look are the well, the pressure tank, and the supply line before any water heater.
- Smell in hot water only: the water heater is a common place to check, since the tank and its parts can react with the water.
- Smell in both: start with the well and the main supply line.
What usually causes a sulfur smell
The most common cause of a rotten-egg smell in well water is hydrogen sulfide, a gas that can occur naturally in groundwater, especially in some deeper bedrock wells. Some sulfur odors come from bacteria that grow in the well, the pressure tank, or a wet part of the plumbing. Hydrogen sulfide is a gas, so the smell can be strong at the tap and weaken once the water sits in an open container. Bacterial odors can linger in fittings and may show up with a slimy film. A smell alone cannot tell you which one you have, so testing is the next step.
Test before you change anything
Treat the smell as a clue, not a diagnosis. Before you buy equipment or start repairs, collect a sample following the lab's instructions and send it to a certified laboratory. Ask for tests covering hydrogen sulfide, total coliform bacteria with E. coli, and the basic minerals that show up in local wells. Lake St. Louis sits within the service area of the former Weldon Spring cleanup site, and homes there may draw on a mix of river and groundwater. Current municipal radium readings are within limits, but a private well is tested separately, so a radium or radionuclide test is worth discussing with the lab.
A simple order of checks
- Write down when the smell appears, such as after the pump runs, after heavy rain, or only in the morning.
- Run the taps for several minutes and note whether the smell fades. A smell that fades quickly can point to a gas that is escaping from the water.
- Check the water heater and its drain valve for odor, and confirm whether the smell is limited to hot water.
- Inspect the pressure tank and well cap for damage, standing water, or debris.
- Collect a sample for a lab test, and keep the report with the date it was taken.
- Compare the results against the tests listed above before deciding on any treatment.
What a softener does and does not do
A water softener is built to reduce hardness minerals, not sulfur gas or bacteria. In a Lake St. Louis home, a softener can help with scale on fixtures and water-using appliances, but it will not fix a rotten-egg smell by itself. Hydrogen sulfide is commonly handled with different equipment, such as an aeration or oxidizing system, and the right choice depends on the lab results and the well. Do not install a softener expecting it to solve a sulfur problem.
When to get professional help
If the lab reports bacteria, or any result you do not understand, contact a certified well professional or the St. Charles County health department for guidance. Ask them to walk you through the report line by line. While you wait for answers, ask them what to do about use of the affected tap. Keep the lab report with your home records so that any future technician has the history in front of them.