Rotten egg smell in well water: hydrogen sulfide levels and what removes it

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Short answer

A rotten egg smell in well water is hydrogen sulfide gas, and the tap it comes from locates the source. After a few hours away from the house, the Minnesota Department of Health says to smell the hot and cold taps. A smell at the hot tap only means the water heater, where Penn State Extension says the magnesium anode rod reduces sulfate to hydrogen sulfide; swapping that rod for an aluminum or aluminum-zinc rod is the fix. A smell at both taps means the well or the plumbing. EPA sets no limit for hydrogen sulfide: its nearest secondary standard is odor at 3 threshold odor number, which lists a rotten-egg smell, and WHO puts the taste and odor threshold at 0.05 to 0.1 mg/L. The concentration picks the treatment class: activated carbon under 1 mg/L, aeration under about 2 mg/L, a manganese greensand filter to about 6 mg/L per MDH, continuous chlorination above 6 mg/L. Test on site, because the gas leaves a shipped sample.

If the cold tap smells clean, you can stop reading after the first table: the well is not the problem, and the fix is a rod, not a filter.

Which tap smells, and what that tells you

Hydrogen sulfide is the gas that gives water a rotten egg odor. The Minnesota Department of Health’s diagnostic is a nose test: after a few hours away from the house, smell the water from the hot and cold faucets, and read the result off the table.

What you notice	Likely cause	Confirm with
Hot tap smells, cold tap is clean	The water heater: Penn State Extension says the magnesium anode rod can chemically reduce sulfate to hydrogen sulfide; Wisconsin DNR names the same rod when the problem is hot water only	Repeat the MDH hot-and-cold check after a few hours away
Both taps smell	Hydrogen sulfide from the well itself, or sulfur bacteria in the well or plumbing	An on-site hydrogen sulfide test in mg/L; Penn State says the test must be done in the home or the sample chemically stabilized before shipping
Smell plus a salty taste	Sulfate, which EPA lists at a secondary standard of 250 mg/L with salty taste as the effect	A lab sulfate result
Smell comes back soon after shock chlorinating the well	Sulfur bacteria re-established: Penn State says the odor returns quickly in most cases	Retest, then choose a continuous treatment row below

The hot-only case is the cheap one. Penn State Extension says replacing the magnesium rod with an aluminum rod should eliminate the odor while keeping the heater’s corrosion protection, and Connecticut DPH adds a rule: if the heater runs above 150 F, only aluminum rods should be used.

Do not pull the rod out and leave the port plugged. Penn State says removal increases corrosion, shortens the heater’s life and will likely void the warranty, and Bradford White’s For The Pro bulletin says complete removal of the anode, magnesium or aluminum, voids the warranty and shortens tank life.

The tank then needs a one-time disinfection, and A. O. Smith’s Bulletin 22 says the smell is bacterial and not a warranty matter. The published procedures differ in dose and time.

Procedure	Dose or setting	Contact time	Who states it
Bleach in the tank	1 gallon of household bleach per 25 gallons of tank capacity	Undisturbed for one hour or more	A. O. Smith Bulletin 23
Bleach in the tank	Half to 1 gallon of bleach	Minimum of 3 hours, a full day preferred	Bradford White For The Pro bulletin
Heat	Heater set to 160 F (71 C)	Several hours	Minnesota Department of Health
Heat	Thermostat at about 160 F	8 hours	Texas A&M AgriLife Extension

If the smell is hot-only

Bradford White names its own A420 rod for this. A generic example is a 44 in flexible aluminum-zinc anode rod , hinged so it fits where there is no ceiling clearance to slide a straight rod in. The flaw to admit: the brand publishes no spec sheet, so confirm the thread and head type against your heater’s manual before ordering, and check whether your rod is a hex head or a combination nipple. If the cold tap smells too, skip this rod entirely: it fixes the heater, not the well.

What the number means

There is no EPA maximum contaminant level for hydrogen sulfide, and the secondary list does not add one. EPA’s secondary standards table has no hydrogen sulfide row; the nearest entries are odor at 3 TON, with a rotten-egg, musty or chemical smell as the listed effect, and sulfate at 250 mg/L.

EPA does not enforce secondary standards. TON is the threshold odor number, EPA’s dilution-based measure of how strong an odor is.

Penn State Extension, Connecticut DPH and Wisconsin DNR all state that no drinking water standard exists. Penn State’s reason is that the water is aesthetically undrinkable at very low concentrations: most people detect hydrogen sulfide well below 0.5 mg/L.

Figure	Value	Who states it
Taste and odor threshold	0.05 to 0.1 mg/L	WHO, which proposes no health-based guideline value
Detection by most people	Well below 0.5 mg/L	Penn State Extension
Corrosion note	1.0 mg/L may be corrosive	Connecticut DPH
Odor, secondary standard	3 TON	EPA
Sulfate, secondary standard	250 mg/L	EPA
Hydrogen sulfide MCL	None	EPA table; Penn State; CT DPH; Wisconsin DNR

The number itself is harder to get than most lab results. Hydrogen sulfide is a gas that leaves the water on the way to the lab, so Penn State says testing must be done in the home, or the sample must be chemically stabilized before it is sent to a commercial lab. A result from an unstabilized bottle shipped across the state is a low number, not a real one.

Which treatment class fits

For a smell at both taps, the on-site mg/L figure picks the class. The ceilings below are the published ones, and they do not all agree, so the row boundaries are ranges rather than lines.

