Rotten Egg Smell in Well Water: Find the Source, Then Fix It

Is it a health problem?

The smell is hydrogen sulfide, a gas that forms when bacteria break down organic matter without oxygen, or turn sulfate into sulfide¹. At the levels you can smell, it is mostly a nuisance: most people detect it well below 0.5 ppm0.5 mg/L, long before it causes health concerns². Health Canada’s aesthetic objective is 0.05 ppm0.05 mg/L¹. The US has no drinking water standard for it², but the EPA’s secondary odour standard lists a “rotten-egg” smell among the effects it covers³.

It does damage plumbing. At about 1 ppm1 mg/L, the water can corrode and tarnish copper pipes and fixtures⁴. With iron, steel or copper pipes, it can turn the water black¹.

Boiling does not remove it and can make the odour stronger⁵.

One case to rule out: in rare instances the gas comes from sewage. Test for coliform bacteria and nitrate to be safe⁶.

Step 1: find where the smell starts

The Minnesota Department of Health gives a simple test. After you have been away from home for a few hours, smell the water at the hot and cold taps, and note which ones smell⁶.

Where the smell is · likely source

You smell it	Likely source	Start with
Hot water only	The water heater: its anode rod reacting with sulfate, or sulfur bacteria in the tank6,7	The water heater fixes below
Hot and cold	Hydrogen sulfide in the groundwater, or sulfur bacteria in the well or plumbing6	A lab test, then whole-house treatment
Only after the softener or filter	Sulfate-reducing bacteria in the treatment equipment4	Disinfecting that equipment
Mostly after the water sat unused	Bacteria in the well or heater; the smell builds with low use7	The same test, after a normal day of use

Hot-only does not always mean the anode. If bacteria in the heater are the cause, Virginia Tech says changing the anode will not solve the problem⁸.

Step 2: test the right things

For a smell in all the water, you need numbers before you buy anything:

• Hydrogen sulfide. The gas escapes from the sample, so the test must be done at home or the sample chemically stabilized before it goes to the lab². Ask the lab for the right bottle and preservative⁴.
• Sulfate. The EPA’s secondary standard is 250 ppm250 mg/L³ and Health Canada’s aesthetic objective is 500 ppm500 mg/L¹. Above about 500 mg/L, sulfate can have a laxative effect, and lower when magnesium is also high¹.
• pH and iron. An oxidizing filter may need an acid neutralizer first if the pH is below 6.7^4,8, and iron changes how a chlorine system is set up⁹.
• Coliform bacteria and nitrate, to rule out sewage⁶.

Mail-in lab test (US)

Tap Score Essential Well Water Test

No NSF/ANSI listing found

Measures

54 items, incl. sulfate, iron, manganese, pH, nitrate

Bacteria

total coliform and E. coli

Hydrogen sulfide

home test strip in the kit

Turnaround

5 business days at the lab

Strengths

• Sulfate, iron, pH, bacteria and nitrate on one report
• Return shipping included for US orders

Drawbacks

• Hydrogen sulfide is a home strip, not a stabilized lab sample
• The partner lab is not named on the product page
• US only: Canadians are sent to separate Canadian kits

Good fit: US well owners who need one baseline test before choosing a sulfur treatment.
Skip it if: you live in Canada, or you need a precise lab value for hydrogen sulfide.

Fixing a smell in the hot water only

A water heater makes the gas in two ways: it is a warm home for sulfur bacteria, and its anode rod, usually magnesium, helps turn sulfate into hydrogen sulfide⁶. Three fixes; Alberta Agriculture suggests disinfecting the tank first⁹:

