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Environmental Health Division
Why Does My Water Smell Like Rotten Eggs?
Hydrogen Sulfide and Sulfur Bacteria in Well Water
Hydrogen sulfide gas (H2S) can give water a “rotten egg” taste or odor. This 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).
May help other bacteria grow
Sulfur bacteria produce a slime and can help other bacteria grow, such as iron bacteria. The slime can clog wells, plumbing, and irrigation systems.
Gas may be harmful
While sulfur bacteria are not harmful, hydrogen sulfide gas in the air can be harmful at high levels. It is important to remove the gas from the water, or vent the gas to the atmosphere. Venting prevents the gas from collecting in low-lying spaces (such as well pits and basements) or enclosed spaces (such as well houses). Only well professionals should enter a well pit or other enclosed space where hydrogen sulfide gas may be present.
How to detect
- 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).
- Have your water tested at a laboratory.
Consider testing your water
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, test your well water for coliform bacteria and nitrate.
What you can do
The first step is to find out what the source of the issue is; that will let you know what treatment option is best.
How to Find the Source
After you have been away from your home for a few hours, smell the water coming out of the hot and cold water faucets. Determine which faucets have the “rotten egg” odor.
If the Problem is in the Water Heater
Unless you are very familiar with water heater operation and maintenance, have a plumber or water system professional to do the work.
- Replace or remove the magnesium anode. 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 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 all bacteria are not destroyed by chlorination, the problem may return within a few weeks.
- 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. 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.
How Hydrogen Sulfide Gas is 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.
If the Problem is in the Well, Plumbing System, or Water Softener
Disinfect the well and plumbing system with a strong chlorine solution. You can hire a licensed well contractor to do this or refer to the Well Disinfection webpage for instructions.
Sulfur bacteria can be difficult to remove once established in a well. Pre-work (such as scrubbing the well casing, using special treatment chemicals, and agitating the water before disinfection) may be necessary—especially if there are also iron bacteria. Contact a licensed well contractor to do this pre-work.
If the bacteria are in the water softener or other treatment devices, contact the installer, manufacturer, or Minnesota Department of Health for disinfection instructions.
If the Problem is in Groundwater
Installing home water treatment or drilling a new well in a different formation are both options. Below are types of home water treatment effective at removing hydrogen sulfide gas. Learn more at the Home Water Treatment webpage.
- Activated carbon filters are effective for hydrogen sulfide levels 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.
- The following are options are effective for levels both below and above 1 mg/L.
- Oxidizing media filtration (such as a manganese greensand filter) are effective for hydrogen sulfide levels up to about 6 mg/L. This type of treatment is 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.
- Aeration and filtration.
- Continuous chlorination and filtration.
- Ozonation and filtration.
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Should I test my well water for anything besides Hydrogen Sulfide?
Yes. Both natural sources and human activities can contaminate well water and cause short-term or long-term health effects. Testing your well water is the only way to detect most of the common contaminants in Minnesota groundwater; you cannot taste, see, or smell most contaminants. Minnesota Department of Health 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.
See Bacterial Safety of Well 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.
See Nitrate in Well Water.
- Arsenic at least once. About 40 percent of wells in Minnesota have arsenic in the water. Drinking water with arsenic in it for a long time can contribute to reduced intelligence in children and increased risks of cancer, diabetes, heart disease, and skin problems.
See Arsenic in Well Water.
- 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. Lead can damage the brain, kidneys, and nervous system. Lead can also slow development or cause learning, behavior, and hearing problems.
See Lead in Well Water Systems.
- Manganese before a baby drinks the water. High levels of manganese can cause problems with memory, attention, and motor skills. It can also cause learning and behavior problems in infants and children.
See Manganese in Drinking Water.
Other contaminants sometimes occur in private water systems but less often than the contaminants listed above. Consider testing for:
- Volatile organic chemicals if the well is near fuel tanks or a commercial or industrial area.
- Agricultural chemicals commonly used in the area if the well is shallow and is near cropped fields or handling areas for agricultural chemicals or is in an area of geologic sensitivity (such as fractured limestone).
- Fluoride if children or teenagers drink the water.
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