Coliform bacteria are indicators of potential contamination of a well water supply and may originate from human, other animal, or soil sources. If coliform bacteria are present, drinking the water may not necessarily result in illness, but that possibility exists. Common causes of coliform bacteria contamination in private water wells include improper well surface seal and well maintenance without disinfection. Proper sealing of the well and disinfection should be performed to ensure a safe water supply and to verify that there are no other sources of contamination that need investigation.

Well Chlorination

1. First, check to see that the well is tightly sealed to prevent the entrance of any surface contamination, either solid or liquid, to the water supply. Vents should be screened, opening down, and above flooding. If the well is not sealed, take measures to have it sealed properly, but allow capped ports for chlorine to be added to the well now and in the future.
2. If applicable, notify tenants in advance that you are chlorinating.
3. Determine the amount of household laundry bleach (5.25% sodium hypochlorite) to be added to obtain 100 parts per million available chlorine in the well. for 6-inch diameter well casing, use 4.5 cups of bleach for every 100 feet depth of water in the well; for 8-inch casing, use 2 quarts of bleach / 100 feet; for 10-inch casing, use 3 quarts of bleach / 100 feet; for 12-inch casing, use 4.5 quarts of bleach / 100 feet.
4. Mix the bleach in approximately 10 gallons of water and pour the mixture into the top of the well.
5. Open the first faucet beyond the water storage tank and start the pump. When chlorine smell is detected from the faucet, connect a hose to the faucet and spray the chlorinated water back into the well for about 5 minutes, trying to have chlorine touch all the casing and piping surfaces inside the well.
6. Open every faucet / outlet in the system (outside first) and let the water run. Limit the amount of chlorine reaching a septic system. Close each outlet when chlorine is smelled from it and seal top of well. Allow to stand 12 to 24 hours. Do not drink or bathe in this strongly chlorinated water.
7. Starting with the outside faucets, open each water outlet and flush until the chlorine is gone. It is best to use a swimming-pool chlorine test kit after the smell is gone. We recommend using a bucket to collect inside chlorinated water to avoid running too much chlorine into the septic tank. Flushing can take many hours or even days. Monitor the use of your well pump during flushing to avoid damage.
8. At least 5 days after the water is free of chlorine, resample the water for coliform bacteria testing. For best results, keep the sample cool and get it to a laboratory within a couple of hours. When a well is badly contaminated, the above procedure may have to be repeated several times. If the well casing is deteriorated or the well is not properly sealed, disinfection will not prevent further contamination. Testing of private water samples may be done at the Sutter County Public Health Laboratory, 1445 Circle Drive, Yuba City, 822-7225.