Bad-smelling water usually comes from bacteria, dissolved minerals, or disinfection chemicals, and the specific smell tells you a lot about what’s causing it. Most water odors are cosmetic nuisances rather than health hazards, but a few deserve immediate attention. Here’s how to identify what you’re dealing with based on what your nose is telling you.
Rotten Egg Smell: Hydrogen Sulfide
The most common complaint about smelly water is a sulfur or rotten egg odor. This comes from hydrogen sulfide gas, which is produced when certain anaerobic bacteria break down sulfur compounds in your water supply. These bacteria, particularly members of the Desulfovibrio genus, convert sulfate (a compound naturally present in groundwater and some foods) into hydrogen sulfide as part of their metabolism. Well water is especially prone to this because groundwater can sit in contact with sulfur-bearing rock for long periods.
If the smell only appears when you run hot water, your water heater is the likely culprit. Most water heaters contain a magnesium anode rod designed to prevent the tank from corroding. Sulfur-feeding bacteria in the tank interact with that magnesium rod, and the warm, enclosed environment accelerates the chemical reaction that produces hydrogen sulfide gas. The fix can be as simple as replacing the magnesium anode rod with an aluminum or zinc one, or flushing the tank with a high concentration of chlorine.
To figure out whether the problem is your water heater or your water supply, run cold water only from a faucet and smell it. If the cold water is fine but the hot water stinks, the issue is in the tank. If both smell, the source is your water supply itself.
Earthy or Musty Smell
Water that smells like dirt, damp basement, or wet wood usually contains a compound called geosmin. It’s the same chemical responsible for the smell of fresh rain on dry soil. Geosmin is produced by soil-dwelling bacteria, especially species of Streptomyces, and by certain types of blue-green algae like Anabaena. These organisms thrive in reservoirs, lakes, and rivers, particularly during warm months when algal blooms are more common.
A related compound called 2-methylisoborneol (MIB) produces a similar musty odor and comes from the same types of organisms. Humans are remarkably sensitive to both of these compounds, detecting them at concentrations as low as a few parts per trillion. That means your water can smell noticeably earthy even when the amount present is far too small to pose any health risk. Municipal water systems use sand filtration and other methods to reduce geosmin and MIB levels, but seasonal spikes can still push enough through to be noticeable at the tap.
Metallic or Bitter Taste and Smell
A metallic smell or taste typically comes from dissolved metals in your plumbing or water source. The EPA’s secondary drinking water guidelines identify the key offenders and the concentrations at which they become noticeable:
- Iron causes a metallic taste and rusty, reddish staining at levels above 0.3 mg/L.
- Manganese produces a bitter metallic taste with black or brown discoloration at levels above 0.05 mg/L.
- Copper creates a metallic taste and blue-green staining above 1.0 mg/L.
- Zinc adds a metallic flavor above 5.0 mg/L.
These are classified as “secondary” contaminants, meaning the EPA considers them aesthetic problems rather than health threats at those levels. Water with a low pH (below 6.5) tends to be more corrosive, which dissolves metals from pipes and fixtures more aggressively. If you notice a metallic smell primarily from the first water out of the tap in the morning, corroding pipes are the likely source. Running the water for 30 seconds to a minute can flush out the standing water that’s been sitting in contact with the metal overnight.
Chlorine or Swimming Pool Smell
Municipal water systems add chlorine or chloramines to kill bacteria, and the maximum allowable level is 4.0 mg/L for both. Most systems aim well below that, but some people can detect chlorine at much lower concentrations. The smell tends to be stronger when water treatment plants increase their disinfection dose, which commonly happens after heavy rain events that wash extra organic matter into reservoirs.
Chlorine evaporates readily, so filling a pitcher and letting it sit in the refrigerator for a few hours will reduce the smell significantly. A basic activated carbon filter (the kind found in most pitcher filters and faucet-mount systems) also removes chlorine effectively. Chloramines are more persistent and require a catalytic carbon filter to remove.
Fishy Smell
A fishy odor in water can come from several sources. Naturally occurring barium, a metal found in mineral ores, can seep into wells and pipes and produce a fishy smell. Cadmium, another metal, can have a similar effect. Algal blooms in a water source are another common cause, particularly when algae die off and decompose. Chloramines, the disinfectant some cities use instead of chlorine, can also react with organic matter in the water to produce compounds with a fishy odor.
If you’re on city water and the fishy smell is new, your utility may have recently switched disinfection methods or may be dealing with seasonal algae in the source water. A call to your water provider can confirm whether anything has changed.
Chemical, Fuel, or Solvent Smell
Water that smells like gasoline, paint thinner, or nail polish remover is the one scenario where you should stop drinking it immediately. These smells indicate volatile organic compounds (VOCs), which can enter water supplies through industrial contamination, leaking underground fuel tanks, or agricultural runoff. Benzene, a known human carcinogen, is one of the more dangerous VOCs that can contaminate water supplies.
Unlike the other odors on this list, fuel-like or solvent-like smells are not cosmetic problems. If your water smells like chemicals, avoid using it for drinking or cooking until you’ve had it tested. Contact your local water utility if you’re on a municipal system, or arrange for a lab test if you’re on a private well.
How Odor Is Measured and Regulated
The EPA sets a secondary standard for odor at a threshold odor number (TON) of 3, which is essentially the point at which most people can detect that something is off. This standard isn’t legally enforceable. It’s a guideline that water utilities are encouraged to meet for customer satisfaction. Primary contaminants with enforceable limits, like certain VOCs and disinfection byproducts, are regulated separately and carry mandatory testing requirements.
Fixing Smelly Well Water
Private wells don’t benefit from municipal treatment, so the responsibility falls on the homeowner. For bacterial contamination causing sulfur or other organic smells, shock chlorination is the standard first step. This involves introducing a concentrated chlorine solution (typically 10 to 50 mg/L, far higher than the trace amounts used in city water) into the well, letting it sit for several hours, and then flushing the system. This kills the bacteria causing the odor.
Shock chlorination is a one-time treatment, not a permanent solution. If the bacteria repopulate, which they often do in wells with high sulfate levels, you may need a continuous disinfection system or a specialized filter. Aeration systems work well for hydrogen sulfide because the gas escapes readily when water is exposed to air. Activated carbon filters handle many organic compounds and chlorine. For metals, a water softener or oxidizing filter is typically more effective.
If you’re unsure what’s causing the smell, a water test from a certified lab will identify the specific contaminants. Many state health departments offer free or low-cost well water testing, and the results will tell you exactly which treatment approach makes sense for your situation.