Why Does My Water Smell Like Rotten Eggs? Causes and Fixes

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Water that smells like rotten eggs almost always contains hydrogen sulfide gas, and the two most common sources are sulfur-reducing bacteria or naturally occurring sulfide in well water, and a chemical reaction between the water heater’s magnesium anode rod and sulfate in the water, which produces the smell only from the hot tap. A five-minute hot-versus-cold test usually tells you which one you have. This guide walks through that test, ranks the causes from most to least likely, and explains what you can safely do yourself and when to bring in a plumber, water treatment specialist or the gas utility.

Part of our Plumbing & Water guide — start there for the full picture →

A critical safety distinction first: if the smell is in the air of the house rather than coming from water at the tap, do not troubleshoot. Natural gas and propane are odorized with a sulfur-like chemical so leaks are noticeable. Leave the home immediately, avoid switches and flames, and call your gas utility or 911 from outside. Our guide to a rotten egg smell in the house covers airborne odors and their causes in detail. Everything below applies only to water that smells when it runs from a faucet. The information reflects general guidance as of 2026.

What Causes the Rotten Egg Smell in Water

Hydrogen sulfide is a gas that dissolves in water. People can smell it at very low concentrations. OSHA’s hydrogen sulfide hazards page lists an odor threshold in air of roughly 0.01 to 1.5 parts per million, the range where the rotten-egg smell first becomes noticeable to some people. That sensitivity explains why water that seems fine to drink can still smell unpleasant enough to be a nuisance.

In household water, hydrogen sulfide usually comes from one of these processes:

• Sulfur-reducing bacteria. These naturally occurring bacteria live in low-oxygen environments such as deep wells, water heaters and some plumbing. They use sulfate in the water as an energy source and give off hydrogen sulfide.
• Geology. Some groundwater picks up sulfide from contact with shale, coal deposits or other sulfur-bearing rock and sediment.
• Water heater chemistry. A magnesium anode rod, installed to protect the tank from corrosion, can react with sulfate in the water and, with help from bacteria, produce hydrogen sulfide inside the tank.
• Drain gas mistaken for water odor. Sometimes the smell is actually rising from the sink drain, not coming from the water itself.

The EPA treats odor in drinking water as an aesthetic issue under its secondary standards. Its guidance lists a recommended odor level of 3 threshold odor number and describes the related effect as a “rotten-egg”, musty or chemical smell. Secondary standards are non-enforceable guidelines for taste, odor and appearance, so a smell alone does not mean water is unsafe. It does, however, deserve investigation, especially in a private well.

The Five-Minute Test: Hot, Cold or Drain?

Before buying any equipment, narrow the source with a simple test. Use a clean glass and step away from the sink before smelling, so you are not picking up drain odor.

1. Run cold water only at a kitchen faucet for a minute. Fill a glass, carry it to another room and smell it.
2. Run hot water only at the same faucet until it is fully hot. Fill a second glass and smell it the same way.
3. Check a different faucet, ideally one on another floor, to see whether the smell is everywhere or in one spot.
4. Smell the drain with the water off. If the drain smells but the water in the glass does not, the problem is in the drain, not the supply.
5. Note timing. Is the smell strongest first thing in the morning, after vacation, or after the water heater has sat unused?

Cause 1: The Water Heater (Hot Water Only)

If only the hot water smells, the water heater is the source in most cases. Tank water heaters contain a sacrificial anode rod, usually magnesium or aluminum, that corrodes so the steel tank does not. In water with sulfate and sulfur-reducing bacteria, a magnesium anode can drive a reaction that produces hydrogen sulfide. The smell often gets worse when the heater sits unused or its thermostat is set low, because bacteria thrive in warm, stagnant water.

What to check: Confirm the hot-only pattern at more than one faucet. Note the heater’s age and whether it has ever been flushed. Ask whether a water softener was recently installed; softened water can change how the anode corrodes.

