Why does my water smell like rotten eggs?

A rotten-egg odor usually indicates hydrogen sulfide gas or sulfur-related bacteria. The source may be the groundwater, well, plumbing, water heater, or another part of the water system.

Check whether the odor affects hot water, cold water, or both. That distinction helps narrow down the source, but water testing and an on-site inspection are the best ways to select the correct treatment.