Recommendations for Private Wells Inundated by Flooding

Disinfection procedures and other guidance if a private well has been contaminated by floodwater


If you live in an area that was recently or is currently flooded, your private well may be in danger of contamination from pollutants carried in floodwaters. If your well cap is or has been underwater, you should assume that your well is contaminated.

Boiling Your Water

If you suspect your drinking water is contaminated, you should obtain water from a known safe source for drinking, cooking and food preparation. You can get water from a neighbor's well you know is safe, from a community water supply or you can purchase bottled water. If you cannot find a convenient source of safe water, you can boil your well water for five minutes before use.

If you have any questions about your well or water quality, please call your local Board of Health or the MassDEP Drinking Water Program at 617-292-5770 during business hours. Use the following procedure to disinfect your well.

Disinfecting a Well

  1. Once the water has subsided below the cap of the well, remove the well cap. Mix the appropriate amount of unscented chlorine bleach (Clorox, Dazzle, or other EPA/NSF approved bleach) from the table below with 5 gallons of water and pour into the well. Make sure the well casing walls are wetted completely with the solution.
    Drilled Well
    (Please note: a greater amount of chlorine may be needed to disinfect the water depending on the degree of contamination)
    Depth Dosage
    50 feet 2  1/2 cups
    100 feet 1  1/2 quarts
    150 feet 2 quarts
    200 feet 2  1/2 quarts
    250 feet 3 quarts
    300 feet 3  1/2 quarts


    Dug Well
    (Approximate amount of water in the bottom of a well and not the total depth of the well.  For a surface SPRING, use 2 gallons)
    Depth Dosage (gal.)
    5 feet 1/2 gallon
    10 feet 1 gallon
    15 feet 1  1/2 gallons
    20 feet 2 gallons
  2. Replace the well cap. In order to distribute the disinfectant, the well should be pumped, re-circulating the water back to the well, for at least 15 minutes.
  3. Open all faucets, sill cocks and similar outlets individually until you smell chlorine in each outlet.
  4. Allow the mixture to stand in the system a minimum of 2 hours, preferably overnight, then flush the chlorine mixture from the system using an outside faucet and garden hose. DO NOT FLUSH THE MIXTURE INTO YOUR SEPTIC SYSTEM. You may resume using the toilet facilities as the septic system is designed to handle this, but the septic system cannot handle the large amount of water needed to flush the chlorine from the well. Since the chlorine will kill grass, be careful where you run the water outside.
  5. After disinfection, the water supply should be sampled and tested for coliform bacteria by a MassDEP certified laboratory. To find a certified lab, see Find a Certified Laboratory for Water Testing, or call the MassDEP Wall Experiment Station at 978-682-5237. Sampling should be done 24 hours after the odor of chlorine disappears. It takes about 3 or 4 days of normal water usage before all of the chlorine smell disappears.

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