A rotten-egg smell from the tap is one of the most common complaints from homeowners who use private wells, and it shows up in O'Fallon homes too. The odor usually comes from hydrogen sulfide, a gas that can form in groundwater or in plumbing where sulfur bacteria grow. You can narrow down the likely source with a few careful observations before you buy any equipment.
Start With Which Taps Smell
Run each tap separately and note where the odor is strongest. The pattern points in different directions.
- Hot water only: the water heater tank, its anode rod, or the hot water lines are the first suspects. Bacteria can grow in a tank that sits unused for stretches, and the heater can add to the problem.
- Hot and cold both: the water source is the more likely cause, which for most O'Fallon homes with a well means the well itself, the pressure tank, or the supply line feeding the house.
- Cold only, faint or only at some times: the smell may be coming from a specific fixture, a drain trap, or a drain that is dry and letting sewer gas in. That is a different problem from water chemistry.
What the Smell Is
Hydrogen sulfide is a naturally occurring gas that gives water its rotten-egg character. It can come from groundwater, or it can be produced by sulfur bacteria that feed on sulfur compounds in a well, a water heater, or pipes. Sulfur bacteria often leave black slime or dark staining in toilet tanks, faucet aerators, and water heater drains. Your nose can also become used to the smell over time, so a faint odor may be easy to miss even when it is present.
Hydrogen sulfide is not the same as chlorine, and it is not removed by simply running a softener. Softeners target hardness minerals, which are a separate issue. Treatment for hydrogen sulfide is usually chosen after testing, and the right approach depends on how strong the gas is and what else is in the water.
Test Before You Treat
Testing gives you a clear answer, and it keeps you from fixing the wrong thing. A practical sequence looks like this:
- Run the cold tap for several minutes, then smell the water right at the faucet. Compare it with a cold tap at a different location, such as an outdoor spigot or a laundry sink.
- Run the hot tap for several minutes, then smell it again. If the odor only appears at the hot tap, the water heater is part of the story.
- Check the aerator screen and the toilet tank for black slime or dark staining. Note what you see and where.
- Ask the Missouri State Public Health Lab about low-cost or free coliform, E. coli, and iron bacteria testing for your well, since those are common companions to sulfur odors in shallow or poorly sealed wells.
- Ask a certified lab whether it can test for hydrogen sulfide directly, and collect the sample exactly as the lab instructs, since the gas can escape from a poorly handled bottle.
Checks for the Water Heater
If the odor is limited to hot water, start with the heater before you assume the well is at fault. Look at the tank's age, any signs of corrosion, and whether the heater has been drained recently. A licensed plumber can inspect the anode rod, which is designed to corrode in place of the tank, and can advise on whether the tank needs cleaning or flushing. Do not try to disinfect a heater on your own without reading the manufacturer's instructions and confirming the steps with a professional.
Checks for the Well and Supply
When both hot and cold taps smell, the source is usually upstream of the house. Ask a well professional to look at the well cap, the seal around the casing, and the pressure tank. Shallow wells near septic systems or farm land deserve extra attention, because surface conditions can affect water quality. Keep copies of every lab result, since a clear record makes it easier to compare results over time and to explain the situation to a technician.
Questions to Settle Before You Call
- Which taps smell, and does the odor change when water runs for a few minutes?
- Is there black slime, dark staining, or a film in the toilet tank or aerator?
- When was the water heater last inspected, drained, or serviced?
- Has the well been tested for coliform, E. coli, and iron bacteria?
- Does the lab report include hydrogen sulfide, and what treatment does the lab recommend after results come back?
A rotten-egg smell is a signal to investigate, not a reason to guess. Start with the taps, let testing tell you where the gas is coming from, and then choose a fix that matches the results you actually have.