Safe DIY steps:

• Flush the tank. Turn off the heater’s power or set a gas unit to its pilot or vacation setting, connect a hose to the drain valve, and drain several gallons to clear sediment where bacteria live. Follow the manufacturer’s instructions and be careful: the water is hot.
• Consider the anode rod. Many plumbers replace a magnesium anode with an aluminum-zinc or powered anode designed to reduce odor. Our guide to the water heater anode rod explains the options, how to locate the rod and why removing it entirely is a bad idea, since that can void the warranty and shorten tank life.
• Avoid running the tank cold for long periods. Extended low settings can favor bacterial growth. Any temperature change should follow manufacturer guidance and scald-safety practices, especially with children or older adults in the home.

When to call a pro: If the anode rod is seized, the heater is old or leaking, or the smell returns quickly after a flush. A plumber can replace the anode, disinfect the tank where the manufacturer allows it, or advise whether replacement makes more sense. Tankless heaters have no anode rod, so a smell from a tankless unit points back to the water supply or plumbing.

Cause 2: Hydrogen Sulfide in Well Water (Hot and Cold)

When both hot and cold water smell throughout the house, and the home is on a private well, the groundwater is the likely source. Hydrogen sulfide can occur naturally in an aquifer, or sulfur-reducing bacteria can colonize the well itself, the pressure tank or the plumbing. A smell that is strongest after the water has sat overnight, then fades, often suggests bacteria in the well or pipes.

Private wells put the responsibility on the owner. The EPA states that “private well owners are responsible for delivering safe drinking water to their households,” and that private domestic wells are not regulated by the federal government under the Safe Drinking Water Act. That means no one else is testing your well for you.

What to check:

• When the well was last tested, and for what.
• Whether the smell started suddenly, which could point to a change in the well, recent flooding, or work on the pump or pressure tank.
• Any accompanying signs: black specks or stains (often from sulfide reacting with metal), slimy buildup in the toilet tank, or a change in taste or color.

Safe DIY step: test the water. The CDC recommends testing private well water “at least once every year” for total coliform bacteria, pH, total dissolved solids and nitrates, and adding other tests based on local concerns. When odor is the complaint, ask a certified laboratory to add hydrogen sulfide, sulfate, iron and manganese, and sulfur or iron bacteria if available. Our well water testing guide explains how to collect samples and read results. Testing first matters because treatment equipment for sulfide differs from equipment for iron or bacteria, and choosing the wrong system wastes money.

When to call a pro: If the test confirms hydrogen sulfide or bacteria, if coliform bacteria show up, or if the smell came on suddenly. A licensed well contractor can perform shock chlorination of the well and plumbing when bacteria are the cause, and a water treatment professional can size equipment for ongoing sulfide.

Treatment Options for Well Water Sulfide

Treatment depends on the concentration and on what else is in the water. The table below summarizes common approaches. Costs are typical installed ranges and vary widely with flow rate, water chemistry and installer.

The CDC’s overview of home water treatment notes that no single system removes every contaminant and that owners should check a product’s label and certification for what it is designed to reduce. For a deeper comparison of whole-house setups, see our well water filtration system guide, and when you are ready to compare equipment, our roundup of the best well water filters has our picks.

Cause 3: Plumbing, Softeners and Drains

A smaller share of cases trace to the home’s plumbing or treatment equipment rather than the source water:

• Water softeners and filters. A softener resin bed or a neglected sediment or carbon filter can harbor bacteria. If the smell appears only on treated lines, sanitize the softener according to the manufacturer and replace filter cartridges on schedule.
• Dead-end lines. Seldom-used guest bathrooms or plumbing serving a former fixture can let water stagnate. Run those taps regularly or have a plumber remove the dead leg.
• Drain biofilm. Bacteria growing in sink drains and overflows produce sulfur odors that are easy to mistake for water odor. Clean the drain, overflow and stopper, and pour water into rarely used drains to keep traps filled.

