A Rotten Egg Smell Has a Few Likely Sources: How to Tell Whether Your Water Carries Sulfur Gas

The smell of rotten eggs in water is most often hydrogen sulfide gas. It can come from the water source, from bacteria living in the well or the pipes, or from a reaction inside a water heater. The smell itself does not tell you which one it is. The pattern of when and where you notice it does, and that pattern is what you should record before you call anyone.

Start with hot and cold

Run the cold tap in the kitchen for a minute, then the hot tap, and sniff each one. If the smell appears only at the hot tap, the source is most likely the water heater. Many tank heaters contain a sacrificial anode rod that is designed to corrode so the tank does not. Under some conditions that reaction produces sulfur compounds. If both taps smell, the problem is more likely in the water supply itself, whether that is a well, a shared line, or the pipes behind the walls.

Watch how the smell changes over time

Note whether the smell is constant, comes and goes, or is strongest after the water has sat overnight or after a period of no use. A smell that fades after a few minutes of running the tap often points to stagnant water in the pipes or heater. A smell that stays steady across the day is more likely to come from the supply. Write down the times you notice it and whether it changed after a recent maintenance visit or a change to the heater.

Look for signs of bacteria

On a private well, sulfur-producing bacteria can grow in the well, the pressure tank, or the plumbing. Clues that point this way include a slimy film in the toilet tank, dark or slippery deposits around the faucet aerator, or a smell that is worse after a long gap in use. Bacteria are not a cosmetic issue, and a smell like this on a well is worth having the water tested rather than guessing at a cause.

Test before you treat

A simple home check can show you whether the problem is worth a professional visit, but it will not tell you what is in the water. A certified laboratory test is the way to confirm sulfide levels and bacterial presence. Ask the lab to test the sample you collect from the tap where the smell is strongest, and note the date and time of collection so the result can be matched to your observations.

What to tell a technician

• Which tap smells, and whether it is hot only, cold only, or both
• How long the water runs before the smell fades, if it fades at all
• Whether you have a private well, a pressure tank, or a water heater with an anode rod
• Any recent heater work, well service, or treatment you have had done
• Whether the smell is constant or appears only after the water sits

A clear record of these points lets a provider decide whether the fix belongs at the heater, the well, or the treatment system, and it keeps you from paying for work that does not match the source.