Well water that smells like rotten eggs contains hydrogen sulfide gas (H2S). It is not a health hazard, but it can corrode plumbing metals, tarnish copper and silverware, stain laundry, affect how well water softeners work and change the appearance and taste of coffee, tea, and cooked foods. Before you can decide on the right treatment option, you need to know the source of the gas.
H2S can naturally occur in groundwater when vegetation or other organic matter decays or because of chemical reactions with sulfur-containing minerals. Sulfate-reducing bacteria can change sulfate that dissolves in groundwater into hydrogen sulfide gas. H2S may also appear in shallow wells located too close to septic systems or sewer lines. Water heaters are another source of H2S, either when the magnesium rod in the water heater reacts with sulfate in the water and converts it to hydrogen sulfide gas or when the temperature is lowered to save energy, allowing sulfate-reducing bacteria to live in the heater.
If you only smell rotten eggs when the hot water is on, the problem is probably coming from the hot water heater. If there's a strong smell when both the hot and cold water are first turned on but it goes away or lessens with time, you probably have sulfate-reducing bacteria in your well. Another indication of bacteria in your well is an odor that varies through time. An odor that is constant and doesn't diminish means there's H2S in the groundwater.
Test your water for H2S levels so you can decide on the best type of treatment. If H2S is caused by the magnesium rod in your water heater, you can replace it with an aluminum rod. If it's caused by sulfate-reducing bacteria in the water heater, you can kill the bacteria by raising the temperature of the water to 140 degrees or more. First, make sure the water heater has an operable pressure relief valve.
If you have sulfate-reducing bacteria in your well or water distribution system, you'll need to flush them with a strong chlorine solution. You may also need to scrub the well casing, agitate the water before you chlorinate it or use special treatment chemicals because the bacteria can be hard to remove.
If H2S gas is present in the groundwater, the only way to get rid of the problem is by connecting to another water source. This might mean drilling a new well into a different formation that is free of gas. Other options include installing an activated carbon filter or an oxidizing filter, using an aeration system or treating your water with constant chlorination. Which treatment you use will depend on how much H2S is present.
H2S gas can be hazardous at high levels. Gas should be removed from the water or vented to the atmosphere so it doesn't collect inside the house, basement or in well pits. In addition, even though sulfate-reducing bacteria aren't harmful, they produce a slime that can encourage other bacteria to grow.