What Causes the Rotten Egg Smell in my Well Water?
Is it Harmful?

The rotten egg (sulfur) smell in your well water is technically called hydrogen sulfide gas (H2S).

Hydrogen sulfide gas can result from a number of different sources.

• It can occur naturally in groundwater.
• It can be produced by "sulfur bacteria" in the groundwater, in the well, or in the water distribution system.
• It can be produced also by sulfur bacteria or chemical reactions inside water heaters.
• In rare instances, it can result from pollution.

The source of the gas is important when considering treatment options.

Are sulfur bacteria or hydrogen sulfide harmful?
In most cases, the rotten egg smell does not relate to the sanitary quality of the water. However, in rare instances the gas may result from sewage or other pollution. It is a good idea to have the well tested for the standard sanitary tests of coliform bacteria and nitrate.

Sulfur bacteria are not harmful, but hydrogen sulfide gas in the air can be hazardous at high levels. It is important to take steps to remove the gas from the water, or vent the gas to the atmosphere so that it will not collect in low-lying spaces, such as well pits, basements, or enclosed spaces, such as well houses.

Only qualified people who have received special training and use proper safety procedures should enter a well pit or other enclosed space where hydrogen sulfide gas may be present.

Are there other problems associated with sulfur bacteria or hydrogen sulfide?
Yes. Sulfur bacteria produce a slime and can promote the growth of other bacteria, such as iron bacteria. The slime can clog wells, plumbing, and irrigation systems. Bacterial slime may be white, grey, black, or reddish brown if associated with iron bacteria.

Hydrogen sulfide gas in water can cause black stains on silverware and plumbing fixtures. It can also corrode pipes and other metal components of the water distribution system.

What causes hydrogen sulfide gas to form in groundwater?
Decay of organic matter such as vegetation, or chemical reactions with some sulfur-containing minerals in the soil and rock, may naturally create hydrogen sulfide gas in groundwater. As groundwater moves through soil and rock formations containing minerals of sulfate, some of these minerals dissolve in the water. A unique group of bacteria, called "sulfur bacteria" or "sulfate-reducing bacteria" can change sulfate and other sulfur containing compounds, including natural organic materials, to hydrogen sulfide gas.

How is hydrogen sulfide gas produced in a water heater?
A water heater can provide an ideal environment for the conversion of sulfate to hydrogen sulfide gas. The water heater can produce 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. The anode is 1/2 to 3/4 inches in diameter and 30 to 40 inches long.

How can I find the source of a hydrogen sulfide problem, and what can I do to eliminate it?
The odor of hydrogen sulfide gas can be detected in water at a very low level. Smell the water coming out of the hot and cold water faucets. Determine which faucets have the odor. The "rotten egg" smell will often be more noticeable from the hot water because more of the gas is vaporized. Your sense of smell becomes dulled quickly, so the best time to check is after you have been away from your home for a few hours.

If the smell is only from the hot water faucet the problem is likely to be in the water heater. If the smell is in both the hot and cold faucets then the problem is likely caused from sulfur bacteria in the well.

What if the smell is just in my hot water? (Unless you are very familiar with the operation and maintenance of the water heater, you should contact a water system professional, such as a plumber, to do any work on your water heater.)
If the smell is just coming from your hot water, and not at all from your cold water, then replacing or removing the magnesium anode rod in your hot water tank may be the solution. Many water heaters have a magnesium anode, which is attached to a plug located on top of the water heater. It can be removed by turning off the water, releasing the pressure from the water heater, and unscrewing the plug. Be sure to plug the hole. Removal of the anode, however, may significantly decrease the life of the water heater. You may wish to consult with a reputable water heater dealer to determine if a replacement anode made of a different material, such as aluminum, can be installed. A replacement anode may provide corrosion protection without contributing to the production of hydrogen sulfide gas.

Disinfect and flush the water heater with a chlorine bleach solution: Chlorination can kill sulfur bacteria, if done properly. If all bacteria are not destroyed by chlorination, the problem may return within a few weeks.

Kill the bacteria with heat: Increase the water heater temperature to 160 degrees Fahrenheit (71 degrees Celsius) for several hours. This will destroy the sulfur bacteria. Flushing to remove the dead bacteria after treatment should control the odor problem.

CAUTION: Increasing the water heater temperature can be dangerous.  Before proceeding, consult with the manufacturer or dealer regarding an operable pressure relief valve and for other recommendations. Be sure to lower the thermostat setting and make certain the water temperature is reduced following treatment to prevent injury from scalding hot water and to avoid high energy costs.

What if sulfur bacteria are present in the well, the water distribution system, or the water softener?
Shock chlorination is the best procedure used to kill bacteria present in your well and/or water system.  If your hot water heater is ruled out as the problem, shock chlorination should be your first course of action before spending large sums of money on a new well.  In many cases, shock chlorination will remove or greatly reduce the problem so that more expensive options are not needed.  We offer an easy to follow step-by-step booklet that shows you how to shock chlorinate your well water system in one Saturday afternoon for less than $13 of materials.  Click here for more information or to purchase our booklet.

What if hydrogen sulfide gas is in the groundwater?
The problem may only be eliminated by drilling a well into a different formation capable of producing water that is free of hydrogen sulfide gas or connecting to an alternate water source, if available. However, there are several options available for treatment of water with hydrogen sulfide gas.

1. Install an activated carbon filter. This option is only effective for low hydrogen sulfide levels, usually less than 1 milligram per liter (mg/L).* The gas is trapped by the carbon until the filter is saturated. Since the carbon filter can remove substances in addition to hydrogen sulfide gas, it is difficult to predict its service life. Some large carbon filters have been known to last for years, while some small filters may last for only weeks or even days.
2. Install an oxidizing filter, such as a "manganese greensand" filter. This option is effective for hydrogen sulfide levels up to about 6 mg/L. Manganese greensand filters are often used to treat iron problems in water. The device consists of manganese greensand media, which is sand coated with manganese dioxide. The hydrogen sulfide gas in the water is changed to tiny particles of sulfur as it passes through the filter. The filter must be periodically regenerated, using potassium permanganate, before the capacity of the greensand is exhausted.
3. Install an oxidation-filtration system. This option is effective for hydrogen sulfide levels up to and exceeding 6 mg/L. These systems utilize a chemical feed pump to inject an oxidizing chemical, such as chlorine, into the water-supply line prior to a storage or mixing tank. When sufficient contact time is allowed, the oxidizing chemical changes the hydrogen sulfide to sulfur, which is then removed by a particulate filter, such as a manganese greensand filter. Excess chlorine can be removed by activated carbon filtration

Eliminate or greatly reduce sulfur bacteria in your well. 
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