Why does my tap water smell musty?

Musty or earthy tap water is almost always caused by geosmin or 2-methylisoborneol (MIB), two compounds produced by algae and cyanobacteria in surface water reservoirs. Humans can detect geosmin at concentrations as low as 4 parts per trillion, which is why even tiny amounts produce a noticeable smell. The problem peaks in late summer when warm water temperatures encourage algae blooms.

What is geosmin in drinking water?

Geosmin is a naturally occurring organic compound produced by algae, cyanobacteria, and certain soil bacteria. It's the same compound responsible for the smell of rain on dry earth, which is called petrichor. In water, geosmin produces an earthy or musty odor detectable at extremely low concentrations. The EPA has no maximum contaminant level for geosmin because it isn't a health hazard, only an aesthetic issue.

Is earthy smelling water safe to drink?

Municipal water that smells musty or earthy is almost always safe to drink. Geosmin and MIB are non-toxic at any concentration found in treated water. The smell is unpleasant but harmless. Well water is a different situation. A musty smell in well water can indicate biofilm, organic contamination, or bacteria in the well casing. Well water that suddenly develops an earthy smell should be tested for bacteria before you assume it's just geosmin.

Does a water filter remove musty smell?

Yes. Activated carbon filtration removes geosmin and MIB effectively. A pitcher filter with activated carbon, an under-sink carbon filter, or a faucet-mount carbon filter will eliminate the musty smell at your drinking tap. Standard carbon filters are actually very well suited to this particular problem. The smell should disappear within the first one or two filtered glasses.

Why does city water smell bad in summer?

Warm summer water temperatures trigger algae blooms in the reservoirs that supply many municipal water systems. The algae produce geosmin and MIB as metabolic byproducts. Water treatment plants use activated carbon, ozone, and algaecides to manage this, but during a heavy bloom year, some of the compounds can make it through treatment and reach your tap. It's more common in July through September and tends to come and go with bloom intensity.

Water Smells Musty or Earthy: What's in There and How to Fix It

If your tap water smells like wet dirt or a damp basement, you’re detecting geosmin. It’s one of the most recognizable smells in nature, and humans are extraordinarily good at detecting it.

That sensitivity is a quirk of evolution. Geosmin smells like stagnant water or mold, two things early humans needed to avoid. So our noses learned to pick it up at vanishingly small concentrations. As low as 4 parts per trillion. The downside is that safe municipal water can smell genuinely bad during algae season even when there’s nothing harmful in it.

What Geosmin Is

Geosmin is an organic compound produced by algae and cyanobacteria (blue-green algae) in surface water reservoirs. A related compound, 2-methylisoborneol (MIB), produces a similar musty or camphor-like smell. Both are non-toxic. The EPA has no maximum contaminant level for either one.

They’re the same compounds responsible for the smell of rain on dry earth. Soil bacteria produce geosmin too, which is why fresh-turned soil and earthworms smell the way they do.

In drinking water, the smell can range from mildly earthy to intensely swampy depending on concentration. Even at very low levels, many people find it unpleasant enough to avoid drinking the water.

Why It Gets Worse in Summer

Algae thrive in warm water. When reservoir temperatures rise from late June through September, algae populations can grow rapidly. A bloom doesn’t have to be visible on the water surface to affect taste and odor. Microscopic algae populations produce enough geosmin to be detectable at the tap.

Municipal water treatment plants use several tools to manage algae, including activated carbon dosing at the plant, ozone treatment, and algaecides. These work well most of the time. But during an unusually warm summer or an especially heavy bloom year, treatment may not fully strip out the geosmin before water reaches homes.

If your water smells musty and it’s late summer, this is almost certainly the cause. Check whether your water utility has issued any taste and odor advisories. Many utilities send alerts during bad bloom years.

Well Water Is Different

Well water doesn’t come from surface reservoirs, so algae blooms aren’t the culprit.

A musty smell in well water usually points to one of three things. Biofilm buildup inside the well casing or pipes. Organic matter from surface water infiltrating the well through a cracked casing or improper seal. Or bacteria in the well, some of which produce earthy-smelling metabolic byproducts.

The important distinction: a musty smell in well water should be investigated, not just filtered away. Standard carbon filtration will remove the smell, but it won’t tell you whether bacteria are present. Test your well for total coliform and E. coli before deciding the smell is cosmetic. See the well water testing guide for how to collect a sample and where to send it.

If your well test comes back clean, a carbon filter handles the remaining smell effectively.

How Activated Carbon Fixes It

Activated carbon filtration is very well matched to geosmin and MIB removal. Both compounds adsorb readily to the carbon surface, meaning they bind to it as water passes through. The filtered water comes out without them.

A basic pitcher filter with activated carbon will remove the musty smell from drinking and cooking water. Most standard pitcher filters, including entry-level Brita models, use activated carbon and work fine for this purpose.

An under-sink carbon filter or a faucet-mount filter extends the benefit to tap water without the pour-and-wait step of a pitcher.

A whole-house carbon filter removes the smell from all taps, including the shower. Some people find the earthy smell in shower steam as bothersome as in drinking water. Whole-house filtration addresses that.

For a full comparison of pitcher filter options, see best pitcher water filters.

The Rotten Egg Exception

Musty and sulfurous are different smells, but occasionally people describe a faint hydrogen sulfide odor as “earthy” in the early stages.

If the smell is more rotten egg than wet soil, the cause is hydrogen sulfide gas, not geosmin. Hydrogen sulfide in well water often comes from sulfur-reducing bacteria or naturally occurring sulfur in the aquifer. It requires a different treatment approach than geosmin. A rotten egg smell that’s stronger from the hot tap than the cold tap usually points to the water heater anode rod, not the well.

Geosmin smells like fresh soil, mushrooms, or a damp basement. If that’s what you’re getting, you’re on the right page.

What to Do Right Now

On city water: run the tap cold for 30 seconds, since chilled water releases less volatile odor. If that helps but the smell is still there, a pitcher filter with activated carbon is a quick and inexpensive fix. Replace the filter on schedule, because a spent carbon filter eventually stops adsorbing and can release compounds back into the water.

On well water: test first. If the test comes back clean, a carbon pitcher filter or faucet filter solves the smell. If you find bacteria, address the contamination before filtering.

Sources: EPA Drinking Water Taste and Odor Issues | CDC Drinking Water Overview