At a Glance: That rotten egg smell coming from your tap water is almost always caused by hydrogen sulfide gas. This gas forms when sulfur bacteria break down organic matter in your plumbing system or water supply. Once you identify the source, there are clear steps to fix it, and a plumber can help if the problem persists.

If you’ve ever turned on your faucet and been hit with that unmistakable sulfur smell, you know how unpleasant it can be. The rotten egg odor is one of the most common water quality complaints homeowners deal with. It can come from your hot water, your cold water, or both, and each scenario points to a different cause.

This guide breaks down exactly why your water smells like rotten eggs, where the problem is coming from, and what you can do about it.

What Causes the Rotten Egg Smell in Water?

The culprit behind that foul odor is almost always hydrogen sulfide gas (H₂S). Hydrogen sulfide is a colorless gas that forms naturally when sulfur-reducing bacteria break down organic matter in water. Even in small amounts, it produces a strong, recognizable smell.

Here’s a quick breakdown of how hydrogen sulfide ends up in your water:

Source	How It Happens
Water heater	Sulfur bacteria react with the magnesium anode rod inside the tank
Well water	Naturally occurring sulfur or sulfur bacteria in the groundwater
Plumbing system	Sulfate-reducing bacteria build up in pipes or fixtures
Water softener	Creates conditions where sulfur bacteria thrive
Stagnant water	Water sitting unused in pipes develops bacteria over time

Is It Only the Hot Water, or the Cold Water Too?

Pay attention to whether the rotten egg smell comes from your hot water, your cold water, or both. This tells you a lot about where the problem is originating.

The Smell Only Comes from Hot Water

If the sulfur smell only appears when you run hot water, your water heater is most likely the source. Inside a conventional water heater tank, there’s a metal rod called the anode rod (often a magnesium anode rod) designed to provide corrosion protection. When sulfate-reducing bacteria interact with this magnesium rod, a chemical reaction occurs that produces hydrogen sulfide gas.

This is one of the most common causes of rotten egg odor in homes. It can often be resolved by replacing the anode rod and flushing the hot water heater tank.

The Smell Comes from Both Hot and Cold Water

If the unpleasant smell shows up in both cold water and hot water, the issue likely goes beyond your water heater. This points to a problem with your overall water supply or plumbing system, which could include:

• Sulfur bacteria or iron bacteria growing in your pipes or water softener
• Well water with naturally high sulfur levels
• Stagnant water in sections of your plumbing that don’t get regular use
• A distribution system problem if you’re on municipal water

The Smell Only Appears at Certain Fixtures

If the rotten egg odor is isolated to one sink or a bathroom that rarely gets used, stagnant water in those pipes may be to blame. A buildup near the garbage disposal or drain can also be the source. Bacteria thrive in slow or non-moving water, especially when there’s organic material present.

Common Causes of Rotten Egg-Smelling Water

Sulfur Bacteria in Your Water Heater

Sulfur-reducing bacteria are naturally occurring organisms that use sulfur compounds for energy. Inside a water heater tank, these bacteria can survive and multiply, especially in tanks set to a lower temperature. When they come into contact with the magnesium anode rod, the result is hydrogen sulfide gas in your hot water.

Signs this is your issue:

• Smell is only present in hot tap water
• Smell is stronger right after the water heats up
• Older water heater that hasn’t been flushed in years

Well Water and Naturally Occurring Sulfur

Homes on well water are more likely to experience high hydrogen sulfide levels in their drinking water. Sulfur occurs naturally in certain rock formations and soil, and groundwater that passes through these areas can carry sulfur bacteria directly into your home’s water supply.

If you’re on well water and notice a consistent sulfur smell in both cold and hot water, testing your water quality is the right first step.

Sulfate-Reducing Bacteria in Your Plumbing

Sulfate-reducing bacteria can also grow inside the pipes of your plumbing system, particularly in older homes or areas with specific water chemistry. These bacteria feed on organic matter and produce hydrogen sulfide as a byproduct. In some cases, coliform bacteria or iron bacteria may also be present, which can point to more serious water quality concerns.

Water Softeners

Water softeners can sometimes create conditions where sulfur bacteria thrive. If your system hasn’t been properly maintained or sanitized, bacteria can colonize the unit and spread hydrogen sulfide gas throughout your plumbing fixtures.

Treatment Methods: How to Fix the Problem

The right fix depends on where the smell is coming from. Here’s a breakdown of the most effective treatment methods.

For Water Heater Issues

• Replace the anode rod: Swapping out the magnesium anode rod for an aluminum or zinc alternative can stop the chemical reaction that produces hydrogen sulfide.
• Flush the tank: A thorough flush of the water heater tank removes sediment and bacteria buildup.
• Raise the water temperature: Setting your hot water heater to 140°F kills most sulfur bacteria. Use caution with higher temperatures to prevent scalding.
• Shock with a chlorine bleach solution: Disinfecting the tank with a chlorine bleach solution eliminates bacteria. This is best handled by a professional.

For Well Water

• Test your water: Get a water quality test to identify the exact cause, whether it’s hydrogen sulfide gas, sulfur bacteria, or iron bacteria.
• Install a treatment system: Options include activated carbon filters, air injection systems, or hydrogen peroxide injection systems that neutralize hydrogen sulfide levels.
• Shock the well: A professional can disinfect your well and distribution system to address bacterial contamination.

For General Plumbing Issues

• Flush unused plumbing fixtures regularly to prevent stagnant water buildup
• Clean or replace your water softener to eliminate bacterial growth
• Have a plumber inspect your plumbing system if the unpleasant odor persists or is widespread

When Should You Call a Plumber?

While some minor cases of rotten egg smell can be addressed with DIY fixes like flushing a water heater or cleaning a drain, there are situations where you need a professional.

Call a plumber if:

• The smell is present in all tap water, both hot and cold, throughout your home
• You’re on well water and suspect bacterial contamination
• You’ve already tried basic fixes and the unpleasant smell keeps coming back
• You notice other signs of water quality issues, like discoloration or an odd taste
• The smell is coming from your irrigation systems or multiple outdoor fixtures
• You’re dealing with an older plumbing system and aren’t sure of the water source

Hydrogen sulfide gas at higher concentrations can be harmful, so don’t ignore a persistent sulfur smell. A licensed plumber can diagnose the issue accurately and recommend the right solution.

Staying on Top of Water Quality

Keeping your water smelling fresh comes down to routine maintenance. Here are a few habits that help:

• Flush your water heater annually and check the anode rod every 3 to 5 years
• Run infrequently used faucets and plumbing fixtures every few weeks
• Test your well water at least once a year
• Keep your water softener clean and properly maintained
• Have your plumbing system inspected if you notice any changes in water smell or quality

A little preventive care goes a long way when it comes to drinking water quality.

Get Your Water Smelling Right Again

That rotten egg odor coming from your tap is frustrating, but it is also fixable. Whether the cause is sulfur bacteria in your water heater, naturally occurring hydrogen sulfide in your well water, or buildup in your plumbing system, there is a solution.

At John’s Sewer and Drain Cleaning, our experienced technicians have been solving plumbing problems in the Boston area since 1976. We’re available 24 hours a day, 7 days a week, and we aim to be at your door within 2 hours of your call. If your water has an unpleasant smell you can’t identify, we’re here to help.

Contact John’s Sewer today. A live person is always ready to take your call.