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New Hampshire Department of Environmental Services

Frequently Asked Questions
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  • How do I disinfect my well?
    Place the following amounts of disinfectant into the well:
    1. For Dug wells or drilled wells less than 200 feet deep, add one gallon of liquid bleach for every thousand gallons of water in your well. The number of gallons in your well can be found by multiplying the diameter of the well (in feet) by itself, then multiplying this result by 5.9. This number is then multiplied by the depth of the water in the well (in feet). The result is the volume of water in your well in gallons.
    2. For Drilled wells deeper than 200 feet, use calcium hypochlorite tablets (Chlorine pool tablets). Place the tablets in a heavy bag and break them into marble size pieces with a hammer (be careful, this material is hazardous). Pour into your well, 2 ounces of calcium hypochlorite for every 100 feet of well depth. Let it sit for several hours then continue with step two. This amount assumes your well casing is 6 inches (as most are) and that you are using 70 percent chlorine tablets (most range between 65 and 95 percent available chlorine).
    Once the disinfectant is in the well run each house faucet until you smell chlorine and then turn off faucet.
    Then run a garden hose from an outside faucet to the well. Turn the faucet on and let the chlorinated water run down the sides of the well. Recirculate the water back to the well for an hour or so.
    Now take the hose out of the well, reinstall the cover and let the whole system sit at least overnight.
    Over the next few days, flush the well by running an outside faucet. Do not run the well pump too long at any one time. Direct the flushing away from plants, since the strong chlorine solution will kill them. Do not drain chlorine into your septic system.
    Keep using the water until you can no longer smell any chlorine in the house (this may take several days). Then we recommend that you use the water for another week in order to allow any possible sources of contamination time to infect the well.
    Finally, have water retested for bacteria.
    Contact: Mona Freese, Microbiology Supervisor ([email protected])
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NH Department of Environmental Services | 29 Hazen Drive | PO Box 95 | Concord, NH 03302-0095
(603) 271-3503 | TDD Access: Relay NH 1-800-735-2964 | Hours: M-F, 8am-4pm

copyright 2016. State of New Hampshire