Why Does My Water Smell Like Rotten Eggs?

1. What Causes the Rotten Egg Smell in Water?

The smell is hydrogen sulfide, a gas that dissolves in water and escapes as soon as the water leaves the tap. Your nose is extremely sensitive to it: most people can smell it at well under one part per million, which is why a tiny amount makes a glass of water seem undrinkable. At low levels it reads as musty or swampy; as concentration rises it becomes the unmistakable rotten egg.

There are three common ways it ends up in a home's water. It can occur naturally in groundwater, from the decay of organic matter or contact with sulfur-bearing rock. It can be made by sulfate-reducing bacteria, harmless microbes that live in low-oxygen places such as wells, pipes, softeners and water heaters and turn sulfate into hydrogen sulfide. Or it can be produced inside a water heater, where the magnesium anode rod drives the same chemistry.

Two other smells get mistaken for it, and they matter. Sewer gas from a drain with a dried-out trap or a slimy drain line also smells of rotten eggs, but it comes from the drain, not the water. And natural gas and propane are given a sulfur-like odour on purpose so leaks are noticed. If the smell is in the air of a room rather than in a glass of water, treat it as a possible gas leak first.

Because the causes are so different, the fixes are too. A new anode rod cures one, shock chlorination of a well cures another, and a whole-house oxidising filter is needed for a third. Spending money before you know which one you have is the most common and most expensive mistake people make with sulfur water. The tap test below takes five minutes and usually settles it.

2. Is It the Hot Water, the Cold Water, or the Drain?

Start with the cold water. Run a cold tap for a few seconds, fill a clean glass, and walk away from the sink into another room before you smell it. Stepping away matters, because a smelly drain under the tap can fool you into blaming the water. Then do the same with hot water from the same tap. Repeat at a second faucet, ideally one on a different floor or side of the house.

If you have a water softener, test water that bypasses it as well: an outdoor spigot is often plumbed before the softener, or you can put the softener in bypass for a few minutes. A smell that appears only after the softener means bacteria are living in the resin or brine tank. The same logic applies to any whole-house filter that has been neglected.

Note whether the smell is strong when you first open the tap and then fades, or stays constant no matter how long the water runs. A smell that fades points at bacteria growing in stagnant water in the well, pipes or tank. A smell that stays steady points at hydrogen sulfide coming in with the groundwater itself. Both are treatable, but the treatment and the cost are different.

Write down what you find. If you end up calling a water treatment company or your county extension office, being able to say that only the hot water smells, or that it fades after two minutes, saves time and steers you away from equipment you do not need.

3. Why Does Only My Hot Water Smell Like Rotten Eggs?

This is the most common version, and it is almost always the water heater. Tank heaters protect their steel with a sacrificial anode rod, usually magnesium. In water that carries sulfate and some sulfate-reducing bacteria, the reaction at the anode supplies what the bacteria need to produce hydrogen sulfide, and the warm, stagnant tank is an ideal home for them. Softened water and well water make it more likely, as does a heater that sits unused for a while.

The standard fix is to replace the magnesium anode with an aluminum-zinc alloy anode, which is far less likely to drive the reaction. It is a part that typically costs tens of dollars and fits the same port. A powered anode, an electronic rod that protects the tank with a small current and does not dissolve, is a more expensive option that both eliminates the smell and never wears out, which makes it popular in softened homes.

Disinfecting the tank helps kill the bacteria already living there. Some extension services describe dosing a drained-down tank with ordinary 3 percent hydrogen peroxide, on the order of a pint or two for a 40-gallon heater, then flushing. Another approach is raising the thermostat to around 160°F for several hours and flushing the hot lines, which kills bacteria but creates a serious scald risk. Check your heater's manual first, and warn everyone in the house.

What you should not do is pull out the anode and leave it out. The smell goes, and so does the tank's corrosion protection, so the heater rusts through years early and the warranty is typically void. We explain the anode's role in how long water heaters last. If the smell started right after a new heater went in, it is usually the anode, and many installers will swap it on request.

Tankless heaters have no anode rod and store almost no water, so they rarely cause this. If you have a tankless unit and the hot water smells, look at the softener, the plumbing or the well instead. If your tank heater is old and smells and also shows rust, it may be time to replace it; our best gas water heaters guide covers what to look for.

4. What If Hot and Cold Both Smell? Well Water and Hydrogen Sulfide

When every tap smells, hot and cold, the hydrogen sulfide is coming in with the water. On a private well that usually means one of two things: the gas occurs naturally in the aquifer, which is common in some regions with shale, sandstone or deposits near coal and oil formations, or sulfate-reducing bacteria have colonised the well, the pressure tank or the pipes.

Test before you treat. Hydrogen sulfide escapes from a sample quickly, so it is usually measured on site or with a sample collected and handled exactly as a certified lab instructs. Ask for sulfate, iron, manganese and pH at the same time, plus total coliform bacteria, which every well owner should test for periodically anyway. Some labs can also test for sulfate-reducing and iron bacteria. The results decide which treatment will work.

