Common Causes (Ranked by Likelihood)
Hydrogen sulfide (H₂S) is the most frequently reported tap water odor. Sources: naturally occurring sulfur in groundwater (well water), sulfate-reducing bacteria in the water supply or pipes, or a degraded magnesium anode rod in your water heater producing H₂S. If the smell is only from hot water, the water heater anode rod is almost certainly the cause. If from cold water too, it's in the supply or pipes.
Municipal water is disinfected with chlorine or chloramines. The EPA allows up to 4 mg/L. A noticeable chlorine smell is typically harmless but indicates higher disinfectant levels — often seasonal (summer algae blooms prompt higher treatment). Chloramine (chlorine + ammonia) smells slightly different — more medicinal. Both are safe at regulated levels but unpleasant.
Musty or earthy smell from tap water usually comes from geosmin and 2-methylisoborneol (MIB) — compounds produced by cyanobacteria and actinobacteria in reservoirs and source water, especially during algae bloom seasons (late summer). These compounds are detectable by humans at extremely low concentrations (5 parts per trillion for geosmin) — far below any health concern level. Also possible: biofilm growth inside pipes or a rarely-used fixture.
What smells like sewage in the tap water is frequently actually the drain. When you run water, air is displaced upward from the drain, carrying sewer smell with it — making it seem like the water smells when it's actually the drain. Test by filling a glass away from the sink and smelling it — if the water in the glass smells fine, the issue is the drain, not the water. Actual sewage smell in the water supply is rare and serious.
Metallic smell comes from dissolved metals: copper (bluish-green corrosion, blue staining in sinks), iron (reddish-brown staining, rusty smell), zinc (from galvanized pipes), or manganese (black staining). Old plumbing, corrosive water chemistry, or standing water in pipes overnight are common causes. The first water drawn in the morning is typically worst (has been sitting in pipes all night).
Fishy smell in tap water can come from barium, cadmium, or certain naturally-occurring organic compounds (particularly in private well water). Chloramines in municipal water can also produce a fishy smell when interacting with organic material. Algae blooms produce compounds with fishy character. Less commonly, contamination from industrial sources or agricultural runoff.
How to Fix It
Water Heater Anode Rod Replacement
$20-50 (DIY) / $100-200 (plumber)For rotten egg smell from hot water only.
- 1Confirm the smell is only from hot water — run cold water at the same tap and check if it smells. If cold is fine, the water heater is the source.
- 2The magnesium anode rod in most water heaters produces H₂S when reacting with sulfur bacteria. Replace with an aluminum/zinc anode rod — produces far less H₂S.
- 3Shut off water heater, connect a hose to the drain valve, turn off cold water supply. Use a 1-1/16" socket to remove the old rod from the top of the heater.
- 4Install aluminum/zinc anode. Consider also flushing the tank — bacteria accumulate in sediment at the bottom.
- 5Add a cup of hydrogen peroxide to the tank after refilling to kill remaining sulfur bacteria.
Activated Carbon Filter
$30-300For chlorine, musty, and general odor from municipal water.
- 1For point-of-use: install an under-sink activated carbon filter ($50-150) or countertop pitcher filter (Brita, Pur). Replace filters per manufacturer schedule.
- 2For whole-house: install a carbon block filter on the main supply line ($200-500 + installation). Removes chlorine, chloramines, VOCs, and most odors before reaching any fixture.
- 3Activated carbon does NOT remove hardness, nitrates, or heavy metals — for those, reverse osmosis is needed.
- 4Replace filters on schedule — an overloaded carbon filter can actually harbor bacteria.
- 5Refrigerator filters also use activated carbon — remember to replace those every 6 months.
Clean Sink Aerators and Drains
$0For musty smell from specific taps or apparent sewage smell.
- 1Unscrew the aerator (the small screen) at the end of the faucet. Soak in white vinegar for 30 minutes, scrub, rinse, reinstall.
- 2Pour 1/2 cup of baking soda followed by 1/2 cup of white vinegar down the drain. Let fizz for 15 minutes, flush with hot water.
- 3Follow with boiling water to kill biofilm in the drain.
- 4For persistent drain smell: use an enzymatic drain cleaner (not bleach, which can damage pipes).
- 5Check the P-trap — a rarely used sink may have an evaporated P-trap. Run water for a minute to refill it.
Water Testing
$20-100For persistent odor or suspected contamination.
- 1Purchase a basic water test kit at a hardware store ($20-30) for immediate results on pH, hardness, chlorine, iron, and bacteria.
- 2For comprehensive testing: send a sample to a certified lab ($50-150). NSF-certified labs test for heavy metals, nitrates, bacteria, and hundreds of compounds.
- 3Well owners: test annually at minimum. Test after flooding, nearby agricultural activity, or unusual odor/color changes.
- 4Municipal customers: request a Consumer Confidence Report (CCR) from your utility — required annually and shows all tested contaminants.
- 5If E. coli or coliform bacteria are detected: do not drink until resolved and consult your local health department.
When to Call a Professional
Sewage smell from ALL taps simultaneously — not the drain; possible sewer gas intrusion into water supply; call your water utility immediately.
Sudden new smell with no recent changes — have water tested; could indicate new contamination.
Rotten egg smell from cold water too, not just hot — sulfur in the water supply; test for H₂S levels.
Metallic smell in home with old plumbing and a child under 6 — lead contamination risk; test immediately and use certified NSF-42/53 filter until results are known.
How to Prevent It
Replace water heater anode rod every 3-5 years — prevents H₂S production before it starts.
Flush rarely-used taps monthly — prevents biofilm buildup in aerators.
Well owners: test water annually — before changes are noticeable.
Replace refrigerator and pitcher filters on schedule — overloaded filters stop working.
Insulate pipes in areas of extreme temperature — prevents leaching from thermal stress.
The Smell Test: Diagnosing Tap Water Odor by Character
Rotten egg from hot water only: water heater anode rod. Rotten egg from cold and hot: sulfur in the supply or pipes. Chlorine/bleach from all taps: municipal treatment, normal or elevated. Musty/earthy from all taps: algae compounds in source water (seasonal). Musty from one tap only: dirty aerator or biofilm in that fixture. Sewage smell that disappears when you fill a glass: it's the drain, not the water. Metallic: pipe leaching (worse after standing overnight).
The fill-a-glass-away-from-the-sink test is the most useful first step for any tap water odor. Fill a glass, walk to another room, and smell it there. If the smell is present, it's in the water. If it's gone, you were smelling the drain, not the water — completely different fixes apply.
Frequently Asked Questions
It depends on the source. If it's only from the hot tap, it's the water heater anode rod — safe to drink but unpleasant. If it's from well water, test for H₂S levels — high concentrations warrant a filter. If from municipal water that normally doesn't smell, contact your utility.
Sudden changes in municipal water smell often reflect changes in the source water (seasonal algae, runoff after rain) or treatment levels. Your utility may have sent a notice. Call your water utility to ask about recent changes. If you're on well water, test immediately — sudden changes can indicate contamination.
Yes — activated carbon filters like Brita effectively remove chlorine smell, musty/earthy geosmin smell, and most VOC odors. They don't remove sulfur bacteria (for that you need a catalytic carbon or KDF filter), heavy metals (except lead in NSF-certified models), or nitrates. For rotten egg smell from the water heater, the filter won't help — address the anode rod.
Fishy smell in tap water usually comes from naturally-occurring organic compounds in source water, algae bloom byproducts, or chloramine interacting with organic material. It's usually harmless. An activated carbon filter typically removes it. If the smell is new and strong, contact your water utility.