Why does my water smell like rotten eggs?

A rotten egg smell in water is usually caused by hydrogen sulfide gas from bacteria in your water heater or well water. For well water, the issue may be in the groundwater itself. For city water, the problem is often in the water heater anode rod, which can be replaced. Flushing the water heater and replacing the anode rod typically solves the problem. If the smell is in both hot and cold water, the source is likely in the supply.