Why your water smells like rotten eggs

Best for

Anyone on a private well who has just noticed a sulfur or rotten-egg smell and wants to know the cause before spending money.

Wrong fit

If you are on city water, start with the city water page instead, because the causes and the people to call are different.

Tradeoff

A smell test narrows the cause for free, but only a lab test tells you how much hydrogen sulfide is there, and that number decides which treatment works.

It’s hydrogen sulfide, and where you smell it matters

The rotten-egg smell is hydrogen sulfide (H2S), a gas dissolved in the water. It takes very little to notice. Penn State Extension says most people can detect it well below 0.5 mg/L (ppm), and UGA Extension says water above about 1 mg/L smells plainly of rotten eggs, while 0.5 to 1 mg/L tends to smell musty or swampy.

The gas can come from four different places, and each has a different fix:

• The water heater. A magnesium anode rod and sulfate in the water make H2S inside the tank.
• The groundwater. Some aquifers carry H2S naturally, so the water smells before it reaches the house.
• Sulfur bacteria living in the well, pressure tank or pipes.
• A drain, where bacteria grow on hair, soap and food waste.

Buying a whole-house filter for a water heater problem, or a new anode for a groundwater problem, wastes money. Spend ten minutes on the smell test below first. The smell diagnosis tool on the homepage walks you through the same questions.

The 10-minute smell test

The Minnesota Department of Health suggests checking after the water has sat unused for a few hours, such as first thing in the morning. Smell the cold tap, then the hot tap, at more than one fixture. If you have a softener, also check a tap that bypasses it, such as an outdoor spigot.

The table follows the troubleshooting steps published by the Minnesota and Vermont health departments and UGA Extension. They agree on the pattern: no smell in cold water points to the heater, a smell that fades with running points to bacteria, and a steady smell in both points to the groundwater.

Cause 1: the water heater (hot water only)

Water heaters have a sacrificial anode rod, usually magnesium, that protects the steel tank from corrosion. Penn State Extension explains that the magnesium rod can chemically reduce sulfate in the water to hydrogen sulfide. Bacteria in the warm tank can do the same, and UGA Extension notes this is more common when hot water sits unused or the thermostat is set low.

The usual fix is replacing the magnesium rod with an aluminum or aluminum-zinc rod. Don’t simply pull the rod out. Penn State warns that removing it can shorten the heater’s life and will likely void the warranty. The full set of options, including a powered anode and a disinfecting flush, is on the hot water smell page.

Cause 2: hydrogen sulfide in the groundwater

When the smell is in both hot and cold water at every tap, and it doesn’t fade as the water runs, the gas is coming from the well. Penn State says H2S problems are most common in wells drilled into acidic bedrock such as shale and sandstone. Virginia Cooperative Extension adds that it also shows up near coal or oil fields, that it is often seasonal, and that it is often found alongside iron, manganese or low pH.

This is the case where a treatment system makes sense. The right one depends on the concentration, so test before you buy. See how to test for hydrogen sulfide.

Cause 3: sulfur bacteria in the well or plumbing

Sulfate-reducing bacteria thrive in low-oxygen water. They can live in the well, the pressure tank, the pipes or a softener, and they turn sulfate into H2S. The giveaway is a strong smell when you first open a tap that fades after a few minutes of running, according to the Minnesota and Vermont health departments. Slime is another sign. Minnesota notes that sulfur bacteria can produce slime that clogs wells and plumbing.

The first fix is a strong chlorine treatment of the well and plumbing, called shock chlorination. It’s cheap, but Minnesota warns that sulfur bacteria can be hard to remove once established, and Virginia Cooperative Extension says control may be temporary. See how to shock chlorinate for sulfur bacteria.

Cause 4: a drain (one sink only)

If only one sink smells, the water may be fine. UGA Extension says bacteria growing in a drain are the most common cause of a sulfurous odor, and Penn State notes that odor at one sink may come from the drain rather than the water.

The glass test settles it. Fill a glass at the smelly sink, step away from the sink, swirl the water and smell it. If the water in the glass doesn’t smell, the drain is the problem. Disinfect and flush the drain. No treatment system is needed.

Cause 5: the water softener

Penn State calls this rare, but a softener can give sulfur-reducing bacteria a place to grow. Minnesota’s test is to compare a tap that bypasses the softener with one that doesn’t. If only softened water smells, disinfect the softener following the installer’s or manufacturer’s instructions.

Is it dangerous?

Usually not at the levels found in household water. Penn State says the water becomes unpleasant to drink long before H2S reaches harmful levels, and that sulfur-reducing bacteria don’t cause human health problems. Two cautions apply. Minnesota notes that in rare cases the smell comes from sewage or other pollution, so test for coliform bacteria and nitrate. And the gas itself can build up in enclosed spaces such as well pits. The details are on is sulfur water safe.

H2S is also corrosive. Virginia Cooperative Extension says it can darken silverware, discolor copper and brass fixtures, and leach metals from plumbing.

What to test before you buy anything

If the smell test points to the well, get a lab test that includes:

• Hydrogen sulfide, collected in the lab’s bottle with a preservative, because the gas escapes quickly
• Iron and manganese, which often come with H2S and change the treatment choice
• pH, because oxidizing filters have pH limits
• Sulfate, which feeds sulfur bacteria and is a separate issue
• Coliform bacteria and nitrate, to rule out contamination

Bring those numbers to the treatment-by-level table. As a rough guide from extension sources, activated carbon handles only trace levels (under about 0.3 to 1 mg/L, depending on the source), air-injection or oxidizing filters cover the middle range, and chlorine injection with a contact tank is the usual choice above about 6 mg/L. If the smell came from the heater, a drain or bacteria, you may not need equipment at all.