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Hydrogen sulfide in well water

H₂S

Measured in ppm (mg/L) · Updated 2026-09-15

The rotten egg smell, and the one water problem people notice instantly. Your nose finds it at concentrations a lab would call trace, and the treatment depends entirely on how much there is.

Why
Sulfur-reducing bacteria and certain geology both produce it. It is corrosive to plumbing and fixtures over time and makes water unpleasant to use.
Watch out
If only the hot water smells, the well is probably innocent. The magnesium anode rod in the water heater can generate sulfide from sulfate in the water, and that is a much cheaper fix than a treatment system.

No federal standard

No federal standard

There is no primary or secondary standard for hydrogen sulfide. The nose detects it well below any level of concern, which is its own kind of warning system.

What treats it

  • Air-injection iron filters for well water

    The default choice for iron-bearing well water: it pulls in a pocket of air, oxidises dissolved iron as the water passes through it, and traps the resulting particles in a media bed that backwashes itself. No chemicals to buy or refill.

  • Chlorine and peroxide injection systems

    A metering pump injects chlorine or hydrogen peroxide, a retention tank gives the reaction time to finish, and a backwashing filter removes what it produced. It handles water that defeats air injection.

  • Activated carbon filters for well water

    Adsorbs organic compounds, chlorine and the things that make water taste and smell wrong, on a bed of carbon with an enormous internal surface area. It is the polishing stage, and it is mandatory behind any chemical injection.

Check the water heater first

The cheapest explanation is also a common one, so eliminate it before anything else.

If the smell is present in hot water and absent in cold, the well is probably not the source. Water heaters contain a sacrificial anode rod, usually magnesium, whose job is to corrode so the tank does not. Minnesota’s health department describes the mechanism: that magnesium can supply electrons that aid the conversion of sulfate already in the water into hydrogen sulfide gas.

The tank is manufacturing the smell out of something harmless.

The fix is replacing the magnesium anode with an aluminium or powered one, which costs a fraction of any treatment system. Removing the anode entirely will stop the smell and shorten the heater’s life considerably, and generally voids the warranty, so replacement rather than removal is the advice.

Flushing and disinfecting the heater is often part of the same job, since sulfur bacteria establish themselves in the tank.

Run both taps and compare before spending anything. It takes a minute.

If both hot and cold smell

Then the source is the well or the plumbing, and concentration decides the approach. The Minnesota figures are the clearest published guidance:

  • Below 1 mg/L, activated carbon is effective.
  • Up to about 6 mg/L, oxidising media filtration works, which in practice means an air-injection or greensand filter, often the same unit already being fitted for iron.
  • Above 6 mg/L, continuous chlorination or ozonation followed by filtration is the answer, because air stops keeping up.

Sulfide is also frequently a package deal with iron, since the conditions that produce one often produce the other. Where both are present, one oxidising filter usually handles both, which makes the combined problem cheaper to solve than two separate ones.

The other clue is whether the smell is steady or intermittent. Geological sulfide is steady. Bacterial sulfide varies, and often worsens after the water has sat still, which points toward shock chlorinating the well rather than filtering its output indefinitely.

Read next

  • Contaminant

    Iron in well water

    The most common well water complaint in the country. Above 0.3 ppm it stains everything water touches, and the form it arrives in decides whether a filter or a softener is the right answer.

  • Comparison

    Air injection vs chemical injection

    Air injection is the default and the right answer for most well iron. Chemical injection is for the specific cases air cannot reach: sulfide above about 6 ppm, very heavy iron, organic iron and iron bacteria.

  • Symptom

    Well water smells like rotten eggs

    Hydrogen sulfide. Run the hot and cold taps separately before doing anything else, because if only the hot water smells, the water heater is manufacturing it and the repair costs a fraction of a treatment system.

  • Treatment

    Acid neutralizers for low-pH well water

    Passes water through a bed of calcite, or calcite blended with magnesia, which dissolves slowly and raises pH. It is the stage that makes an oxidising filter downstream work at all on acidic water.

  • Treatment

    Reverse osmosis for well water

    Pushes water through a membrane fine enough to reject dissolved salts and most contaminants. Under the kitchen sink it is the most capable treatment available for water that is going to be drunk.

  • Treatment

    Water softeners for well water

    Trades the calcium and magnesium that cause scale for sodium, on a resin bed that regenerates itself with brine. It is the right answer for hard water and the wrong answer for an iron problem, though it is sold for both.

Not sure what is in your water yet?See which tests to order, thenplan the treatment from your results.

Questions

Only my hot water smells. Why?
Almost certainly the water heater rather than the well. Minnesota's health department explains that the magnesium anode many heaters use can supply electrons that help convert sulfate into hydrogen sulfide. Replacing or removing that anode is the standard response, and it is worth ruling out before pricing whole-house treatment. Removing it outright shortens the heater's life and may void the warranty, so replacement with an aluminium or powered anode is the usual advice.
What treats it at what level?
Minnesota's health department puts activated carbon as effective below 1 mg/L, and oxidising media filtration such as a manganese greensand filter as effective up to about 6 mg/L. Above that, continuous chlorination or ozonation with filtration takes over.
Is it dangerous?
In water at well concentrations it is a nuisance and a corrosion problem rather than a health hazard, and there is no federal standard for it. As a gas at high concentration in an enclosed space it is dangerous, which is a consideration for well pits and confined spaces rather than for a kitchen tap.
Can the smell come and go?
Yes, and that is a useful clue. Sulfide produced by bacteria varies with conditions in the well, while sulfide from geology tends to be steady. Intermittent smell, particularly after periods of low use, points at biology rather than rock.

Sources