Why does my water smell like rotten eggs?

What people usually report

The smell is usually noticed in the shower or at a hot tap, sometimes only first thing in the morning or after the house has been empty for a few days. Some people notice it only on well water. Others notice dark or yellowish stains on fixtures at the same time, or that the odor fades after the water runs for a while.

What official sources say the causes can include

• Sulfur-reducing bacteria in groundwater. Hydrogen sulfide occurs naturally in groundwater as a result of these bacteria (Penn State Extension).
• The water heater’s magnesium rod. Water heaters are fitted with a magnesium rod to inhibit corrosion, and that rod can chemically reduce sulfates to hydrogen sulfide (Penn State Extension). When the heater is the source, the odor is present in hot water but not cold (Virginia Cooperative Extension).
• The well’s geology. Virginia’s extension service says hydrogen sulfide is often present in wells drilled in shale or sandstone, or near coal or oil fields (Virginia Cooperative Extension).
• Bacteria in a drain. A rotten egg or sewage-like odor at a sink can come from bacterial activity in the drain rather than the water (University of Georgia Extension).

EPA lists a “rotten-egg” smell among the effects of its secondary standard for odor. Secondary standards cover taste, odor, and appearance, and EPA does not enforce them (U.S. EPA).

What the sources say about levels and side effects

Most people can smell hydrogen sulfide well below 0.5 mg/L, according to Penn State Extension, and Virginia’s program puts the usual detection range at 0.5 to 1.0 mg/L. Penn State describes the gas as poisonous and flammable, while noting that the nose detects it well before it causes health concerns. It can also corrode metal plumbing and leave yellow or black greasy stains on fixtures (Penn State Extension; Virginia Cooperative Extension). None of this tells you the level in your own water. Only a test does.

How to find out

1. The glass test. Fill a glass at the sink that smells, step away from the sink, and swirl the water. If the water in the glass has no odor, the problem is in the drain (University of Georgia Extension).
2. Hot versus cold. Run only the cold tap, then only the hot. A smell in hot water alone points to the water heater (Virginia Cooperative Extension). A smell in both points to the water supply.
3. Test the water. Penn State Extension says hydrogen sulfide has to be tested in the home, or the sample has to be chemically stabilized before it goes to a commercial lab, because the gas escapes (Penn State Extension). Ask the lab how it wants the sample taken.

Official places to start:

• On public water: read your utility’s Consumer Confidence Report. EPA requires community water systems to send one to customers every year by July 1, and you can look yours up with EPA’s CCR search tool or call the utility (U.S. EPA).
• For a lab test: EPA links each state’s drinking water laboratory certification program, which lists certified labs you can pay to test a sample (U.S. EPA).
• On a private well: EPA and CDC both say to test at least once a year for total coliform bacteria, nitrates, total dissolved solids, and pH, and to test again after any change in taste, odor, or color (U.S. EPA; CDC). EPA does not regulate private wells; your state or local health department can help.

If you are on public water and the smell is in both hot and cold water at every tap, call your water utility and describe it. If the smell is only in hot water, see our page on water heater costs before you get quotes.