Why well water smells bad
Barb Liukkonen (Reviewed 2011)
If you have a private well and the water smells musty or like "rotten eggs," you may have a bacteria problem. Bacteria that cause this smell live in the soil or aquifer. They are not a health risk. However, once introduced to a well, they may multiply rapidly and cause odor problems.
Sometimes odor-producing bacteria indicate that disease-causing bacteria are also present. This is especially true if there has been a sudden change in water quality. Have your well water tested through a certified lab to make sure there is not a health problem. Tests usually identify potential health problems but may not detect odor-causing bacteria. The lab lets you know if water is safe to drink. If it is safe but smells bad, you can probably reduce or eliminate the odor.
Odor-producing bacteria are often referred to as "iron" or "sulfur-reducing" bacteria. They use iron or sulfur in their life cycle and give off hydrogen sulfide gas. That's the rotten egg odor. The bacteria may form slimy colonies in pipes or toilet tanks and can stain laundry. However, the odor is usually the most objectionable problem.
Iron bacteria can get into wells when maintenance is done on piping, well casing, or pumps. Bacteria can get in when work is done on indoor plumbing, or when a water heater is installed. They may be introduced if your well is flooded. Finally, they may get into water supplies through a direct connection to surface water or shallow groundwater seepage.
Once established in your well or water supply, bacteria can be very hard to eliminate. After work is done on your well or plumbing, thoroughly disinfect the system. This kills bacteria that enter the water before they get a foothold. If there is a connection to surface water or bacteria are strongly established, repeated disinfection may be necessary. Disinfection eliminates or reduces bacteria to tolerable levels.
Shock chlorination is the most common way to disinfect a well. You need to calculate carefully how much chlorine is appropriate for your well size and depth. Make sure the entire system gets disinfected and that chlorine remains in the system 6 to 8 hours. For directions on how to disinfect your well, see Treatment Systems for Household Water Supplies: Chlorination.
Sometimes rotten egg odor comes from rock the well is drilled into. In these cases, the smell is generally present when the well is first used. The smell may decrease over time. Odor that appears later is from bacterial growth and may appear suddenly. The odor may begin after the well has been unused for an extended period of time, following floods or drought, or after maintenance. Again, a sudden change in water quality may signal potential health risks. Test the water to make sure it's safe to drink. To get a water test kit, contact your county health department.