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Rotten-Egg Smell in Well Water: Causes and Fixes
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Step 1: Hot only, or hot and cold?
- Hot water only: The water heater may be the source. Penn State Extension says its magnesium anode rod can turn sulfates in the water into hydrogen sulfide. The fix it describes is replacing the magnesium rod with an aluminum rod. It warns that removing the rod entirely can increase corrosion and may void the warranty. Have a licensed plumber do the work.
- Hot and cold: The gas is in the water coming from the well. Go on to step 2.
Step 2: What causes it in the well
Penn State Extension says sulfur-reducing bacteria in groundwater produce the gas. They thrive in low-oxygen conditions in wells and plumbing, and the problem is most common in wells drilled into acidic bedrock such as shale and sandstone. Its page also notes the gas can corrode plumbing metals and leave yellow or black greasy stains on fixtures and inside pipes.
Step 3: Test
Penn State Extension says test results help you choose a treatment device. Ask a state-accredited commercial lab whether it tests for hydrogen sulfide. The same page says to test in the home or chemically stabilize the sample before sending it to a lab, so ask the lab how to collect it. Add a total coliform test while you are at it. See the testing guide.
Step 4: Match the treatment to the level
| Treatment | Works for (Penn State Extension) | Notes |
|---|---|---|
| Carbon filter | Below about 1.0 mg/L | Needs a large unit (1.0 to 1.5 cubic feet) and periodic carbon replacement. |
| Catalytic carbon | Higher levels than standard carbon | Needs at least 4.0 mg/L dissolved oxygen in the source water. |
| Oxidizing filter (manganese greensand) | Up to 2 to 3 mg/L | Higher levels need more frequent regeneration and backwashing. Birm media is not recommended. |
| Aeration | Not normally above about 2 mg/L | No chemicals, but may form sulfur particles that need filtering. Disinfection is recommended. |
| Continuous chlorination plus filter | Most often above 6.0 mg/L | At least 20 minutes of contact time. Needs significant maintenance. |
| Continuous potassium permanganate plus filter | Above 6.0 mg/L | At least 15 minutes of contact time. The chemical is an irritant and poison. |
| Ion exchange | Works like a softener | Needs salt regeneration. May need pretreatment for solids and iron. |
| Shock chlorination | Kills bacteria in the well | Odor usually returns quickly. |
Source: Penn State Extension, Hydrogen Sulfide (Rotten Egg Odor) in Water Wells.
Penn State Extension also lists changes to the supply: connect to a nearby municipal system, drill a shallower new well, or develop a spring or rainwater cistern. Each has its own water quality or quantity trade-offs.
If it smells like sewage or metal instead
- Sewage smell: Wisconsin DNR says sewage contains bacteria and viruses that are a hazard to your health, so do not use the water for drinking, bathing or cooking until it has been tested for coliform bacteria and confirmed safe. It suggests sending a sample to a lab certified for coliform, and asking for a septic system inspection. It also asks: was there a sudden change after heavy rain, is the casing shallow or thin-walled, is the well close to a sewer pipe or leaking sump? Iron bacteria are listed as another possible cause and are a nuisance, not a health threat. See the bacteria guide.
- Metal: Penn State Extension says iron and manganese give a strong metallic taste and cause orange, brown or black stains. See the iron guide. We did not find a source on a metallic smell, so we only cover the taste.
Not sure what's wrong with your water?
Our free checker matches what you see, smell or taste to the likely cause, the test to ask for and the type of treatment that fits.
Open the well water checkerNeed a system for hydrogen sulfide?
Treatment should match a lab report, not a guess. Once you know what is in your water, a licensed water treatment professional can size a system for it. You can also look at whole-house systems sold online.
How well water systems workSee whole-house system optionsCommon questions
Is rotten-egg smell in well water dangerous?
Penn State Extension says the sulfur bacteria that cause it do not cause human health concerns, but the gas itself is poisonous and flammable. Odor usually warns people first, and most people can smell hydrogen sulfide well below 0.5 mg/L. There is no drinking water standard for it because water becomes unpleasant long before reaching harmful levels.
Why does only my hot water smell like sulfur?
If the smell is only in hot water, the water heater may be the source. Penn State Extension says the heater's magnesium anode rod can convert sulfates into hydrogen sulfide.
Will shock chlorination fix the smell?
Penn State Extension says shock chlorination kills bacteria in the well, but the odor usually returns quickly, and continuous treatment may be needed.
Can a water softener cause the smell?
Rarely. Penn State Extension says a softener can create conditions that let the bacteria grow.
Why does my well water smell like rotten eggs?
Usually hydrogen sulfide gas. Penn State Extension says sulfur-reducing bacteria in groundwater, wells and plumbing produce it, and they thrive where oxygen is low.
Why does my hot water smell like rotten eggs but not the cold?
Penn State Extension says hydrogen sulfide is sometimes noticeable only in the hot water, and chemical reactions in the water heater may be the source. Its magnesium rod can chemically reduce sulfates to form hydrogen sulfide, and replacing it with an aluminum rod should eliminate the odor. Removing the rod entirely could increase corrosion and likely void the warranty.
Why does my well water smell like sewage?
Wisconsin DNR lists sewage as a possible cause, which carries a health concern because sewage contains bacteria and viruses. It says not to use the water for drinking, bathing or cooking until it has been tested for coliform bacteria and confirmed safe, and to ask about a septic inspection. Iron bacteria can also cause a sewage-like smell, and the DNR says they are a nuisance but not a health threat.
Why does my well water smell or taste like metal?
Penn State Extension says iron and manganese impart a strong metallic taste, and iron can leave orange or brown stains. Its page describes taste and stains, not smell. A lab test can tell you which is in your water.