On-site reading	Treatment class	Why, and the conditions
Trace to under 1 mg/L	Activated carbon	MDH: effective under 1 mg/L. Penn State: under about 1.0 mg/L, in a 1.0 to 1.5 cubic foot unit. Texas A&M: trace amounts of 0.05 to 0.3 mg/L
Under about 2 mg/L	Aeration	Penn State: not normally used above about 2 mg/L. CT DPH: effective below 2.0 mg/L. Texas A&M: less than 2 mg/L. MDH lists it for levels both below and above 1 mg/L
About 1 to 6 mg/L, some sources to 10	Oxidizing filter with manganese greensand	Penn State: 2 to 3 mg/L. MDH: up to about 6 mg/L. CT DPH: 1 to 10 mg/L. Texas A&M: up to 10 mg/L. Inversand GreensandPlus sheet: pH 6.2 to 8.5, service 2 to 12 gpm/sq ft, backwash minimum 12 gpm/sq ft at 55 F, 30 in minimum bed, chlorine demand of 6 mg/L per mg/L H2S, 0.5 to 1.0 mg/L free chlorine residual
Higher levels, only if dissolved oxygen is 4.0 mg/L or more	Catalytic carbon	Penn State: catalytic carbon requires a minimum of 4.0 mg/L dissolved oxygen. Calgon’s Centaur 12x40 sheet lists hydrogen sulfide removal from potable water with contact time typically under seven minutes; Jacobi’s AquaSorb CX-MCA coconut carbon is certified to NSF/ANSI 61 and 42. Neither sheet publishes a mg/L ceiling
Any level; the usual choice above 6 mg/L	Continuous chlorination, contact tank, then a filter	Penn State: any level, most often above 6.0 mg/L, at least 20 minutes of contact. CT DPH: oxidizing chemical injection at or above 6 mg/L. Texas A&M: up to 75 mg/L, dosed at 2.2 mg/L of chlorine per mg/L of H2S, with a tank sized for 20 minutes of contact
Sulfur bacteria in the well	Shock chlorination of the well and plumbing	PA DEP: chlorine solution in the entire system at least four hours, preferably overnight, then pumped until no chlorine odor remains. Penn State: the bacteria and the odor return quickly in most cases

Two media notes. Penn State says oxidizing filters with Birm media are not recommended for hydrogen sulfide removal, so an iron filter sold on Birm is the wrong tank here. The NSF/ANSI 42 mark on Jacobi’s sheet is a certification, not a hydrogen sulfide ceiling; the ceiling still comes from the extension bulletin.

Before you buy anything

Do not order a filter from the smell. The greensand ceiling runs from 2 to 3 mg/L in Penn State’s bulletin to 10 mg/L in Connecticut DPH’s, and a carbon unit that MDH rates under 1 mg/L is a wasted tank at 4. An on-site hydrogen sulfide test in mg/L, with pH and sulfate, decides the row, and a dissolved oxygen reading decides whether catalytic carbon is even eligible. The rod link above is the only purchase on this page that a smell alone justifies, and only when the cold tap is clean.

What to do, in order

Leave the house for a few hours, then smell the hot and cold taps, as MDH describes.
Hot only: replace the magnesium anode with an aluminum or aluminum-zinc rod, then chlorinate the tank per A. O. Smith Bulletin 23 or the Bradford White bulletin.
Both taps: get an on-site hydrogen sulfide test in mg/L, with sulfate and pH, and dissolved oxygen if catalytic carbon is on the table.
If sulfur bacteria are suspected, shock chlorinate the well and plumbing, hold at least four hours or overnight, flush until the chlorine odor is gone, and expect the smell to return.
Pick the treatment row by the on-site number, then retest at least once each year and after any change in the water’s smell, per CDC.

Frequently asked questions

Why does only my hot water smell like rotten eggs?

Penn State Extension says the magnesium anode rod inside a water heater can reduce sulfate to hydrogen sulfide, so only the hot tap smells. The Minnesota Department of Health's check is to smell the hot and cold taps after a few hours away. A clean cold tap points at the heater, and an aluminum or zinc rod is the fix named by Penn State, Connecticut DPH and Wisconsin DNR.

Is there an EPA limit for hydrogen sulfide in drinking water?

No. EPA's secondary standards table has no hydrogen sulfide row; its nearest entries are odor at 3 threshold odor number, which lists a rotten-egg smell, and sulfate at 250 mg/L, and EPA does not enforce secondary standards. Penn State Extension and Connecticut DPH both state that no standard exists. WHO puts the taste and odor threshold at 0.05 to 0.1 mg/L and proposes no health-based guideline value.

Can I just take the anode rod out of the water heater to stop the smell?

Not without a cost. Penn State Extension says removal increases corrosion, shortens the heater's life and will likely void the manufacturer's warranty, and Bradford White's service bulletin says complete removal of the anode voids the warranty and shortens tank life. The Minnesota Department of Health says the same about heater life. Swap the magnesium rod for an aluminum or aluminum-zinc rod instead, which keeps the corrosion protection.

Does shock chlorinating the well get rid of the rotten egg smell for good?

Usually not. Shock chlorination kills the sulfur bacteria in the well and plumbing, and Pennsylvania DEP says the chlorine solution should stay in the whole system at least four hours and preferably overnight, then be pumped out until no chlorine odor remains. Penn State Extension says that in most cases the sulfur-reducing bacteria and the odor return quickly, so a persistent smell needs one of the continuous treatment classes.

How much hydrogen sulfide can a manganese greensand filter handle?

The published ceilings disagree, which is why an on-site number matters. Penn State Extension says oxidizing filters remove up to 2 to 3 mg/L, the Minnesota Department of Health says up to about 6 mg/L, and Connecticut DPH and Texas A&M AgriLife Extension say up to 10 mg/L. Inversand's GreensandPlus sheet adds pH 6.2 to 8.5 and 6 mg/L of chlorine per mg/L of hydrogen sulfide.