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NPS Arrowhead National Park Service
US Department of the Interior
Office of Public Health 1201 Eye Street, NW
Room 1131
Washington, DC 20005

Phone: 202-513-7215
Fax: 202-371-1349
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Office of Public Health - Drinking Water System Superchlorination Factsheet
Points Of Contact
(202) 513-7217
Assistant to Director for Science
(202) 513-7097
(505) 248-7806
Assistant to Director for Field Operations
(202) 513-7056
National Capitol Region
Northeast Region
(215) 597-5371
Southeast Region
(404) 507-5730
Mid-West Region
(402) 661-1718
Intermountain Region
(505) 988-6040
Pacific West Region
(510) 817-1375
Alaska Region
(206) 220-4270


Complete all repairs before decontaminating the system. Prevent surface water and near surface ground water from entering the well in the future. Seal pipes and wires entering the well and seal well casing joints near the ground surface in bored wells

High concentrations of chlorine in water is toxic and can burn the skin on contact and cause illness if consumed. Warn all potential users of the water about the dangers.


Small water systems, such as churches and day care centers should disinfect the plumbing system before disinfecting the well. The following steps are suggested:

1. Pour 3 to 4 gallons of bleach into the well and recirculate the water to the well. Use a clean garden hose to discharge water from any convenient spigot into the well. Continue the process until you smell bleach in the recirculating water.

2. Stop recirculation. For all remaining spigots (hot, cold and fountains), open the spigot until you smell bleach and then close the spigot.

3. Turn off water heaters to prevent damage from accidental system drainage.

4. Disinfect the well as described below.


Calcium hypochlorite tablets are recommended for the superchlorination (disinfection) of a well. Chlorine must be distributed from top to the bottom of the well. Liquid chlorine bleach or granular chlorine powder will usually not distribute chlorine over the entire water column. Use tablets with “Calcium hypochlorite” as the active ingredient; other forms of chlorine tablets are not suitable for use in wells. Unbroken tablets are oval shaped, about 1 1/4" long and 3/4" across, have rounded ends and are usually sold under the “H.T.H.” brand name. Sources of tablets include water treatment supply vendors, building supply centers and hardware stores and sold in 5 or 6 pound containers.

1. Break the tablets into different sized pieces by placing them on a solid surface (brick or concrete) and tapping with a hammer. Read and observe container safety precautions, use eye protection and wear rubber gloves. Do not wrap tablets for breaking

2. Pour broken tablets directly into the bored well. Remove the well air vent and insert the broken tablets through the vent opening in a drilled well. A rod or screwdriver may be used to force the particles through the opening.

3. Recirculate water to wash down the well walls if this was not done during the plumbing system disinfection. Replace the vent if removed for chlorine addition or recirculation. Turn the pump off and allow the chlorine to stand in the well and system for at least 24 hours.

Clean Up:

1. Pump the chlorinated water to waste starting with the outside spigot. Pump until the water no longer smells of bleach and then flush through the remaining outside spigots.

2. Flush through all other spigots, fountains and connections until you do not smell bleach.

3. Before resampling for bacteria, test to confirm that all chlorine has been removed (a swimming pool test kit can be used in this case)

Caution: Highly chlorinated well water can damage grass and plants. Do not discharge significant amounts of highly chlorinated water into a septic tank. Any trace of chlorine in a water sample invalidates laboratory test results. Repeat tests are costly and delays inconvenient.

Tablets Needed to Obtain 200 mg/l (ppm) Chlorine
Drilled Well Bored Well
Diameter of well Tablets per 100 ft Diameter of well Weight per 50 ft
2 inches 4 tablets (1 oz.) 22 inches 2.5 lbs.
4 inches 12 tablets (3 oz.) 24 inches 3 lbs.
6 inches 24 tablets (6 oz.) 28 inches 4 lbs.
8 inches 43 tablets (11 oz.) 32 inches 5.5 lbs.
    36 inches 7 lbs.

If you have any questions, please contact the nearest Point of Contact, park sanitarian or the Washington Office.

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