Water that smells like rotten eggs is carrying hydrogen sulfide gas, and the smell tells you almost nothing about where the gas came from until you narrow it down. The three usual sources are sulfur in the aquifer itself, sulfate reducing bacteria growing in the well or the plumbing, and a reaction inside the water heater. Each has a different fix, so diagnosis comes before equipment.
Hot only, cold only, or both
This single test does most of the work. If the smell appears only on hot water, the water heater is the source and the magnesium anode rod is usually reacting with sulfate in the tank. If both hot and cold smell everywhere in the house, the gas is arriving with the water and the well or the source is responsible. If one fixture smells and the rest do not, suspect that fixture, its aerator, or a rarely used branch of pipe where water sits.
When the water heater is the culprit
A standard magnesium anode rod protects the tank by corroding sacrificially, and in some water chemistries that reaction feeds bacteria that release hydrogen sulfide. Replacing the rod with an aluminum or zinc alloy version usually stops it, and flushing and disinfecting the tank clears the existing population. Do not remove the rod entirely to solve a smell, because the tank then corrodes instead of the rod and you trade an odor for a leak.
When the well is the source
Sulfur bearing groundwater and sulfate reducing bacteria both produce the smell at the tap, cold and hot alike. Shock chlorination will knock back a bacterial population and tells you a great deal: if the smell disappears for weeks and then returns, you have bacteria recolonizing, and if it never really goes, the gas is dissolved in the source water and needs continuous treatment. Private wells are not federally regulated under the Safe Drinking Water Act, so nobody is testing yours unless you arrange it.
Continuous treatment for dissolved sulfide
For a well that genuinely carries hydrogen sulfide, the standard approach is oxidation followed by filtration: air injection or a chemical oxidant converts the dissolved gas to elemental sulfur, and a media bed then filters the solid out and backwashes it to drain. Catalytic carbon handles lighter cases on its own. A plain softener does not remove sulfide, and running sulfide through one can foul the resin, so treat the odor upstream of the softener rather than expecting the softener to cope.
Do not skip the test just because you can smell it
The nose detects hydrogen sulfide at very low concentrations, which is useful for noticing a problem and useless for sizing equipment. A lab test tells you the sulfide level, the iron and manganese that usually travel with it, the pH that determines which oxidizer works, and whether coliform bacteria are present. Our well water testing guide covers which panel to order. Sizing an oxidation and filtration system without those numbers is guesswork that you pay for twice.
Questions people ask about water smells like rotten eggs
Is water that smells like rotten eggs dangerous to drink?
Hydrogen sulfide at household levels is mainly a nuisance for taste and odor, but it often signals bacterial activity or well problems worth investigating, so test for coliform bacteria rather than assuming the smell is only cosmetic.
Why does the smell come back after shock chlorination?
Because the bacteria recolonize from the aquifer or from a section of the system the chlorine did not reach. Repeat returns mean you need continuous treatment rather than another round of disinfection.
Will a carbon filter fix the smell?
Standard carbon handles only light odor and exhausts quickly. Catalytic carbon does better, but persistent or strong sulfide needs oxidation ahead of the filter so the gas is converted to a particle the media can catch.
Can the smell come from the drain rather than the water?
Yes, and it is worth ruling out. Fill a glass, step away from the sink, and smell it there. If the glass is clean and the sink stinks, the problem is biofilm in the drain, not your supply.