Drain-related smells are usually local to one sink. If several drains smell and you notice gurgling or slow drainage, the vent or sewer line may be involved, and that is a plumbing call rather than a water treatment one.

Is Rotten-Egg Water Safe to Drink?

At the levels that typically cause a nuisance odor in tap water, hydrogen sulfide is mainly an aesthetic problem: it smells and tastes bad, can tarnish silverware and copper, and can leave black stains. The bigger concern is what else the odor might indicate. In a private well, a new smell can accompany changes that allow bacteria or other contaminants into the water. That is why testing comes first.

Practical guidance while you wait for results:

• If the smell is new and you are on a well, consider using bottled water for drinking and cooking until test results come back, especially for infants, pregnant people and anyone with a weakened immune system.
• Do not mask the odor with a pitcher filter and assume the problem is solved.
• Ventilate bathrooms during showers if the odor is strong. Hydrogen sulfide in the air at high concentrations is a recognized hazard, and OSHA notes that at higher exposures people can lose the ability to smell it, so odor fading is not proof it is gone. Concentrations from household water are generally far lower than workplace hazard levels, but strong, persistent odor indoors warrants a professional look.

Front Range Considerations

Most homes in Front Range cities draw treated municipal water, and rotten-egg odors from city water are uncommon. When they happen in a city home, the water heater is the usual explanation. Rural and foothill properties in Boulder, Larimer, Jefferson, Douglas and El Paso counties are more likely to rely on private wells drilled into fractured rock or alluvial aquifers, where sulfide, iron and manganese vary from one well to the next. Homes that sit vacant during a sale can also develop stronger odors because water stagnates in the heater and lines.

Buyers of well properties should treat a water quality test as a standard part of due diligence rather than an add-on, and should ask the seller for any past test results and treatment equipment service records.

What a Home Inspector Can and Cannot Tell You

During a standard home inspection, the inspector runs water at fixtures, looks at the water heater, notes its approximate age and condition, and identifies visible treatment equipment such as softeners and filters. An inspector who smells sulfur at the taps will typically note it and recommend further evaluation. What a visual inspection cannot do is measure hydrogen sulfide, sulfate or bacteria. Those answers come only from laboratory testing of a properly collected sample.

For a well property, ask about adding a water quality test to the inspection, and ask the seller for the well permit, construction details, pump and pressure tank age, and any treatment service records. Together those documents show whether an odor is a known, managed condition or a new problem the seller may not have noticed. If treatment equipment is present, find out what it was installed to remove and when the media or cartridges were last changed, since a neglected system can create odors of its own.

Prevention

• Flush the water heater annually, or as the manufacturer recommends, to limit sediment and bacteria.
• Ask your plumber about an odor-reducing anode if your water is high in sulfate.
• Test private well water at least once a year, plus any time taste, odor or color changes.
• Keep the wellhead cap sealed and the ground sloped away from the well casing.
• Service softeners and replace filter cartridges on schedule.
• Run water in rarely used fixtures regularly.

When to Call a Professional

Bring in help if any of these apply:

• The air in the home smells of rotten eggs or gas. Leave and call the gas utility first.
• The smell is new, sudden or accompanied by color, cloudiness or illness.
• Well tests show coliform bacteria or sulfide at levels that need treatment.
• The water heater is old, leaking or the anode rod cannot be removed.

Our plumbing and water hiring guide explains which specialist to call for each situation. If you are buying a home on a well or want a second set of eyes on water quality, contact our team to arrange an inspection and water testing.

References

• Secondary Drinking Water Standards: Guidance for Nuisance Chemicals — U.S. Environmental Protection Agency
• Private Drinking Water Wells — U.S. Environmental Protection Agency
• Guidelines for Testing Well Water — Centers for Disease Control and Prevention
• About Home Water Treatment Systems — Centers for Disease Control and Prevention
• Hydrogen Sulfide: Hazards — Occupational Safety and Health Administration

Dealing with smelly water in a home you own or are buying? Get in touch with our team to arrange a Front Range inspection and water test.