Rotten egg smell in well water: hydrogen sulfide
Hydrogen sulfide is the gas that makes water smell like rotten eggs. Ontario's aesthetic objective is 0.05 mg/L. It isn't usually a health risk at levels you'd drink, but the smell can be strong.
- US (EPA)
- No limit.
- Ontario
- 0.05 mg/L aesthetic objective.
What it is
Hydrogen sulfide forms when sulfur bacteria break down sulfur in the well or plumbing, or when water reacts with the magnesium anode rod in a water heater. It's different from sulfate, which has no smell.
What your number means
- Within limitsBelow 0.05 mg/L
Below the level most people can smell.
- Worth a look0.05 to 1 mg/L
Noticeable rotten egg smell, can tarnish silver and stain fixtures black.
- Worth a lookAbove 1 mg/L
Strong smell and corrosion. Treatment is usually needed.
What to do
- If the smell is only in hot water, ask a plumber about replacing the water heater anode rod.
- If it is in hot and cold water, shock chlorinate the well to knock back sulfur bacteria.
- For ongoing problems, ask about aeration, an oxidizing filter or chlorination.
- Have a bacteria test done too, since a well with sulfur bacteria can let other bacteria in.
How often to test
Whenever the smell appears or changes, together with a bacteria test.
Common questions
Is rotten egg smelling water safe to drink?
At the levels people tolerate, hydrogen sulfide is not usually a health risk, but test for bacteria to be sure the well is sound.
Why does only my hot water smell?
Usually a reaction with the water heater's magnesium anode rod. An aluminum or powered anode often fixes it.
Sources: Ontario Drinking Water Quality Standards (O. Reg. 169/03); Ontario: testing and treating private wells. Limits checked November 2026.