Well Water Smells Musty, Earthy or Moldy
A musty basement smell in the water, worst in late summer, is usually harmless organic matter or bacteria growing where water sits. It is annoying, easy to diagnose and usually easy to end.
Likely causes
- Decaying organic matter in a shallow well
- Iron or sulfur bacteria in the well or tank
- Bacteria growing in a softener or carbon filter
- Algae in a spring-fed or surface-influenced well
Tests to order
- Total coliform
- Iron bacteria check
- Total organic carbon
- Turbidity
Treatment options
- Shock chlorination of the well and plumbing
- Sanitize the softener and replace filter cartridges
- Activated carbon filter
- Wellhead inspection
Find where the smell starts
Smell a glass from the outside spigot before any treatment, then one from the kitchen tap. If the raw water is clean and the treated water smells, the culprit is growing inside a softener resin bed, a carbon filter or the pressure tank. If the raw water smells too, it is coming from the well: organic matter from a shallow aquifer, iron or sulfur bacteria, or, if the well is near a pond or stream, algae.
Why a musty smell deserves a bacteria test
Earthy smells themselves are not dangerous, but a shallow well that is drawing in organic matter is a well that is open to the surface. Order total coliform along with the checks above. If the coliform comes back present, follow the coliform steps first.
Clearing it
A shock chlorination of the well, the pressure tank and the house plumbing clears most musty smells for months. Replace every filter cartridge and run the softener through a sanitizing cycle with the resin cleaner the maker recommends. For a persistent smell from the aquifer, a whole-house activated carbon filter, $600 to $1,500, absorbs the compounds that cause it. Change the carbon on schedule, because a spent carbon filter becomes the best bacteria habitat in the house.
Why is it worse in August?
Warm groundwater and low water tables concentrate organic matter and speed up bacterial growth. Shallow wells are most affected.
Is the musty smell mold?
Not in the water. The compounds, mostly geosmin, are made by soil bacteria and algae. They smell like mold but are not.