A sulfur smell in well water is almost always hydrogen sulfide gas, and it smells like rotten eggs because that is exactly what the gas is. It comes either from the aquifer itself, from sulfate-reducing bacteria living in the well or plumbing, or from a reaction inside the water heater, and the three have different fixes.
Find out where the smell starts
Run the cold tap at an outside spigot or the closest fixture to the well and smell it. If cold water at that point smells, the gas is coming in with the water. If only the hot side smells, the culprit is usually the magnesium sacrificial anode in the water heater reacting with sulfate in the water; swapping it for an aluminum or powered anode normally ends it. If the smell appears only after the water has stood in the pipes overnight, suspect bacteria colonizing the plumbing or the well casing itself.
The treatment depends on the concentration
Low levels of hydrogen sulfide can be handled by a backwashing carbon or catalytic carbon filter, which adsorbs the gas and periodically rinses itself. Higher levels need oxidation first: an air injection or oxidizing media filter converts the dissolved gas to elemental sulfur particles, which the same filter bed then catches and flushes to drain. Persistent, heavy sulfur, especially where iron and manganese come with it, is normally handled by chemical injection followed by a contact tank and a filter.
Bacteria need shocking, not just filtering
If sulfate-reducing bacteria are living in the well, a filter treats the symptom while the source keeps producing. Shock chlorination of the well and the plumbing kills the colony, and the smell often disappears for months before returning if the conditions that fed it remain. A well that regrows the problem quickly usually needs continuous disinfection, either chlorine injection or ultraviolet with proper pretreatment, rather than repeated shocking.
Test before choosing equipment
The EPA is clear that private wells are not federally regulated under the Safe Drinking Water Act, so no utility is sampling your water and no annual report is coming. A USGS study of 2,100 private wells found about one in five carried at least one contaminant above a human-health benchmark. A sulfur smell often travels with iron, manganese and bacteria, and the right system depends on which of those are present and at what strength, so get a well panel run before you buy anything.
Questions people ask about sulfur smell in well water
Is a sulfur smell in well water dangerous to drink?
Hydrogen sulfide at household levels is mainly a taste, odor and corrosion problem rather than a toxic one, but the same conditions often carry bacteria, so test rather than assume.
Why does only my hot water smell of rotten eggs?
That points to the water heater's magnesium anode rod reacting with sulfate. Replacing it with an aluminum or powered anode usually clears the smell for good.
Will a water softener remove the sulfur smell?
No. A softener exchanges hardness minerals and does nothing for dissolved gas. You need carbon, oxidation and filtration, or chemical injection, depending on strength.