If your tap water smells musty or earthy, the cause is almost always organic, not chemical. The three most common sources are a municipal algae bloom in the water supply, a biofilm building up in your home’s pipes, or a problem with your water heater or drain nearby. Here’s how to tell them apart and what to do.
Why Water Smells Earthy or Musty
The odor comes from compounds called geosmin and 2-methylisoborneol (MIB). These are byproducts of algae, cyanobacteria, and certain soil bacteria. They’re not toxic at typical drinking-water concentrations, but they’re detectable by smell at extremely low concentrations (sometimes just a few parts per trillion). So even a perfectly safe municipal supply can smell like a pond after an algae bloom.
Your nose is more sensitive to these compounds than most lab tests, which is actually useful for diagnosing the source.
The Most Likely Causes, in Order
Municipal algae bloom. Most common in late summer and early fall, when warm reservoir water allows algae to multiply. Your water utility treats for this, but treatment takes time to work through the distribution system. If your neighbors notice the same smell, this is probably it. Check your utility’s website or call them — most post bloom advisories.
Biofilm inside your pipes. A thin layer of bacteria and organic matter can form on the inside of older pipes, especially if water sits in them for a while (vacation homes, infrequently used fixtures). The smell tends to be worse after the water has been sitting overnight and gets better after you run the tap for 30 seconds to a minute. This is more common with older plumbing, particularly galvanized steel pipes.
Hot water heater (even when running cold). If your water heater is set too low, sulfur-reducing bacteria can grow inside the tank. You’ll usually notice a rotten-egg smell more than a musty one, but sometimes both happen together. Also worth knowing: the anode rod in a water heater can react with water chemistry to produce odors. Running the cold water tap separately rules out the heater.
A nearby drain. Sometimes the smell isn’t coming from the water at all. A dry or dirty P-trap under the sink can release sewer gas that mixes with the air while you’re running the faucet. Easy to check: fill a glass of water, walk to another room, and smell it there. If the glass smells fine, the problem is likely the drain, not the water.
Private well. If you’re on a well, earthy or musty smells are more significant and more common. Shallow wells pick up surface runoff, and bacterial contamination from soil is a real possibility. This warrants testing before you draw conclusions.
How a Tech Diagnoses It
A plumber or water quality tech will usually start by isolating the source:
- Cold only, or both hot and cold. Cold only points toward the supply or pipes; both sides points toward the water heater or a whole-house issue.
- One fixture or every tap. One fixture (especially a bathroom rarely used) usually means biofilm or a dry trap at that location.
- Better after flushing or worse. Stagnant water in the line worsens biofilm odors; if flushing helps, that’s a strong clue.
- Smell the water away from the sink. Rules out the drain.
For well water, a licensed water quality lab test is the proper starting point. Your county health department can usually refer you to a certified lab.
What You Can Do Yourself
A few things are reasonable to try before calling anyone:
Flush your cold water lines. Run every cold tap in the house for two or three minutes. If you’ve been away, this clears stagnant water. If the smell improves or goes away, you’ve found the culprit.
Check your P-traps. Run water in every sink, tub, and floor drain you haven’t used recently. A dry trap lets sewer gas back in. Pour a cup of water down infrequently used drains.
Check with your water utility. Call or look up their water quality page. Utilities are required to publish annual water quality reports, and many post bloom alerts in real time. If there’s a known algae event, you’re done diagnosing.
Smell the water in a separate glass, away from the sink. This takes 30 seconds and rules out drain interference.
What not to do yourself: Don’t start cutting into pipes or disassembling your water heater on the strength of a smell. Flushing lines and checking traps is well within DIY territory. Anything involving your water heater’s anode rod, pressure relief valve, or the pipes behind walls is not.
When to Call a Licensed Plumber
Call a licensed plumber if:
- The smell is coming from every tap and your utility confirms there’s no known bloom
- The problem doesn’t clear up after flushing and checking traps
- You’re on a well and haven’t had the water tested recently (or ever)
- The odor is getting worse over time, which can indicate active bacterial growth inside the plumbing system
- You notice the smell only from hot water, which is worth having your water heater inspected
In California, verify any plumber’s license at cslb.ca.gov before hiring. A licensed plumber can pull water samples, scope pipes if needed, and give you a real answer instead of a guess.
A Note on Water Filters
Activated carbon filters (pitcher filters, under-sink filters) do reduce geosmin and MIB, so a filter can mask the smell. That’s fine for a confirmed municipal algae bloom. It’s not a fix for biofilm in your pipes or a contaminated well. If the smell persists after flushing and your utility shows no bloom advisory, call a licensed plumber to find the actual source. You can verify any plumber’s license at cslb.ca.gov before committing.