Well water that smells like rotten eggs is hydrogen sulfide gas. The first job is not buying a filter: it is working out whether the smell comes from the well itself, from bacteria in the water heater, or from a drain, because each has a completely different fix and two of them are cheap.
Locate the smell before you spend anything
Run the cold tap at an outside spigot or the closest fixture to where the water enters the house, fill a glass and smell it away from the sink. If cold water smells at every tap including the one nearest the pressure tank, the source is the well or the aquifer. If only hot water smells, the culprit is almost always the magnesium anode rod in the water heater reacting with sulfate-reducing bacteria, and replacing that rod with an aluminum or powered one usually ends the problem. If the smell only appears at one sink and the glass of water smells clean, the drain trap is your answer.
Where the gas comes from
Hydrogen sulfide forms where sulfate-reducing bacteria live without oxygen, either in the aquifer, in the well itself, or in the plumbing. Deep wells in shale or sandstone and wells with a lot of organic matter in the formation are the common geologies. Because private wells are not regulated under the federal Safe Drinking Water Act, no one is monitoring this and the smell is usually the first anyone hears of it. It is worth testing at the same time for iron, manganese and bacteria, since sulfide problems commonly travel with iron and iron bacteria, and the treatment choice depends on which of them are present.
Treatment that works, in order of severity
For a light odor, an air injection or catalytic carbon filter at the point of entry oxidizes and traps the sulfide with no chemicals to buy, and this is by far the most common household solution. For stronger odor, or where iron and manganese are present alongside, chemical injection using chlorine or hydrogen peroxide ahead of a contact tank and filter is the reliable route. Where iron bacteria are also present, a one-time shock chlorination of the well and plumbing is needed as well, because filtration downstream will not touch a colony living in the casing. Under-sink reverse osmosis fixes only the drinking tap.
What will not fix it
A plain sediment cartridge does nothing: hydrogen sulfide is a dissolved gas, not a particle. A basic granular carbon cartridge will strip a faint smell for a short while and then break through, which is why households replace them constantly and conclude filters do not work. A water softener does not remove sulfide either, and running sulfide-bearing water through a softener can foul the resin. Bleach poured in the well kills what is in the well today and does nothing about the aquifer feeding it. Get iron, manganese, sulfide, pH and bacteria measured first; our well water testing guide covers the sample.
Questions people ask about well water smells like rotten eggs how to fix
Why does only my hot water smell of rotten eggs?
That points at the water heater, specifically the magnesium anode rod reacting with sulfate-reducing bacteria in the tank. Swapping to an aluminum or powered anode usually ends it.
Is hydrogen sulfide in well water dangerous?
At household concentrations the concern is taste, odor and corrosion of metal fittings rather than acute toxicity, but it makes water unpleasant to drink and is a sign worth investigating.
Will a carbon filter fix the smell?
Catalytic carbon in a properly sized point-of-entry tank handles light odor well. A small granular carbon cartridge under the sink will break through quickly on a real sulfide problem.
Does shock chlorinating the well solve it permanently?
It clears bacteria living in the well and plumbing, which helps if that is the source. It does nothing if the sulfide is coming from the aquifer, and the smell returns within weeks.
Can a softener remove the sulfur smell?
No. A softener exchanges hardness ions and leaves dissolved gas untouched, and sulfide-bearing water can foul the resin, so treatment should sit ahead of the softener.