1. Disinfect the tank with chlorine bleach. Bradford White, a water heater maker, gives the steps: turn off the water and the power or gas, drain a few gallons, pour ½ to 1 gallon of bleach in through the hot water outlet, run each hot tap until it smells of chlorine, and leave it at least 3 hours, ideally a full day, before flushing¹⁰. If some bacteria survive, the smell can return within a few weeks⁶.
2. Change the anode. Penn State says an aluminum anode in place of the magnesium one should remove the odour while still protecting the tank². Wisconsin says zinc greatly reduces the bacteria without removing them all⁷. Check with the heater’s maker first so the warranty is not affected⁸. Connecticut adds that above 150°F, only aluminum should be used⁴.
3. Heat the tank. The Minnesota Department of Health suggests 160°F (71°C) for several hours, then flushing; check that the pressure relief valve works and turn the thermostat back down afterwards⁶. Wisconsin says this reduces the bacteria but does not eliminate them⁷, and Bradford White says these bacteria can survive up to about 225°F¹⁰. Water at 140°F causes serious burns in 6 seconds; the US Consumer Product Safety Commission recommends 120°F for everyday use¹¹.

Never just pull the anode out. It shortens the tank’s life and voids the warranty^2,10. Powered anode rods are also sold for this problem, but we found no maker or agency data on how well they work.

Fixing a smell in all the water: treatment by level

Sources agree on the order (carbon for traces, oxidizing filters in the middle, chlorine at the top) but not on the cut-offs. We show each one.

Hydrogen sulfide · usual treatment by level

Treatment	Range given by the sources	Watch for
Reference: Health Canada aesthetic objective, 0.05 ppm0.05 mg/L1
Activated carbon	Under 1 ppm1 mg/L2,6; only a few tenths of a mg/L8	Limited capacity: a small filter may last only weeks6
Aeration	Under about 2 ppm2 mg/L2,4	Smell near the unit; needs venting4
Oxidizing filter (manganese greensand)	2–3 ppm2–3 mg/L2; under 5 ppm5 mg/L1; up to about 6 ppm6 mg/L6,8	pH below 6.7 needs a neutralizer first8; Birm media is not recommended for sulfur2
Chlorine injection, then a filter	Above about 6 ppm6 mg/L, and works at any level2,8	About 2 mg/L of chlorine per 1 mg/L of hydrogen sulfide1,9; 20 minutes of contact2,4; a carbon filter after it removes leftover chlorine2

Our take: the middle range is where people get burned. If your test shows 3 mg/L, a filter rated “up to 3” is a gamble. Pick one rated well above your number, or go straight to chlorine injection.

Air-injection filter (iron, manganese, sulfur)

SpringWell WS1 / WS4 Well Water Filter

No NSF/ANSI listing found

Hydrogen sulfide

up to 8 ppm · 8 mg/L

Iron

up to 7 ppm

Manganese

up to 1 ppm

pH

6.5–10 (WS4 manual: 6.8–10)

Flow

WS1 12 gpm · WS4 18 gpm

Backwash

nightly, to a drain; needs power

Strengths

• Sulfur, iron and manganese in one tank, with no salt or chemicals
• Automatic nightly backwash

Drawbacks

• 8 ppm is the maker's figure; extension services give 2 to 6 mg/L for oxidizing filters
• Needs a drain and an outlet
• Not for sulfur bacteria in the well: shock chlorinate first

Good fit: a sulfur smell in all the water, at a measured level well under 8 ppm, with pH 6.5 or higher.
Skip it if: your test shows bacteria, a pH under 6.5, or a smell in the hot water only.

If you also have hard water, SpringWell sells the same filter paired with a softener; see our water softener guide.

Sulfur bacteria in the well

When bacteria in the well make the gas, shock chlorination is the usual first step, and Alberta Agriculture suggests trying to control the bacteria before buying equipment⁹. Expect it to be temporary. Penn State says that in most cases the bacteria and the smell return quickly after shock chlorination², and Virginia Tech says routine shocking may be required⁸. Once established, the bacteria can be hard to remove and may need a licensed well contractor⁶. See our shock chlorination guide for the steps.

As a last resort, a new well drilled into a different rock formation can avoid the gas⁶.

Safety

• Hydrogen sulfide can collect in well pits and basements. Only well professionals should enter a well pit or other enclosed space where the gas may be present⁶.
• In very rare cases, gas released from water into a closed space can build up to toxic or flammable levels⁸.
• Heating a tank to 160°F is a scald risk: warn everyone in the house and turn it back down^6,11.