The "rotten egg" odor in water may be caused by the presence of sulfate reducing bacteria in distribution lines or water heaters or by the presence of dissolved hydrogen sulfide gas. In addition to an odor, hydrogen sulfide gas can cause black stains on plumbing fixtures and tarnish silverware. EPA has not set a standard for hydrogen sulfide in drinking water.
Additionally, a "rotten egg" odor associated with hot water may be due to magnesium rods in hot water heaters. The rod is a component of the water heater and the odor may be eliminated by removing the rod (Manual of Small Public Water Supply Systems, EPA570-9-91-003; May 1991).
To help determine the cause(s) of aesthetic or cosmetic effects from your drinking water, contact your local drinking water system. Additional guidance for household well owners is available at http://water.epa.gov/drink/info/well/index.cfm. General information on nuisance chemicals is available at http://water.epa.gov/drink/contaminants/secondarystandards.cfm .