Why Does My Well Water Smell Like Rotten Eggs? In Plain Words

What the smell is

Hydrogen sulfide is a gas that gives water a distinctive “rotten egg” odor. Penn State Extension says hydrogen sulfide in water is an aesthetic concern that causes a disagreeable taste and odor to the water.

Most people can detect hydrogen sulfide levels well below 0.5 mg/L, and Penn State Extension notes that the human nose can detect it well before it causes health concerns.

Hydrogen sulfide does not have a drinking water standard, because it makes the water aesthetically undrinkable long before it reaches harmful concentrations.

The U.S. Environmental Protection Agency (EPA) lists “Odor” in its secondary drinking water standards table at 3 threshold odor number (TON), with the noticeable effect above that level listed as a “rotten-egg”, musty or chemical smell. EPA says its secondary standards are non-mandatory water quality standards, established as guidelines for aesthetic considerations such as taste, color, and odor, and that these contaminants are not considered to present a risk to human health at the secondary level.

Where the gas can come from

The Minnesota Department of Health says hydrogen sulfide gas can occur in wells anywhere and be:

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

Penn State Extension explains the bacteria side this way: hydrogen sulfide occurs naturally in groundwater as a consequence of the activities of sulfur-reducing bacteria. These bacteria feed on small amounts of sulfur in the water and thrive in the low-oxygen environments present in groundwater wells and plumbing systems.

Hydrogen sulfide problems are most common in wells drilled into acidic bedrock such as shale and sandstone.

In rare cases, Penn State Extension says, adding water treatment equipment, like a water softener, may cause hydrogen sulfide production, because the softener provides a favorable environment for sulfur-reducing bacteria to grow.

What the pattern can mean

Which water smells can point to where the gas is being made. This is what the pattern can mean — it is not a repair procedure.

• Smell only in the hot water: Penn State Extension says that when hydrogen sulfide is noticeable only in the hot water in the home, chemical reactions within the water heater may be the source of the rotten egg odor. The Minnesota Department of Health’s source chart points the same way: if there is no smell from the cold water faucet, the problem is likely in the water heater.
• Smell in the cold water too: The Minnesota Department of Health’s chart says that if the smell is still there after the water runs for a few minutes, the problem is likely hydrogen sulfide gas in the groundwater, and if the smell gets less noticeable after the water runs for a few minutes, the problem is likely sulfur bacteria in the well or plumbing system.
• No smell at faucets that are not connected to the water softener: The Minnesota Department of Health’s chart says the problem is likely sulfur bacteria in the water softener.

The Minnesota Department of Health says the first step is to find out what the source of the issue is; that will let you know what treatment option is best. Choosing and doing that treatment is a job for a professional, not for this page.

How a water heater can make the gas

The Minnesota Department of Health says a water heater can provide an ideal environment for the conversion of sulfate to hydrogen sulfide gas, in two ways: creating a warm environment where sulfur bacteria can live, and sustaining a reaction between sulfate in the water and the water heater anode.

A water heater usually contains a metal rod called an “anode,” which is installed to reduce corrosion of the water heater tank. The anode is usually made of magnesium metal, which can supply electrons that aid in the conversion of sulfate to hydrogen sulfide gas.

Penn State Extension says the same thing in shorter form: water heaters are fitted with a magnesium rod to inhibit corrosion of the heater, and the magnesium rod can chemically reduce sulfates to form hydrogen sulfide.

Signs that fit the smell

The Minnesota Department of Health lists these signs:

• Bacterial slime may be white, grey, black, or reddish brown if associated with iron bacteria (signs of sulfur bacteria)
• Black stains on silverware and plumbing fixtures (signs of hydrogen sulfide gas)
• Corrosion on pipes and metal components of the water distribution system (signs of hydrogen sulfide gas)

Sulfur bacteria produce a slime and can help other bacteria grow, such as iron bacteria. The slime can clog wells, plumbing, and irrigation systems.

Penn State Extension adds that hydrogen sulfide can cause corrosion of metals in a plumbing system, and yellow or black greasy stains on fixtures or inside pipes when it forms metallic sulfides.

Is it harmful?

While sulfur bacteria are not harmful, the Minnesota Department of Health says hydrogen sulfide gas in the air can be harmful at high levels. Penn State Extension says that although sulfur-reducing bacteria can impart taste and odor in the water, they do not cause human health concerns, and that while the gas is poisonous and flammable, the human nose can detect it well before it causes health concerns.

In most cases, the rotten egg smell does not relate to the sanitary quality of the water. In rare instances, the gas may be from sewage or other pollution. To be safe, the Minnesota Department of Health says, test your well water for coliform bacteria and nitrate.

EPA also cautions that odor and taste are useful indicators of water quality, even though odor-free water is not necessarily safe to drink.

Why testing is the route, not the smell

The Minnesota Department of Health says testing your well water is the only way to detect most of the common contaminants in groundwater, and that you cannot taste, see, or smell most contaminants. It recommends testing for:

• Coliform bacteria every year and any time the water changes in taste, odor, or appearance. Coliform bacteria can indicate that disease-causing microorganisms may be in your water.
• Nitrate every other year. Bottle-fed infants under six months old are at the highest risk of being affected by levels of nitrate higher than 10 milligrams per liter in drinking water.
• Arsenic at least once.
• Lead at least once. The well and water system may have parts that have lead in them, and that lead can get into drinking water.
• Manganese before a baby drinks the water.