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Social Media links WEBSITE OF THE STATE OF SOUTH DAKOTA DEPARTMENT OF HEALTH
Doneen Hollingsworth, Secretary of Health
South Dakota Department of Health

Flood Response - Water for Drinking and Cooking

Disinfect: WellCisternSmall Quantities of Water | Biological Water Sampling |

Public and private water supplies may be contaminated in a flood. After a flood, consider all water unsafe. Listen for public announcements on the safety of your area’s water supply and follow the instructions of local authorities.

Private water wells should be pumped out, allowed to recharge naturally, disinfected and the water tested before drinking or being used for cooking.

The safest approach is to drink and cook with bottled water or water previously stored in the refrigerator. If you have to use tap water, follow the recommendations for disinfection of small quantities of drinking water in the following sections.

Do not use contaminated water to make ice, brush your teeth or wash dishes. If there is a shortage of safe drinking water, use disposable eating utensils, plates and napkins.

Disinfect a Well

Adequate disinfection requires a certain chlorine dosage for a minimum contact time. We recommend 50 parts per million for eight hours or overnight. The tables that follow are based on the overnight contact time. We also recommend using household bleach for the disinfectant. Household bleach is a 5.25% hypochlorite solution commonly sold under brand names such as "Hilex", "Clorox" or "Purex." These products are available at all grocery stores and most discount marts.

To disinfect your well, please follow the recommended procedure:

  • Determine the chlorine dosage for your well from the following table. Again, this is the amount of chlorine to add for an eight-hour (overnight) contact time.
Inside diameter
of well (inches)
Quantity of chlorine (bleach) for
each 10 foot of water in well

2 1/4 ounce

4 3/4 ounce

6 2 ounces

8 3 1/2 ounces

10 5 ounces

12 1 cup

18 2 1/4 cup

24 3 3/4 cup

36 8 3/4 cup

 

  • Lift the well pump and pour the required amount of chlorine into the well.
  • Lower the pump and open every faucet in your system and let the water run until you can smell the chlorine. Close all the faucets and let the chlorine stand in the well overnight. Do not use the water during this period.
  • The next morning, again turn on all the faucets and pump water until all the chlorine odor disappears.
  • Forty-eight hours after chlorination collect a water sample for bacteriological testing. (return to top)

Disinfect a Cistern, Reservoir or Spring Box

Adequate disinfection requires a certain chlorine dosage for a minimum contact time. We recommend 50 parts per million for eight hours or overnight. The tables that follow are based on the overnight contact time. We also recommend using household bleach for the disinfectant. Household bleach is a 5.25% hypochlorite solution commonly sold under brand names such as "Hilex", "Clorox" or "Purex." These products are available at all grocery stores and most discount marts.

To disinfect your storage unit follow the recommended procedure:

  • Determine the chlorine dosage for your unit from the following table. This is the amount of chlorine to add for an eight-hour (overnight) contact time.
Volume of box, basin,
reservoir or cistern (gallons)

Quantity of bleach


50 3/4 cup

200 3 cups

500 7 1/2 cups

1,000 1 gallon

2,000 2 gallons

5,000 5 gallons

 

  • Make sure that your storage unit is completely full. Mix the required amount of chlorine throughout the unit.
  • Let the chlorine stand in the unit overnight. Do not use the heavily chlorinated water.
  • In the morning, remove the water from the unit. Do not allow the water to enter a river, lake or stream.
  • Fill the unit with fresh water.
  • Forty-eight hours after chlorination collect a water sample for bacteriological testing. (return to top)

Disinfect Small Quantities of Drinking Water — Chlorination or Boiling

Adequate disinfection requires a certain chlorine dosage for a minimum contact time. We recommend 50 parts per million for eight hours or overnight. The tables that follow are based on the overnight contact time. We also recommend using household bleach for the disinfectant. Household bleach is a 5.25% hypochlorite solution commonly sold under brand names such as "Hilex", "Clorox" or "Purex." These products are available at all grocery stores and most discount marts.

There are many occasions where you may want to disinfect a small quantity of water. This discussion is for those times you wish to disinfect less than 5 gallons. The water must not be cloudy or dirty, the chlorine must be mixed thoroughly with the water, at least 30 minutes of contact time must be provided and the treated water must be protected from further contamination.

To decontaminate the drinking water, place it in a clean container. Add 6 drops (1/8 teaspoon) of unscented chlorine (bleach) for each gallon. If you have one gallon of water to be disinfected, you would add 1/8 teaspoon of bleach. If you have four gallons of water, add 24 drops. After the chlorine is added, mix thoroughly. Cover or cap the container and allow to sit undisturbed for 30 minutes. A slight odor and taste will be apparent but are entirely harmless to humans and will disappear with time.

This water should not be used by pregnant women and mothers that are either nursing or mixing formula, unless the nitrate level has been determined.

Boiling is an excellent option to chemical disinfection, but again, it should not be used unless the nitrate concentration is known. If you know that the nitrate level is low, less than 5 mg/l for example, you may disinfect by allowing the water to boil for three minutes after it has reached a rolling boil. Allow the water to cool and store in a well covered container.(return to top)

Biological water sampling

A water sample should be collected about two days after the disinfection of the water supply is complete. Sample containers are available from your community health nurse, county extension agent, or the Public Health Laboratory. PLEASE USE ONLY THE CONTAINERS PROVIDED AND FOLLOW DIRECTIONS CAREFULLY. (return to top)

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