Why does my well water smell like rotten eggs?

A rotten-egg smell is hydrogen sulfide gas. If only the hot water smells, the water heater's anode rod is usually reacting with the water, and the fix is at the heater. If both hot and cold taps smell, the gas is in the well water itself, from the aquifer or from sulfur bacteria, and treatment is chosen after testing. About one Suffolk household in five reported this smell in a Virginia Tech clinic.

Wellborne hears about the rotten-egg smell often. In the Virginia Cooperative Extension clinic of 204 Suffolk households on private water, 19% reported a sulfur odor and about 9% a sulfur taste. It is common, usually harmless in itself, and almost always fixable once you know where it comes from.

The one test that matters most

Run the cold water and smell it. Then run the hot water and smell it.

• Only the hot water smells. The cause is almost certainly inside the water heater.
• Hot and cold both smell. The gas is in the water coming from the well.
• Only one tap smells. The cause is local to that fixture or drain, often bacteria in a sink drain rather than the water.

Hot water only: the water heater

Most water heaters have a magnesium anode rod that corrodes instead of the tank. In some well water, sulfate-reducing bacteria react with the anode and produce hydrogen sulfide inside the heater. The cold water is fine; the hot water stinks, especially after the heater has sat unused.

The fixes are at the heater: replacing the magnesium anode with an aluminum-zinc or powered anode, and flushing or disinfecting the tank. Removing the anode entirely is not recommended, because it protects the tank from corrosion. This is plumbing work, not well work.

Hot and cold: the well water

When both taps smell, hydrogen sulfide is in the groundwater. It forms naturally where water sits in contact with organic material and sulfur-bearing minerals with little oxygen, conditions common in parts of the Coastal Plain aquifers. Sulfur bacteria living in the well can produce it too.

The smell can come and go. It is often strongest first thing in the morning or after the water has sat, and in some wells it changes with the season or with how hard the well is pumped.

Testing before treating

A lab test measures the hydrogen sulfide level and, just as important, checks for bacteria, iron and other constituents that affect the treatment choice. The smell itself does not tell you whether the water is safe. If coliform bacteria turn up, that points to a wellhead problem to fix first.

Treatment options

The right choice depends on the measured level and what else is in the water:

• Shock chlorination of the well can clear a smell caused by sulfur bacteria, though it often returns.
• Activated carbon handles low levels at a single tap or for the whole house.
• Oxidation and filtration (with air, chlorine or other oxidizers) handles moderate to high levels and often removes iron at the same time.
• Aeration strips the gas from the water in a tank before it is repressurized.

Equipment has to be sized for the household’s flow, or it will reduce water pressure. The well water treatment page covers sizing and installation.

When the smell appears suddenly

A sulfur smell that was never there before deserves attention. It can follow a long period when the well sat unused, a pump replacement that changed how hard the well is pumped, or new treatment equipment that changed the water’s chemistry. Occasionally it signals a change in the well itself or nearby, such as surface water entering. A sudden change is a good reason to test for bacteria as well as hydrogen sulfide, and to have the wellhead checked.

Smell and iron together

Sulfur and iron often appear together, and some sulfur bacteria and iron bacteria live alongside each other. Black staining or black specks usually mean iron or manganese sulfides. The iron guide covers those. To get the smell investigated, call (757) 919-1972.