If the problem is bacteria, shock chlorination is the usual first step: chlorine is added to the well and circulated through the plumbing, left for a period, then flushed out. State health departments and extension services publish step-by-step procedures, and following one carefully matters, because too little chlorine does little and the flushing step must keep heavily chlorinated water out of septic systems and away from plants. Many people hire a well contractor for it.

Shock chlorination is not always a permanent cure. Bacteria can recolonise over weeks or months, and some wells need treatment repeated periodically. If the smell is constant and does not fade after running, and testing shows dissolved hydrogen sulfide in the groundwater, shocking the well will not fix it, because the gas is in the aquifer. That calls for continuous treatment, covered below.

One practical note for well owners: a power cut that stops the pump also stops any treatment system that relies on it, and stagnant water in the pressure tank and pipes can make a sulfur smell worse for a while afterwards. If you run a generator during outages, our guide to what size generator runs a well pump covers sizing.

5. What If You Are on City Water?

Municipal water rarely arrives carrying hydrogen sulfide, because treatment removes it and the disinfectant in the pipes oxidises what little might form. So if you are on city water and smell rotten eggs, the source is far more likely to be inside your house: the water heater, a softener or filter, a dead-end pipe that rarely sees use, or a drain.

Run the glass test carefully. On city water, the drain is a frequent culprit, especially a bathroom sink overflow channel, a shower drain or a floor drain whose trap has dried out and lets sewer gas through. Pouring water into rarely used drains refills their traps, and cleaning the sink overflow and drain pipe with a brush often removes a smell that seemed to come from the tap.

If the smell is in the hot water only, the anode explanation applies on city water too, although it is less common than on wells. If it appears only after a softener or whole-house filter, that equipment needs sanitising or new media; our guide to how long water softeners last covers resin condition, and the softener's manual will describe its sanitising procedure.

If cold water smells at every tap and you have ruled out your own equipment, call your water utility. Ask whether there has been work on the mains, a change in treatment, or similar complaints nearby. Utilities take odour reports seriously because they can point to a problem in the distribution system, and they can often test at your meter for free.

6. How Do You Get Rid of the Rotten Egg Smell for Good?

Match the fix to the source and the concentration. For the water heater: an aluminum-zinc or powered anode plus a tank disinfection. For bacteria in a well or plumbing: shock chlorination, repeated as needed. For a softener: sanitise it and replace exhausted resin. For hydrogen sulfide in the groundwater itself, you need continuous treatment, and the right equipment depends on how much is there.

Extension services commonly describe activated carbon as suitable only for very low levels of hydrogen sulfide, around 1 part per million or less, because the carbon fills up and the smell returns. Oxidising filters handle more. Air-injection oxidising filters, which hold a pocket of air at the top of the tank to oxidise the sulfide before a filter bed captures it, are popular on wells because they use no chemicals and also deal with iron and manganese.

At higher concentrations, injecting chlorine or hydrogen peroxide into the water and giving it contact time before a filter is the most reliable approach. It also disinfects, which helps if bacteria are part of the problem. The trade-off is ongoing chemical refills and pump maintenance. Our best whole-home water filtration systems guide covers which well-water systems we would trust with sulfur and iron.

One thing that will not fix it: a pitcher filter or refrigerator filter. A carbon pitcher can soften a faint odour in drinking water, but it does nothing for showers, laundry or the hot water, and moderate hydrogen sulfide will use up a small cartridge quickly. If you rely on one in the meantime, change it more often than the box suggests; see how often to change a Brita filter.

7. Is Water That Smells Like Rotten Eggs Safe?

At the concentrations normally found in household water, hydrogen sulfide is generally treated as a nuisance rather than a health hazard: the smell makes water unpleasant long before it reaches a level that would harm you. The EPA does not set an enforceable health-based limit for it in drinking water. Showering and washing in it are not considered dangerous at typical household levels, even if they are unpleasant.

The smell can still be a signal worth acting on. On a private well, sulfur bacteria are not a health problem in themselves, but a well that has bacteria growing in it should be tested for coliform bacteria too. High sulfate, which often comes with hydrogen sulfide, can have a laxative effect on people not used to it, and the EPA's secondary guideline for sulfate is 250 milligrams per litre. Infants and visitors are the most likely to notice.

Hydrogen sulfide is also corrosive. It tarnishes silver, blackens copper and brass, and combined with iron it leaves black stains on fixtures and laundry. Over time it attacks plumbing and water heater components, which is another reason to fix the source rather than live with the smell.

Two genuine safety warnings. In confined spaces, such as well pits or enclosed treatment rooms, hydrogen sulfide gas can build to dangerous concentrations, and at high levels it deadens your sense of smell, so you cannot rely on the odour to warn you. Do not climb into a well pit. And if the sulfur smell is in the air of your home rather than in the water, leave the house and call your gas utility from outside.