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Will's Windmill column: Why water in private wells can smell bad

If you have a private well and the water smells musty or like "rotten eggs," you may have a bacteria problem. The bacteria that cause this smell live in the soil or aquifer. They do not pose a health risk. However, once they are introduced to a well, they may multiply rapidly and cause odor problems.

Sometimes odor-producing bacteria are indicators that disease-causing bacteria are also present. This is especially true if there has been a sudden change in your water quality.

You should have your well water tested through a certified lab to make sure there is not a health problem. The tests that are typically run to identify potential health problems may not detect odor-causing bacteria. The lab will let you know whether water is safe to drink. If the water is safe but smells bad, there are steps you can take to reduce or eliminate the problem.

Odor-producing bacteria are often referred to as "iron'" or "sulfur-reducing" bacteria. That is, they use iron or sulfur in their life cycle and give off hydrogen sulfide gas. That's the rotten egg odor you smell. The bacteria may form slimy colonies in pipes or toilet tanks and can stain laundry. However, the odor is usually the most objectionable problem.

These iron bacteria can get into your well when you have maintenance on piping, well casing, or your pump. They may even be introduced when you have work done on indoor plumbing, or when a new hot water heater is installed. Finally, they may get into the water supply through a direct connection to surface water or shallow groundwater seepage.

Once these bacteria get established in your well or water supply, they can be very hard to eliminate. After work is done on your well or plumbing, the system should be thoroughly disinfected. This kills any bacteria that enter the water before they get a foothold. If there is a connection to surface water or the 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 private well. Although not difficult, you need to calculate carefully how much chlorine is appropriate for your well size and depth. Make sure the entire system gets disinfected to protect your septic system. For directions on how to disinfect your well, contact your Extension Office and request publication no. 5941, "Treatment Systems for Household Water Supplies: Chlorination."

In some cases, a rotten egg odor comes from the rock or soil the well is drilled into. In these cases, the smell is generally present when the well is first used. Odor is from bacterial growth and it may appear suddenly. Usual times for this odor to occur are 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. For more information on water testing or chlorination please contact me: Will Yliniemi, Hubbard/Becker County Extension educator at 1-218-732-3391 or 1-218-846-7328, by cell at 1-218-252-1042, or by e-mail at