Rotten Egg Smell in Well Water: What Hydrogen Sulfide Is and How to Get Rid of It
A rotten egg odor from the tap is almost always hydrogen sulfide gas (H2S). It is common in private wells, it is unpleasant at very low concentrations, and it is frequently misdiagnosed. Before buying any equipment, you need to figure out where the smell is coming from.
Three possible sources
1. The well itself
Sulfate-reducing bacteria living in groundwater convert naturally occurring sulfates into hydrogen sulfide. This is the most common cause in areas with shale, sandstone, or organic-rich aquifers. The smell is present in both hot and cold water, at every tap.
2. The water heater
If the smell is only in hot water, the culprit is usually the magnesium anode rod inside the tank reacting with sulfate-reducing bacteria that colonize warm, stagnant water. Cold water smells fine. This is a heater problem, not a well problem.
3. The drain
If the smell appears only at one sink and does not come from the water itself, it is likely biofilm in the drain or overflow. Fill a glass, walk away from the sink, and smell it. If the glass is fine, the drain is the source.
Quick diagnosis
| Where you smell it | Likely source |
|---|---|
| Hot and cold, every tap | Well water (H2S in groundwater) |
| Hot water only | Water heater anode rod |
| One fixture only, not in a glass of water | Drain biofilm |
| Comes and goes, worse after the well sits unused | Bacteria in the well casing or pressure tank |
Is hydrogen sulfide dangerous?
At the concentrations found in well water, no. The human nose detects H2S at around 0.5 parts per million or lower, far below any health threshold. It is a nuisance contaminant: it corrodes copper and iron plumbing, tarnishes silverware, stains fixtures black, and ruins the taste of coffee and tea. The one real concern is that its presence often signals bacterial activity in the well, which is worth a coliform test.
How to treat it
Water heater smell
Flush the tank and replace the magnesium anode with an aluminum-zinc or powered anode. A plumber can do this in under an hour. No water treatment equipment needed.
Low levels in the well (under about 1 ppm)
Activated carbon adsorbs small amounts of H2S. A whole-house carbon stage, such as the CTO carbon block in the AquaVerde 3-Stage Stainless Steel system, will handle light odor, though the carbon will exhaust faster than it would on chlorine alone.
Moderate to high levels
Hydrogen sulfide needs to be oxidized into elemental sulfur and then filtered. Options include catalytic carbon, manganese dioxide media, air injection, or chlorine injection followed by carbon. This is the same class of treatment used for iron and manganese, and the three problems frequently occur together. A multi-media conditioning system such as the AquaVerde HYPER is designed to address that combination in one tank; for a well with heavy H2S, ask us to review your lab report to confirm the media is sized correctly for your load.
Bacteria in the well
Shock chlorination of the well and pressure tank can knock down sulfate-reducing bacteria, but it is usually temporary. A UV system prevents bacteria from reaching the house but does not remove the gas already formed, so it is paired with oxidation or carbon, not used alone.
What will not work
- A water softener. Ion exchange does not remove dissolved gas. Softener resin can even be fouled by H2S.
- Sediment filters. Nothing to catch.
- Reverse osmosis alone. RO membranes do not reliably remove dissolved gases, and H2S can degrade the membrane. Treat at the point of entry first.
Frequently asked questions
Why does the smell get worse after vacation? Stagnant water in the well and pressure tank gives sulfate-reducing bacteria time to work. Running the water clears it temporarily.
Can I just add bleach to the well? Shock chlorination helps if bacteria are the cause, but it does not fix sulfate-rich groundwater and must be done correctly to avoid damaging the pump. Follow your state health department's procedure or hire a well contractor.
Will the smell go away on its own? Rarely. Groundwater chemistry does not change quickly.
Should I test for anything else? Yes: iron, manganese, total coliform, and sulfate. They often travel together and determine which treatment fits.
Start with the AquaVerde Well Water Solutions overview, or send us your lab results and we will tell you which stage handles the odor.
