The rotten egg smell is hydrogen sulfide, and it reaches your tap by one of three routes that need completely different responses. Two checks, taking about five minutes, will tell you which one you have, and doing them first is what separates a targeted repair from an expensive system bought for the wrong problem.
Check one: hot side, cold side, or both
Run the cold tap and smell it, then the hot, at two different fixtures well apart in the house. Smell on the hot side only means the water heater is producing the gas, where sulfate reducing bacteria interact with the magnesium sacrificial anode rod. That is an appliance problem and the water arriving at the house is fine. Smell on both sides means the supply itself is carrying it. Smell at one fixture only usually means something local to that tap or its drain rather than the water at all.
Check two: gritty or slimy
Look inside the toilet cistern and at the fixtures. A dark, slippery film that reforms within days of cleaning points at sulfur bacteria living in the well or the plumbing, which produce the gas biologically and regrow from whatever a filter leaves behind. Clear water with a smell and no film points at hydrogen sulfide dissolved in the groundwater itself, which is chemistry rather than biology. The distinction decides whether disinfection is part of the answer or the whole of it.
What each cause needs
Heater related smell is fixed at the appliance: a disinfection cycle at the temperature the manufacturer permits, a tank flush, and if it returns, an aluminium zinc alloy anode in place of the magnesium one. Removing the anode is not the answer, since the tank then corrodes. Bacteria in the well need shock chlorination and often continuous chlorination afterwards. Dissolved sulfide needs oxidation and filtration, or aeration, sized to the load.
Why the smell is not a good measure
Hydrogen sulfide is detectable by nose at concentrations far below those that decide equipment size, so a strong smell does not necessarily mean a heavy load. The EPA notes private wells are not federally regulated under the Safe Drinking Water Act, so a laboratory test is both your own responsibility and the only sound basis for specifying treatment. Test for sulfide alongside iron, manganese and pH, since the treatment interacts with all three. Our sulfur filter guide covers sizing from that result.
Questions people ask about why does my well water smell like sulfur
Why does only the hot water smell?
Sulfate reducing bacteria in the water heater are producing hydrogen sulfide from the hydrogen released by the sacrificial anode. The supply is fine.
What does a slimy film in the cistern mean?
Sulfur bacteria in the well or plumbing, which regrow from whatever a filter leaves and therefore need disinfection as well as filtration.
Should I remove the anode rod to stop the smell?
No. It protects the tank from corroding. Replace magnesium with an aluminium zinc alloy rod instead.
Can I judge the severity by how strong the smell is?
No. Sulfide is detectable well below the levels that decide equipment size, so a laboratory test is what sizes treatment.