Why your well water smells like rotten eggs, and how to fix it
The smell is hydrogen sulfide gas. If only the hot water smells, the usual cause is the magnesium rod in your water heater, and swapping it for an aluminum rod usually fixes it. If hot and cold both smell, the gas is in the well, and a lab test tells you which filter fits.
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Step 1: Smell the hot and cold taps separately
The Minnesota Department of Health suggests a simple test. After the water has sat unused for a few hours (first thing in the morning works), run the cold tap and smell it. Then do the same with the hot tap.
Where the smell shows up tells you where it starts:
| What you notice | Most likely source |
|---|---|
| Hot water only | Your water heater |
| Hot and cold, at every tap | The well or the groundwater |
| One sink only | Bacteria growing in that drain |
A smell in hot water only usually comes from the water heater
Many water heaters have a magnesium rod inside called an anode rod. It protects the tank from corrosion. In water with some sulfate, Penn State Extension explains, that rod drives a reaction that makes hydrogen sulfide gas.
The usual fix is to replace the magnesium rod with an aluminum one. Penn State says this removes the smell and still protects the tank. Taking the rod out with no replacement also stops the smell, but it can void the heater's warranty and shorten its life.
Minnesota also lists disinfecting the heater with a chlorine bleach solution, or raising the heater to 160°F for several hours to kill the bacteria. The Wisconsin DNR warns not to try the high-heat method unless your heater has a working pressure relief valve. Minnesota adds that water that hot can scald, so turn the thermostat back down afterward. Minnesota suggests having a plumber do any of this water heater work unless you know water heaters well.
If cold water smells too, the gas is coming from the well
Hydrogen sulfide forms underground when sulfur-reducing bacteria feed on small amounts of sulfur in water with little oxygen. Penn State says it's most common in wells drilled into shale and sandstone.
Other signs that point to the well: black stains on silverware and fixtures, black greasy stains in sinks, and white, gray, black, or reddish-brown slime where water sits.
Step 2: Check one sink at a time if only one tap smells
Penn State notes that a smell at a single sink may come from bacteria in the drain. Fill a glass at that tap, carry it to another room, and smell it. If the water in the glass smells fine, the drain is the source, and Penn State says disinfecting the drain removes the odor.
Is the rotten egg smell dangerous?
At the levels found in well water, it's a nuisance more than a health risk. Most people can smell hydrogen sulfide well below 0.5 mg/L (milligrams per liter, the same as parts per million), long before it could harm you. Penn State says that's why there's no drinking water standard for it.
The gas itself is flammable and poisonous in high amounts, so Minnesota advises venting it outside so it can't collect in well pits, basements, or well houses. Minnesota says only well professionals using proper safety steps should go into a well pit or other closed space where the gas may be.
Step 3: Test before you buy a filter
A lab test for hydrogen sulfide, iron, and sulfate tells you how strong the problem is and which treatment fits. Penn State notes that hydrogen sulfide has to be tested at your home or the sample has to be preserved before it goes to the lab, so ask the lab how to collect it.
Minnesota says that in rare cases the gas comes from sewage, so it recommends a coliform bacteria and nitrate test too. If the smell appeared suddenly, test for bacteria before you assume it's harmless sulfur. The EPA notes your local health department may test for free.
Pick the fix that matches your level
Penn State and Minnesota match treatment to the amount of hydrogen sulfide in the water:
| Hydrogen sulfide level | Treatment | What the sources say |
|---|---|---|
| Under about 1 mg/L | Activated carbon filter | Both list it for low levels. |
| Up to about 2 mg/L | Aeration | Penn State says it usually isn't used above about 2 mg/L. |
| Up to about 2 to 6 mg/L | Oxidizing filter (manganese greensand) | Penn State says up to 2 to 3 mg/L. Minnesota says up to about 6 mg/L. |
| Over 6 mg/L | Continuous chlorination or a potassium permanganate feed, followed by filtration | Penn State says chlorination works at any level and is most often used above 6 mg/L. |
Penn State adds that catalytic carbon handles higher levels than plain activated carbon, but it needs oxygen in the water to work.
Shock chlorinating the well kills the sulfur bacteria for a while. Penn State warns that in most cases the bacteria and the smell come back quickly. See our shock chlorination guide and shock calculator.
When to call a pro
Call a licensed well driller or pump installer if the smell keeps coming back after shocking, if you see slime clogging fixtures, or if you need a treatment system sized. Wisconsin requires a licensed plumber to install treatment devices, though in some cases the owner of a single-family home can do it. Other states have their own rules.
Questions people ask
Is it safe to shower in water that smells like rotten eggs?
At the levels found in wells, hydrogen sulfide isn't considered a health risk. If the smell started suddenly, test for coliform bacteria first.
Why does the smell come and go?
The smell from sulfur bacteria is often strongest after the water sits unused. Wisconsin and Penn State both note it, for example first thing in the morning or after a trip.
Will a water softener get rid of the smell?
A regular softener isn't on Penn State's list of treatments for hydrogen sulfide. The options are carbon filters for low levels, aeration, oxidizing filters, and chlorination for higher levels. Penn State also notes that in rare cases a softener gives sulfur bacteria a place to grow and causes the smell.
Can I just remove the anode rod?
You can, and the smell will stop if the rod was the cause. Penn State says removing it can void the warranty and shorten the heater's life. An aluminum replacement rod avoids both problems.
Where this comes from
- Penn State Extension, Hydrogen Sulfide (Rotten Egg Odor) in Water Wells
- Penn State Extension, Common Drinking Water Problems and Solutions
- Penn State Extension, Iron and Sulfur Bacteria, a Slimy Problem
- Minnesota Department of Health, Hydrogen Sulfide and Sulfur Bacteria in Well Water
- Wisconsin DNR, My Water Smells Like Rotten Eggs
- EPA, Protect Your Home's Water