Rotten-egg odor: anode, well, or aquifer?

Hydrogen sulfide gas (H₂S) can make water taste or smell like rotten eggs. Minnesota Department of Health says the first job is to find the source — water heater, well/plumbing, or groundwater — because the treatment is not the same.

Primary source: MDH, Why Does My Water Smell Like Rotten Eggs? last updated 18 October 2024. Retrieved 29 August 2026. EPA’s private-well chart also maps “objectionable taste or smell” to hydrogen sulfide, corrosion, and metals.

Where the gas comes from

MDH: hydrogen sulfide can occur in wells anywhere, and can be:

• Naturally occurring — decay and chemical reactions with soil and rocks
• Produced by certain “sulfur bacteria” in groundwater, the well, or plumbing
• Produced by sulfur bacteria or chemical reactions inside water heaters
• From pollution (this is rare)

Sulfur bacteria produce slime and can help other bacteria grow, such as iron bacteria. The slime can clog wells, plumbing, and irrigation. Signs of the gas include black stains on silverware and fixtures, and corrosion on pipes. Signs of the bacteria include white, grey, black, or reddish-brown slime.

Find the source (MDH method)

After you have been away for a few hours, smell hot and cold water. That tells you which path to take.

If the problem is in the water heater

MDH: unless you are very familiar with water heater operation, have a plumber or water-system professional do the work. Options MDH lists:

• Replace or remove the magnesium anode. Many heaters have a magnesium anode that can supply electrons that convert sulfate to hydrogen sulfide. Removal may shorten heater life. Ask a dealer whether an anode of a different material, such as aluminum, can be installed.
• Disinfect and flush with a chlorine bleach solution to kill sulfur bacteria. If bacteria remain, the odor may return in a few weeks.
• Raise heater temperature to 160°F (71°C) for several hours to destroy sulfur bacteria, then flush. MDH caution: this can be dangerous. Consult the manufacturer about the pressure-relief valve. Lower the thermostat afterward to prevent scalding and high energy costs.

How the heater makes the gas: it is a warm environment for sulfur bacteria, and the magnesium anode can react with sulfate in the water. MDH describes the anode as typically ½ to ¾ inch in diameter and 30 to 40 inches long.

If the problem is in the well, plumbing, or water softener

MDH: disinfect the well and plumbing with a strong chlorine solution (licensed well contractor, or MDH well-disinfection instructions). Sulfur bacteria can be hard to remove; pre-work such as scrubbing the casing, special chemicals, and agitation may be needed — especially if iron bacteria are also present. If bacteria are in a softener or other device, contact the installer or manufacturer.

If the problem is in groundwater

MDH options: home treatment, or a new well in a different formation. Treatment types MDH lists as effective:

• Activated carbon — effective for hydrogen sulfide less than 1 mg/L. Service life is hard to predict because carbon also removes other substances.
• For levels both below and above 1 mg/L: oxidizing media filtration (such as manganese greensand) up to about 6 mg/L; aeration and filtration; continuous chlorination and filtration; ozonation and filtration.

In Minnesota, MDH says treatment professionals should be licensed plumbers or licensed water conditioning contractors (Minnesota Department of Labor and Industry license lookup).

Still test the well

MDH: in most cases the rotten-egg smell does not relate to sanitary quality. In rare instances the gas may be from sewage or other pollution. To be safe, test for coliform bacteria and nitrate. MDH also recommends the usual private-well panel (coliform yearly; nitrate; arsenic at least once; lead at least once; manganese before a baby drinks the water). See how to test and Minnesota labs.

EPA independently maps “objectionable taste or smell” to tests for hydrogen sulfide, corrosion, and metals: Protect Your Home’s Water.