Last checked: October 22, 2026

Rotten-egg smell in well water

A rotten-egg or sulfur smell almost always means hydrogen sulfide gas. It can come from the groundwater itself, from sulfur bacteria living in the well or plumbing, or from a reaction inside your water heater. The fix depends on which one it is, and you can often tell with a simple faucet test before you spend anything.

What causes the smell

Minnesota's Department of Health lists four sources of hydrogen sulfide in well water:¹

• Naturally in the groundwater, from decay and chemical reactions with soil and rock.
• Sulfur bacteria in the groundwater, the well or the plumbing.
• The water heater, through sulfur bacteria or a chemical reaction inside it.
• Pollution, which is rare.

Penn State Extension says sulfur-reducing bacteria thrive in the low-oxygen conditions inside wells and plumbing, and that hydrogen sulfide problems are most common in wells drilled into shale and sandstone.² In rare cases a water softener gives the bacteria a place to grow.²

Is it harmful?

• In the water: Penn State calls hydrogen sulfide an aesthetic problem. There's no drinking water standard for it, because people can smell it well before it reaches harmful levels; most can detect it well below 0.5 mg/L.² Sulfur bacteria aren't harmful either.¹
• In the air: the gas can be harmful at high levels. Minnesota says it should be removed or vented so it doesn't build up in well pits, basements or well houses, and only well professionals with proper safety precautions should enter a well pit or other enclosed space where it may be present.¹
• Around the house: it can corrode metal plumbing, tarnish silverware, and leave yellow or black stains on fixtures and laundry.²³
• Sulfate is different. Sulfate is a related mineral, not a gas. EPA's non-enforceable secondary standard for sulfate is 250 mg/L.⁵ UGA Extension says levels above that can taste bitter and have a laxative effect.³

Find the source: the faucet test

Minnesota suggests waiting until the water hasn't been used for a few hours, then smelling the hot and cold water at the faucets.¹ Florida's Department of Health explains what each result likely means:⁴

Other clues: white, gray, black or reddish slime points to sulfur bacteria (reddish when iron bacteria are also present), and black stains on silverware and fixtures point to hydrogen sulfide gas.¹

What to test

• Coliform bacteria and nitrate. Minnesota recommends these because the gas can rarely come from sewage or pollution.¹ Florida also recommends regular testing for total coliform or E. coli.⁴ UGA notes many labs treat a positive coliform result plus a rotten-egg smell as a sign of sulfur-reducing bacteria.³
• Hydrogen sulfide level, if you're choosing treatment for the groundwater. Penn State says it has to be measured at the home, or the sample has to be chemically stabilized before it goes to the lab, so ask the lab first whether it can do this test.²
• Sulfate, since the water heater reaction turns sulfate into hydrogen sulfide.²¹ Add iron and manganese if you also see orange or black staining (see iron in well water).

Use a state-certified lab. Our county pages list official lab options where we cover, and you can compare your numbers with EPA benchmarks in the Well Test Results Checker.

If it's the water heater

Have a plumber or water system professional do this work unless you're very familiar with water heaters.¹

• Replace the magnesium anode rod. Most water heaters have a magnesium rod to limit tank corrosion, and it can turn sulfate in the water into hydrogen sulfide.²¹ Penn State says an aluminum rod should eliminate the smell while keeping corrosion protection. UGA says aluminum or zinc, but only aluminum if the water is kept above 150°F.²³
• Don't just pull the rod out. Removing it can shorten the heater's life and will likely void the warranty.²¹
• Disinfect and flush the heater with a chlorine bleach solution. If some bacteria survive, the smell may come back within a few weeks.¹
• Heat treatment: Minnesota describes raising the heater to 160°F for several hours to kill sulfur bacteria, then flushing. It warns this can be dangerous (scalding, and the pressure relief valve has to work), so check with the manufacturer or dealer first and turn the temperature back down afterward.¹

If it's the well, plumbing or softener

• Shock chlorinate the well and plumbing. A licensed well contractor can do it, or see our shock chlorination guide.¹ Sulfur bacteria can be hard to remove once established, and scrubbing the casing or other pre-work by a contractor may be needed, especially if iron bacteria are also present.¹
• Expect it may come back. Penn State says the odor often returns quickly after shock chlorination.² UGA says that if two attempts fail, continuous chlorination or another method may be needed.³
• Softener or other treatment equipment: ask the installer or manufacturer how to disinfect it.¹

If it's in the groundwater: treatment by level

Penn State says most hydrogen sulfide treatment is whole-house (point-of-entry), because the smell affects every tap, and that the hydrogen sulfide concentration is what decides the most efficient option.² The sources don't agree exactly on the cutoffs, so the ranges below show each one.

Penn State also mentions hydrogen peroxide and ozone as oxidants, and ion exchange units that swap hydrogen sulfide for chloride.² Minnesota lists ozonation with filtration as well.¹

Before you buy: Penn State suggests comparing units and prices from several dealers, understanding the maintenance, and getting a written warranty.² In Minnesota, the health department suggests checking that a treatment professional is a licensed plumber or water conditioning contractor.¹ See whole-house vs. point-of-use for how to think about the choice.

Other options: a new well in a different formation (Penn State says a shallower well can sometimes avoid sulfur-bearing rock; a consulting hydrogeologist can assess this), or connecting to a public water line if one is nearby.²¹

Caveats

• Thresholds differ by source. Penn State, Minnesota and UGA give different upper limits for carbon and greensand, so treat the ranges as rough guides and ask for the installer's numbers in writing.
• More than one problem at once. Iron, manganese and sulfur bacteria often show up together, and they change which system works. See iron in well water.
• The smell isn't a safety test. A well with no odor can still have bacteria, nitrate, arsenic or other contaminants you can't smell. See how often to test.

Frequently asked questions

Is it safe to shower in water that smells like rotten eggs? We can't say for any given well. Penn State calls hydrogen sulfide in water an aesthetic problem, but Minnesota warns the gas in air can be harmful at high levels in enclosed spaces, so it should be vented.²¹ If the smell is strong or you feel unwell, ask your local health department.

Why does only my hot water smell? Usually the water heater: its magnesium anode rod or sulfur bacteria growing in the warm tank.⁴¹

Will a water softener fix it? A standard softener isn't designed for it. Penn State describes a special ion exchange unit that swaps hydrogen sulfide for chloride, and notes that in rare cases a softener gives sulfur bacteria a place to grow.²⁴

Will shock chlorination get rid of it for good? Sometimes, if bacteria are the cause. Often it returns, and continuous treatment is the next step.²³

Can a lab test hydrogen sulfide from a regular sample bottle? Not usually. It needs an at-home test or a chemically stabilized sample, so check with the lab before you sample.²

Where to get tested and treatment options

Related guides

• Free tool: Well Test Results Checker: type in your lab numbers and see each one next to the EPA benchmark.
• Iron in well water: what to test, and what to buy
• Shock chlorinating a well
• How to read your well water test results
• Whole-house vs. point-of-use treatment
• Best way to test well water