COLUMBIA, Mo. - When large amounts of rain fall over a short period of time, private wells may be flooded by rising rivers or runoff. "Wells could be contaminated with bacteria, viruses or parasites that can make you ill," said Bob Broz, an MU Extension water quality specialist.

Water from a flooded well should not be used for drinking or food preparation until the well and plumbing system have been disinfected and the water has been tested for safety.

In addition to dangerous pathogens, floodwater can carry abrasive sediment, debris and other contaminants that can damage well equipment, Broz said.

If you must use the water, check with your local health department for recommendations on how long to boil water before using. Consider using alternative water sources such as bottled water.

If your well does have run-in water, you should take steps to ensure the safety of the water and minimize damage to the well.

Turn off the electricity to the pump and inspect the well and pumping system for run-in and signs of damage. If the well cap is missing or is not watertight, debris or sediment may have entered the well. Starting the pump under such conditions could damage the pump. If necessary, have a certified well installer look at the well and have an electrician examine the wiring and power unit for the well.

"Choosing not to check and clean flooded wells can do damage to the equipment and could lead to health concerns," said Broz.

You can disinfect contaminated well water through a process called shock chlorination:

1. With the electricity off, remove as much sediment from around the well casing as possible. Clean the well cap and the outside of the casing with a solution of 1 ounce of laundry bleach in 2 gallons of clean water. Rinse with clean water and make sure that the casing and pumping system are completely dry before continuing.

2. Once everything is dry, turn on the electricity to the well pump. If the pump works, open an outside faucet and run water onto the ground for 15 to 60 minutes or until the water runs clear. Check each faucet in the home until it runs clear. Close all faucets and turn off the electricity to the pump.

3. Disconnect any household water filters or water softeners and drain the water heater. If you have a gas water heater, put out the pilot light. If your water heater is electric, turn off the power to the heater. Open the well by removing the well cap or the threaded plug in the cap.

4. Prepare a solution of household bleach and water. If your well is 3 to 4 inches in diameter, mix 2 quarts bleach in 10 gallons of clean water. For a well 5 to 6 inches in diameter, mix 1 gallon of bleach with 10 gallons of clean water. Be sure to use eye protection and rubber gloves when mixing.

5. Pour the diluted bleach solution into the well. Avoid pouring directly onto the pump wiring if possible. After turning on the electricity, circulate the solution in the well either by placing a garden hose into the top of the well and running the water for 15 minutes or by starting and stopping the pump several times.

6. Open every water outlet on the system one at a time. Run the water until you can smell the chlorine, then close the faucet. Flush the toilets, refill the water heater and allow the chlorine solution to remain in the system for at least eight hours.

7. After eight hours, run the chlorinated water from the system and have the water tested for bacterial safety. Obtain a water test kit from your county health department. You should continue to use an alternative water source or boil your water for a minimum of one minute until the laboratory reports that the water is safe.

8. Once you receive a safe test result, the water can be consumed. You should have the well tested again in about two weeks to make sure that the disinfection has been completely effective. If the water still contains dangerous amounts of bacteria, repeat the shock chlorination process.

For more information on shock chlorination, contact your county health department or your local University of Missouri Extension office. Bob Broz can be contacted at 573-882-0085.