Well water smells like rotten eggs: causes and what to test
A rotten-egg taste or odor is caused by hydrogen sulfide gas (H₂S). Minnesota's Department of Health says it can be naturally occurring, produced by sulfur bacteria in groundwater, the well or plumbing, produced inside water heaters, or, rarely, from pollution. [source]
Hot water only, or hot and cold?
| Symptom | Common cause (Penn State Extension) |
|---|---|
| Odor in hot water only | Reaction with the magnesium anode rod in the water heater. Replacing the rod may void the heater warranty. |
| Odor in hot and cold water, black greasy stains | Hydrogen sulfide, sulfides or sulfate-reducing bacteria in groundwater |
| Odor at one sink only | May be bacteria in the drain rather than the water |
What to test
MDH says that in most cases the smell isn't related to the sanitary quality of the water, but to be safe you should test your well water for coliform bacteria and nitrate. [source] The EPA lists hydrogen sulfide, corrosion and metals as tests for objectionable taste or smell. [source]
Safety note
MDH warns that hydrogen sulfide gas in the air can be harmful at high levels. Only well professionals with proper precautions should enter a well pit or enclosed space where the gas may be present. [source]
FAQ
Is rotten egg smell in well water dangerous?
Minnesota's Department of Health says in most cases the smell is not related to the sanitary quality of the water, but recommends testing for coliform bacteria and nitrate to be safe.
Why does only my hot water smell like rotten eggs?
Penn State Extension says hot-water-only odor is usually a reaction with the water heater's magnesium anode rod.
Sources
- Minnesota Dept. of Health – Why Does My Water Smell Like Rotten Eggs? (accessed 2026-10-21)
- Penn State Extension – Common Drinking Water Problems and Solutions (accessed 2026-10-21)
- US EPA – Protect Your Home's Water (accessed 2026-10-21)
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