Why your well water smells musty, and how to fix it
A musty or swampy smell usually comes from iron bacteria, and sometimes sulfur bacteria. Neither is known to cause disease, and neither has a drinking water standard. The smell is often worst after water sits unused. Check the toilet tank for slime, test for coliform bacteria if the smell is new, and shock chlorinate the well.
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Step 1: Smell the water after it sits, then after it runs
Smell the cold tap first thing in the morning, then again after the water has run for a few minutes. Minnesota, Wisconsin, and Penn State all say iron and sulfur bacteria smells are stronger after a period of little or no water use.
Step 2: Check one sink at a time
Penn State Extension says an odor at a single sink can come from bacteria in the drain rather than the water. Fill a glass, take it to another room, and smell it. If the glass is fine, clean and disinfect that drain.
Step 3: Look inside the toilet tank
Wisconsin's first question for a musty smell is whether there's slime in the toilet tank. Penn State and Minnesota list these signs of iron bacteria:
- Orange, red, yellow, brown, or gray slime or stringy strands in the tank
- Red or brown stains on fixtures and laundry
- An oily-looking sheen on standing water
Penn State and Minnesota describe iron bacteria odors as swampy, musty, oily, cucumber, sewage, or rotten vegetation. Sulfur bacteria lean toward a rotten egg smell, with dark slime.
Is a musty smell harmful?
Minnesota says iron bacteria aren't known to cause disease, and Wisconsin says the same of sulfur bacteria. There's no drinking water standard for either. Penn State calls them more of a nuisance than a health hazard.
A new smell is still a reason to test. Minnesota recommends a coliform bacteria test any time water changes in taste, smell, or look, and adds that iron bacteria can create conditions where disease-causing organisms grow. It suggests testing for nitrate too.
The EPA notes your local health department may test for free. If the test is positive, stop drinking the water and see our bacteria page.
Step 4: Shock chlorinate the well
Shock chlorination is usually the first fix people try, per Penn State. Use our shock calculator and shock chlorination guide. Minnesota says the chlorine should be close to, but not over, 200 ppm.
Penn State warns that the bacteria's slime protects them from chlorine, so getting rid of them completely can be impossible. Minnesota says you may have to treat more than once.
If the smell keeps coming back
| Option | What to know |
|---|---|
| Use more water | Wisconsin lists flushing the system and using more water as the first step. |
| Periodic shock chlorination | Repeat when the smell returns. Penn State says frequent shocking can damage the water system, so if you need it more often than every 6 months, look at other options. |
| Well cleaning | A licensed driller or pump installer pulls the pump and scrubs the well. |
| Continuous pellet chlorination with filtration | Penn State lists it for severe cases, followed by a whole-house media filter, with carbon to remove the chlorine taste. In Wisconsin it needs DNR approval first. |
When to call a pro
Call a licensed well driller or pump installer if shocking doesn't help, if water flow has dropped, or if you see heavy slime. Wisconsin says they have the knowledge and equipment for well cleaning.
Questions people ask
Why does my well water smell musty only in the morning?
Water sat still overnight. Wisconsin and Minnesota both say these bacteria odors are stronger after a period of low use.
Can a carbon filter get rid of a musty smell?
Virginia Cooperative Extension says carbon filters treat general taste and odor problems. For iron bacteria, Penn State starts with shock chlorination of the well and uses carbon afterward to remove chlorine taste.
Is musty well water safe to drink?
The iron and sulfur bacteria that cause musty smells aren't known to cause disease. If the smell is new or came on suddenly, test for coliform bacteria before drinking it.
Where this comes from
- Wisconsin DNR, My Water Smells Musty
- Wisconsin DNR, My Water Smells Like Swamp or Bog Water
- Penn State Extension, Iron and Sulfur Bacteria in Water Supplies
- Penn State Extension, Common Drinking Water Problems and Solutions
- Minnesota Department of Health, Iron Bacteria in Well Water
- Minnesota Department of Health, Bacterial Safety of Well Water
- EPA, Protect Your Home's Water
- Virginia Cooperative Extension, Household Water Treatment