Water that is giving off a distinctive smell is most likely contaminated with hydrogen sulfide. Hydrogen sulfide does not usually pose immediate health problems at the levels it is found in domestic drinking supplies. However, it is certainly an inconvenience — especially to one’s nose, as it creates a “rotten egg” smell.

What is Hydrogen Sulfide?
Hydrogen sulfide is a colorless gas that can exist naturally in groundwater. Sulfur-reducing bacteria present in groundwater use sulfur as an energy source to chemically change sulfates to hydrogen sulfide. The bacteria use sulfur from decaying plants and other organic matter in oxygen-deficient environments. They can occur in deep or shallow wells, and reside in plumbing systems.

Hydrogen sulfide crops up in other ways too. The magnesium rod used in water heaters for corrosion control can chemically reduce sulfates to hydrogen sulfide, and sewage pollution can be a source. Hydrogen sulfide also can enter surface water through springs.

Is my private well at risk?
Hydrogen sulfide, if present, will vary by well due to the varying geology. It is most common in shales and sandstones. The occurrence of hydrogen sulfide gas has been correlated to groundwater with low pH and groundwater with high levels of iron and/or manganese. It also may be associated with hydrocarbons and peat formations.

To test for hydrogen sulfide, contact a certified drinking water laboratory. Visit http://water.epa.gov/scitech/drinkingwater/labcert/statecertification.cfm for state-specific information on certified drinking water testing labs. The certification officer in your state can provide a list of certified labs in your area.

Hydrogen sulfide escapes (volatizes) from water easily, so samples should be tested on-site or immediately stabilized for laboratory analysis. Laboratory instructions should be followed very carefully.

If you do have a hydrogen sulfide level in your water that is higher than you would like, there are water treatment technologies available to address the problem.

What types of treatment solutions are available to private well owners?
There are various methods. They should be chosen based on the level of hydrogen sulfide, the amount of water being treated, the levels of iron and manganese, and the water pH, among other factors. Methods to reduce or remove hydrogen sulfide include activated carbon filtration, shock chlorination, ion exchange, manganese greensand filtration, oxidation, oxidizing filtration, ozone treatment, and water heater modification.

More specifically:

  • Activated carbon filters can be effective when hydrogen sulfide is present in low levels (up to about 0.3 ppm). The hydrogen sulfide is absorbed onto the surface of the carbon particles. Filters require periodic replacement and can harbor sulfate-reducing bacteria.
  • Shock chlorination may reduce the hydrogen sulfide-producing bacteria. It’s most effective in water with a pH between 5 and 7 and ineffective in alkaline (higher pH) water. Contact time depends on concentration of chlorine in the water, water temperature, and pH. An activated carbon filter may be necessary to remove residual chlorine or small amounts of remaining hydrogen sulfide.
  • Ion exchange resin absorbs hydrogen sulfide until the resin is exhausted, then the resin is regenerated with a salt. Suspended solids and precipitated iron can clog the unit and may require pretreatment.
  • Manganese greensand filtration used to remove iron and manganese can also be effective with concentrations of hydrogen sulfide less than 5 ppm. These systems are specialized and require frequent maintenance.
  • Oxidation removes hydrogen sulfide concentrations exceeding 6 ppm. It can be done by aeration, chlorination, and potassium permanganate.
  • Oxidizing filters will work for concentrations up to 6 ppm. The filter contains sand with a manganese dioxide coating that changes hydrogen sulfide gas to tiny particles of sulfur that are trapped inside the filter.
  • Ozone treatment creates a chemical reaction that precipitates sulfur. Ozone is effective for concentrations up to 10 ppm.
  • Water heater modification is necessary when hydrogen sulfide is causing an odor within the water heating system. Replacing the magnesium corrosion control rod with one made of aluminum or other metals usually improves the situation. However, such action could void the manufacturer’s warranty on the water heater.
How does Hydrogen Sulfide enter a private water system?
Bacteria in groundwater are responsible for most of the sulfide smells detected while sampling water wells. These are not often associated with high enough concentrations to be a health issue. In rare cases, sulfide presence may be due to sewage pollution. If you experience a hydrogen sulfide odor suddenly, consult with a water well system professional.
What is the measurement of Hydrogen Sulfide?
Hydrogen sulfide is quite pungent at low concentrations and 85% of people can detect ambient concentrations above 0.03 ppm. There is no Maximum Contaminant Level established by the USEPA.
Do I need to be concerned about health effects?
Hydrogen sulfide gas is flammable and poisonous at high concentrations. Usually it is not a health risk at concentrations present in household water. Buildup of hydrogen sulfide concentrations in confined areas has been known to cause adverse health effects.
Are there other effects?
Hydrogen sulfide is corrosive and can leach metals from plumbing systems into the water. Corrosion of metals by hydrogen sulfide forms a black precipitate that can stain laundry and bathroom fixtures, darken silverware, and discolor copper and brass utensils.
Where can I get more information?
For more information on your private water well, contact your local contractor. Also, visit the Web site of the National Ground Water Association, www.ngwa.org, and its site just for well owners, www.wellowner.org.