If you notice this smell in your water, it probably contains hydrogen sulfide (H2S) gas. Hydrogen sulfide gas can result from a number of different sources. It can occur naturally in groundwater. It can be produced by certain "sulfur bacteria" in the groundwater, in the well, or in the water distribution system. It can be produced also by sulfur bacteria or chemical reactions inside water heaters. In rare instances, it can result from pollution. The source of the gas is important when considering treatment options.