Well Disinfection (Chlorination)

If your well water has tested positive for coliform bacteria, you may want to disinfect (chlorinate) your well. Please read and follow these instructions carefully.


  • 1 gallon regular unscented liquid bleach or
  • Clean hose that extends from an outside spigot to the well
  • Wrench to remove well cap
  • 5-gallon bucket
  • Small cup
  • Water sample bottle(s)
  • Location of ON/OFF switch for well pump
  • Eye and skin protection


  1. Plan ahead for bacteria testing after you have finished disinfecting the system. Pick up a coliform bacteria water sample bottle from our office, located at 705 N. Zeeb Road, Ann Arbor, MI 48103. Note: It is best to collect the sample just before bringing it to our office. Samples must be delivered to our office before 3:00 p.m., Monday through Friday.
  2. Draw off about 4 gallons of water into a 5-gallon bucket. Mix 1 gallon of regular unscented liquid bleach with the 4 gallons of water. Note: Liquid bleach should not be used with a steel well casing; it can break loose accumulated corrosion and cause pump failure. Instead, use granular chlorine in a steel well casing.
  3. Turn off the power to your well pump. Remove the well cap. Be careful if set screws or bolts on your well cap are rusty. Note: If the well cap has one large bolt in the center of the cap, you should not attempt to remove it. Have a licensed well contractor disinfect your well. In addition, if your well head or top is buried, or if you have a shallow well installation, your well does not meet current well construction code requirements. Chlorinating these wells is difficult and in many cases impossible. If you have an unsafe water sample and you have a well that fits this description, contact a Washtenaw County Sanitarian at 734-222-3800.
  4. Pour the bleach water mixture into the well between the casing and the cross bar or "T" bar. Newer wells will often have markings or a statement next to the correct hole, indicating where to pour the chlorinating solution. Do not pour the mixture into the 1-inch center hole. Avoid contact of the solution with the wire connections inside the well, as they could corrode.
  5. Connect a clean hose to an outside spigot and extend it into the well approximately 4 feet.
  6. Turn the power supply back on to the well pump. Turn on the outside spigot that is connected to the hose in the well. Allow the hose to run in the well for 20 minutes. This distributes the disinfectant throughout the well system. Turn the spigot off when done.
  7. Turn off the power supply to the well pump. Once the power is off, remove the hose from the well and put the well cap back onto the well.
  8. Turn on the power to the well pump. All work is now completed on the wellhead.
  9. Now it's time to distribute the bleach water solution to the pipes in the home. Turn on each indoor and outdoor water faucet and allow cold water to run until a bleach odor is detected. Be sure to run the shower, clothes washer, dishwasher, and any outside hydrants or plumbing fixtures in other buildings. Also, flush each toilet a couple of times. Once you detect a bleach odor, turn off all faucets.
    Note: Some water softening units should not be chlorinated. Contact your water softening company before disinfection to see if you should bypass the softening system.
  10. Allow the solution to remain in the system for a minimum of eight hours, or overnight. During this time you should not drink, bathe, wash clothes, or cook with the water, but you can use it for toilet flushing.
  11. After the solution has been in the system for a minimum of eight hours, it is time to flush the system. Connect a hose to an outside spigot and allow water to run onto the ground for 20 to 30 minutes to remove the bleach water solution from the system. Try to keep the water away from your drainfield and any plants or trees. After 20 to 30 minutes, turn off the spigot and then run each indoor and outdoor water faucet for 2 to 3 minutes to remove the bleach water solution that was in the pipes.
  12. Now it's time to collect the bacteria water sample. Wash your hands with warm water and soap before collecting the sample. Do not open the bottle until you are ready to collect the sample. Choose a clean, indoor faucet (usually a kitchen faucet). Do not collect the sample from hose lines, from leaking/dirty faucets, or from reverse osmosis treated faucets. Turn on the COLD water and let it run at full stream for 5 minutes. Turn down the faucet to a small stream of the cold water. Open the bottle. Do not set the cap down. Do not touch the inside of the cap or bottle. Do not rinse out the bottle. Do not dump out any powder or pill that is inside the bottle, as this helps preserve the sample. Collect the water sample directly into the sample bottle. Fill to the neck of the bottle. Do not overflow. Do not allow the cap or bottle to touch the faucet. Replace the cap securely on the bottle. Place the bottle in a cooler with an ice pack during transport. Fill out the water sample form completely. Attach the form to the bottle with a rubber band.
  13. Samples that are going to be analyzed by our laboratory must be delivered to our office by 3:00 p.m. Monday through Friday. Bring the bottle to our office at 705 N. Zeeb Road, Ann Arbor, MI 48103 and place the sample bottle with the attached form in the small refrigerator in our lobby. On dates prior to holidays, no samples will be accepted.
  14. If well disinfection and resampling was required as part of a Time of Sale inspection, the follow-up sample (after disinfection) must be obtained by either a registered well driller or a Washtenaw County Certified Time of Sale Inspector (PDF).
  15. If you have any questions about the well disinfection procedure, please contact a registered well driller. You can also contact the Washtenaw County sanitarian assigned to your township area


This procedure is intended to be used for chlorinating 5-inch PVC wells with submersible well pumps. If your well has a jet pump (either shallow or deep), or if your casing is 4-inch steel or smaller, we advise that you consult with a licensed well driller prior to chlorination. These instructions are provided as public information based on conditions found in Washtenaw County. There is no implied guarantee, and the procedure may need to be repeated. Washtenaw County accepts no responsibility for the outcome, or for any damage incurred.