Disinfecting Your Drinking Water

Water that is contaminated with fecal coliform, or E.coli bacteria, can cause health problems if ingested.

What should I do if my drinking water is contaminated?

  1. Boil your water for one minute before using it. All water used for drinking; making ice, juice or baby formula; and for washing fruits and vegetables should be boiled first.
  2. Look at the location and construction of your water source and try to identify and eliminate the source of contamination.
  •  Wells and springs should be isolated, preferably uphill from septic systems and other potential contamination sources, such as barnyards and pasture.
  •  Dug wells and springs should be made of concrete and have tight-fitting, lipped covers and sealed joints. Drilled wells should have casings that extend 18 inches above ground, surrounded by mounded clay to prevent surface water from entering.
How to Disinfect Your Water System

Before you begin, disconnect or remove any water treatment devices, such as activated carbon filters, water softeners or reverse osmosis units. Highly chlorinated water can make them less effective or even damage them.

Dug or Drilled Wells
Add one gallon of household laundry bleach (5.25%) for every 525 gallons of water. See the section below on how to calculate the amount of chlorine needed.

Overflowing Springs
Use fast-dissolving 65% calcium hypochlorite pellets (3 ounces for every 100 gallons of water or about 2 feet in depth). Pellets are available at pool supply or hardware stores.

  • CAUTION: The pellets should contain ONLY calcium hypochlorite—NOT algicides, chlorine stabilizers/conditioners, acids, or other disinfectants. (These may be acceptable to use in swimming pools, but are not safe for drinking water).

Whenever possible, run the chlorinated water through a garden hose back into the well for an hour so the chlorinated water washes down the inside of the casing. Scrub the sides of a dug well or spring with a clean brush and a chlorine solution (mix one part household laundry bleach to four parts of water). Use rubber gloves and eye protection.

After you recap the well, open one faucet at a time throughout the house. Run the water until you smell a strong chlorine smell, then turn the tap off.

  • CAUTION: Strong chlorine solutions may damage rubber and polybutylene gaskets and fittings.

It takes time for chlorine to disinfect, so it is important to keep the chlorinated water in your system for 12 hours. After 12 hours, connect a hose to a tap or outside faucet and drain the chlorinated water to a safe, outdoor location. Don’t drain highly chlorinated water onto a lawn or garden or into the septic system. Dispose of the water onto a safe area, such as a graveled driveway or into a brushy area. Keep children and pets away from the discharge, and don’t allow the chlorinated water to reach a stream, because it could kill fish. Skin and eyes may be sensitive to strong chlorine solutions. Avoid contact during disinfection.

To avoid overstressing your water source, run the water at less than full flow or turn off the tap periodically and allow the source to recover for several hours. Continue flushing until the chlorine smell is gone.

Re-test the water for bacteria two to three days after the chlorine smell has disappeared. Continue to either boil water or get drinking water from a safe source until test results indicate the absence of bacteria.

How to Calculate the Amount of Chlorine Needed to Disinfect a Well

Using the Chlorine Concentration Calculator, enter in the:

  1. Depth of your well
  2. Depth to the water table
  3. Diameter of your well

Be sure to check the concentration of the bleach you use. The calculator will give you the amount of chlorine to use for 5.25% and 8.25% solutions. Many household bleach products are concentrated at 8.25%. Remember to use ordinary household bleach without any added chemicals.

Use the Chlorine Concentration Calculator

Emergency Disinfection

To make water from a spring or well of unknown quality safe to drink on a temporary basis, mix the water with liquid chlorine bleach as shown in the Table of Chlorine Dosages below.

This method will protect against most, but not all, disease-causing organisms. (NOTE: These directions do not apply for disinfection of newly constructed, repaired, or bacteria-contaminated springs and wells. See “How to Disinfect Your Water System” above).

Mix the water and chlorine thoroughly and then let it stand for 2 to 3 hours. Store the water in a human-made covered reservoir, tank or other sealed clean container to protect against recontamination.

Spring, pond, stream or well water that is turbid (cloudy or containing particles) must be boiled for five minutes before using it for drinking; making ice, juice or baby formula; or washing fruits or vegetables.

Chlorine or any other disinfectant will not work well when water is turbid. The chlorine cannot always go through the suspended particles and may not get rid of organisms that cause sickness.

Table of Chlorine Dosages
wATER vOLUME LIQUID BLEACH*
1 gallon 5 drops
10 gallons 1/2 teaspoon
50 gallons 2 teaspoons
100 gallons 1 tablespoon
500 gallons 5 tablespoons
1,000 gallons 3/4 cup

* Household laundry bleach (5.25% chlorine concentration)

Frequently Asked Questions

My water is contaminated. Why hasn’t it made me sick?
Coliform or other bacteria will not necessarily make you sick. However, since these organisms have been able to enter your water system other disease-causing organisms (bacteria, viruses and protozoa) could enter as well. To prevent sickness, the Health Department recommends that you boil your water for one minute before drinking.

Can water tests pinpoint the exact source of contamination?
No. Water tests show that coliform bacteria have entered your water system, either at the source or between the source and the faucet. The tests can't tell you where the source of the contamination is.

I disinfected my well and consulted with the Health Department, but my water is still contaminated. What should I do now?
You may need to hire an environmental engineer or hydrogeologist to find the problem. The Health Department can give you a list of water system consultants and water treatment options.

Watch this video to learn how to shock chlorinate your well. The video will help you understand your well water system, how to calculate the volume of bleach needed for disinfection, and how to safely apply it.

If you have questions, call the Drinking Water Program at 802-863-7220 or 800-439-8550 (toll-free in Vermont) or email AHS.VDHEnvHealth@Vermont.gov.