Water that smells like rotten eggs almost always contains hydrogen sulfide gas. This gas forms when sulfur-containing minerals or organic matter break down, and even tiny amounts produce that unmistakable stench. The good news: the source is usually easy to narrow down, and most fixes are straightforward.
What Creates the Smell
Hydrogen sulfide gas is the culprit. It forms when naturally occurring sulfur compounds in soil and rock dissolve into groundwater, or when certain bacteria feed on sulfate minerals and release the gas as a byproduct. These “sulfur bacteria” thrive in oxygen-poor environments, which is exactly what you find underground, inside well casings, and in the warm, enclosed tank of a water heater.
The process works like this: as groundwater moves through rock formations, it picks up dissolved sulfate minerals. Sulfur-reducing bacteria, which are extremely common in groundwater, convert those sulfates into hydrogen sulfide. A U.S. Geological Survey study in Montana found these bacteria in 25 of 26 groundwater samples tested. They’re not a sign of contamination so much as a normal part of underground biology, but when conditions favor them, the gas they produce becomes noticeable.
The Three Most Common Sources
Your Water Heater
If the smell comes only from hot water, your water heater is the likely source. Inside every standard tank water heater is a metal rod called an anode, usually made of magnesium. Its job is to corrode slowly so your tank doesn’t. The problem is that sulfur bacteria in your water react with that magnesium rod, producing hydrogen sulfide gas. The warm, enclosed tank creates the perfect breeding ground.
This is the single most common reason for a rotten egg smell in homes with otherwise clean water. It’s especially prevalent if you have a water softener, which can increase the sulfate concentration reaching the heater.
Your Well or Groundwater
If both hot and cold water smell, the gas is likely entering your water before it reaches the house. Wells drilled into sulfur-rich rock formations or areas with high organic matter are prone to this. The deeper the well and the less oxygen available, the more active sulfur-reducing bacteria tend to be.
Your Drain, Not Your Water
Sometimes the smell isn’t coming from the water at all. Organic material like food scraps, soap residue, and hair accumulates inside drains and P-traps over time. Bacteria feed on this buildup and release hydrogen sulfide in the process. If you only notice the odor when standing near a particular sink, the drain is a strong suspect.
How to Pinpoint the Source
You can figure out where the smell originates with a few simple tests, no equipment needed.
First, run only the hot water and then only the cold water. If the hot water smells but the cold doesn’t, your water heater is the problem. If both smell, the issue is in your water supply.
Next, fill a clean glass with cold water and carry it to another room. If the smell disappears once you walk away from the sink, you’re dealing with a drain odor, not a water supply issue. If the glass still smells in the other room, the hydrogen sulfide is dissolved in the water itself.
Finally, run a cold water faucet for two to three minutes. If the smell fades as fresh water flows in, bacteria may be growing in your pipes or faucet aerators rather than in the water supply. If it persists, the source is your well or municipal water.
Fixing a Water Heater Smell
The most effective fix is replacing the standard magnesium anode rod with one that doesn’t fuel hydrogen sulfide production. You have two main options. An aluminum-zinc anode rod contains zinc, which doesn’t promote hydrogen sulfide formation the way magnesium does. It’s a simple swap that reduces or eliminates the smell for many households. A powered (or “impressed current”) anode rod goes further: it protects the tank electronically rather than by corroding, so it never reacts with sulfates in the water at all. Powered rods are more expensive but tend to eliminate the odor completely and last longer.
Flushing the water heater with a bleach solution can also kill off sulfur bacteria temporarily, but the smell often returns within weeks or months if the anode rod isn’t changed.
Fixing a Well Water Smell
When hydrogen sulfide is in the water supply itself, filtration or chemical treatment at the point of entry is the standard approach.
For very low levels, below about 0.3 parts per million, a standard granular activated carbon filter can absorb the gas and remove the odor. These filters have limited capacity for hydrogen sulfide, though, and exhaust quickly at higher concentrations. Catalytic carbon filters handle much more. They work by converting hydrogen sulfide into solid elemental sulfur on the filter surface, which means they don’t just trap the gas but chemically neutralize it. They’re effective at concentrations well above what standard carbon filters can manage.
For heavier contamination, oxidation systems inject a chemical like hydrogen peroxide or chlorine into the water before it enters the house. These systems convert the dissolved gas into odorless sulfur particles, which are then filtered out. Hydrogen peroxide is effective against hydrogen sulfide and breaks down into water and oxygen, leaving no chemical taste. These injection systems require a holding tank and a carbon filter downstream, making them a more involved installation.
Shock chlorination of the well itself can knock out sulfur bacteria, but like flushing a water heater, the results may be temporary if conditions underground continue to support bacterial growth.
Fixing a Drain Smell
If your detective work points to the drain, the fix is the simplest of all. Clean the drain stopper and faucet aerator, flush the P-trap and garbage disposal with a dilute household bleach solution, and run water through any drains that don’t get used regularly. Infrequently used drains are especially prone to this because the water in the P-trap evaporates, breaking the seal that normally blocks sewer gases from rising into the room. Simply running water for 30 seconds refills the trap and stops the odor.
Is the Smell Dangerous?
At the concentrations typically found in household water, hydrogen sulfide is a nuisance, not a health hazard. You can detect the rotten egg odor at extremely low levels, well below any dangerous threshold. The EPA lists odor under its secondary drinking water standards, meaning it’s a quality issue rather than a safety violation.
Airborne hydrogen sulfide becomes a different story at high concentrations. Levels of 50 to 100 parts per million in air can irritate the eyes and respiratory tract, and anything above 100 ppm causes a dangerous effect: your nose stops detecting the smell entirely, a phenomenon called olfactory fatigue. Concentrations this high are essentially impossible from a household tap, but they can occur in enclosed spaces like well pits or pump houses with heavy contamination. If you ever enter a confined space with an overwhelming sulfur smell that suddenly disappears, leave immediately.
For everyday household exposure, the main concern is that hydrogen sulfide corrodes copper and silver, which means it can tarnish fixtures, darken silverware, and leave black stains on laundry and bathroom surfaces over time. Addressing the source protects your plumbing and appliances as much as your nose.