A rotten egg smell in water is easy to notice and harder to explain. The smell usually comes from hydrogen sulfide, a gas that can form in well water, in a water heater, or in pipes where bacteria act on sulfur compounds. Where the smell shows up tells you where to start looking.
Check hot and cold separately
Run the cold tap in the kitchen for a minute and smell the water in a clean glass. Then run the hot tap and do the same. If only the hot water smells, the water heater is the most likely source, and the anode rod inside the tank is a common cause. If both hot and cold smell, the source is more likely upstream, such as the well, the pressure tank, or the supply line. If only one tap smells and the others do not, look at that fixture and its drain.
Notice when the smell appears
Timing matters. A smell that shows up after the house has been empty for a few days, or first thing in the morning, suggests water that has sat in the system. A smell that comes and goes with heavy use may point to the well or to a change in the source. Keep a simple log with the tap, the time, the water temperature, and whether the house had been quiet.
Look for other signs
Discoloration, black or slimy deposits, and a film in the toilet tank can point to bacteria or minerals. Those are separate problems with separate tests, so note them alongside the smell rather than assuming one cause explains everything.
Test before you buy equipment
A smell tells you there may be a problem, not what the problem is. Ask a certified lab to test for hydrogen sulfide and bacteria, and ask exactly what else the test covers. If someone in the home has a health concern, raise it with a healthcare provider. The lab result, not the smell alone, should decide whether you buy a treatment unit, service the water heater, or treat the well. Ask the lab to explain the results in plain language and keep the report with your other household papers.