MDH Fact Sheet/Brochure
Why Does My Water Smell Like Rotten Eggs?
HYDROGEN SULFIDE AND SULFUR BACTERIA IN WELL
Hydrogen sulfide gas (H2S) can occur in wells anywhere in
Minnesota, and gives the water a characteristic "rotten egg" taste or odor.
This brochure provides basic information about hydrogen sulfide gas and sulfur
bacteria and discusses actions that you can take to minimize their effects.
What are the sources of hydrogen
sulfide in well water and the water distribution system?
Hydrogen sulfide gas can result from a number of different sources. It can
occur naturally in groundwater. It can be produced by certain "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
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 in 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. You can also have the water tested for hydrogen
sulfide, sulfate, sulfur bacteria, and iron bacteria at an environmental testing
laboratory. The cost of testing for hydrogen sulfide ranges from $20 to $50
depending on the type of test.
- 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, but only from the water treated by a water softener and not in the
untreated water the problem is likely to be sulfur bacteria in the water
- If the smell is strong when the water in
both the hot and cold faucets is first turned on, and it diminishes or goes
away after the water has run, or if the smell varies through time the
problems is likely to be sulfur bacteria in the well or distribution system.
- If the smell is strong when the water in
both the hot and cold faucets is first turned on and is more or less
constant and persists with use the problem is likely to be hydrogen sulfide
gas in the groundwater.
What can I do about a problem 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
- 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 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.
- 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?
- Have the well and distribution system
disinfected by flushing with a strong chlorine solution (shock chlorination)
as indicated in the "Well Disinfection" fact sheet from the Minnesota
Department of Health (MDH). Sulfur bacteria can be difficult to remove once
established in a well. Physical scrubbing of the well casing, use of
special treatment chemicals, and agitation of the water may be necessary
prior to chlorination to remove the bacteria, particularly if they are
associated with another type of bacteria known as "iron bacteria". Contact
a licensed well contractor or a Minnesota Department of Health (MDH) well
specialist for details.
- If the bacteria are in water treatment
devices, such as a water softener, contact the manufacturer, the installer,
or the MDH for information on the procedure for disinfecting the treatment
What if hydrogen sulfide gas is in the
The problem may only be eliminated by drilling a well into 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.
- 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 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.
- 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.
- 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.
Other related references that are available
from MDH include:
Iron Bacteria in Well Water
Sulfate in Well Water
Well Owner's Handbook
If you have any questions, please contact a
licensed well contractor, a reputable water treatment company, or a well
specialist at one of the following offices of the MDH:
Minnesota Department of Health
Well Management Section
PO Box 64975
St. Paul, Minnesota 55164-0975
Source: Minnesota Department of Health
Fact Sheet/Brochure "Why Does My Water Smell Like
Rotten Eggs? Hydrogen Sulfide and Sulfur Bacteria in Well Water"